The vacuum system serves three functions:
By the standards of the vacuum industry, our requirements are modest and are generally called a "medium" vacuum - not the sort of situation where every last "particle" of air needs to be removed but still 1/100th to 1/10,000th of normal atmospheric pressure. These are not generally considered a "high" vacuum (though that's what may be found in catalog listings for the types of mechanical vacuum pumps and gauges that are used) but they are still not the sort of thing you come across in daily life. However, to be able to fill the laser with (mostly) pure gases, it is desirable for the vacuum system to go substantially lower in pressure than that at which the laser operates. And this includes the upper end of the high vacuum range - down to perhaps 1,000,000th of normal atmospheric pressure.
I am gradually putting together a vacuum system (or at least acquiring parts!) and may have an interest in your cast-off or excess vacuum equipment (relatively small items, not complete ion beam machines!) or accessories. Please see: Sam's Classified Page, "Wanted to Acquire" near the end, if you have anything available.
Its parent site, The Bell Jar has an index to many additional articles available only in hard copy and/or by subscription, vacuum equipment suppliers and links. and a Users' page which includes vacuum equipment "For Sale and Wanted" but doesn't appear to have been updated in a couple of years.
These include:
Try to locate "Procedures in Experimental Physics" by John Strong (who wrote many of the SciAm Amateur Scientist articles). In particular, see Chapter 3: Technique of High Vacuum.
"I highly recommend a really nifty book also from the Society of Amateur Scientists. It's one of the old reprints that Lindsay books does in cooperation with SAS called Procedures in Experimental Physics.
The vacuum section is EXCELLENT for the science hacker. Everything from building vacuum gauges from vacuum tubes to desktop thin film sputtering to CVD coatings to making your own diffusion pumps from available materials.
The earth is covered with a vast ocean of air. Despite common experiences, even air has mass and mass implies weight. We know it has volume or else your automobile would have a real problem with flat tires. Most of the volume (the contribution from the volume of the the protons, neutrons, and electrons in the atoms are negligible but not precisely zero) results from the constant motion of the molecules (in air or other gas) bouncing against each-another due to their thermal motion. This also keeps the air in a gaseous state. At really low temperatures, the motion is reduced resulting in liquid and solid phases of even air. At exactly absolute zero (0 °K or -459 °F or -273 °C) all motion ceases. However, even then most of the volume of the frozen air is still empty space - but that is another story.
At sea level under average conditions, the column (actually an inverted truncated pyramid if you want to be strictly correct) of air above 1 square inch of area would weigh 14.7 pounds if you could capture, compress, and package them and plop them down on a delicatessen scale! As you move away from the earth, this 'column' of air becomes increasingly rarified approaching a prefect vacuum at 50 miles or so - else low earth orbit satellites would not stay up very long due to air friction.
It turns out that a column of mercury with an area of 1 square inch and 29.92 inches (760 mm) high weighs exactly 14.7 pounds as well (what a coincidence, huh?). So, if you take a closed-end tube a little more than 30 inches long, fill it with mercury, and invert it in a pool of mercury, the pressure of the surrounding air will be able to support a column of mercury 30 inches high. The space above the mercury will be a decent vacuum. You have made a mercury barometer. (Strictly speaking, there will be mercury vapor in that space but it won't affect the height by much.)
If you were to take this barometer and place it inside a vacuum vessel and start up the pump, the column would go down until at the point of a perfect vacuum (not achievable but close), it would be precisely level with the surrounding pool of mercury.
Note that the diameter of the tube doesn't matter - wider implies a heavier column of mercury but the area of the air acting on the column changes by the same factor. In fact, it can have pretty much any convoluted shape you want (except that if portions are too thin, surface tension becomes a factor) as long as it is sealed and totally filled with mercury. Why this is so is left as an exercise for the student!
The corresponding height of 1 atmosphere for water is about 34 feet - a column of water with a cross sectional area of 1 inch and height of 34 feet weighs 14.7 pounds. This also means that for a diver, the water pressure increases by 1 atm for each 34 feet of depth. Thus it is not surprising that there are significant problems in deep sea diving! You have to go up by MILES in air for the pressure to decrease by a substantial fraction of 1 atm but need only go down 34 feet in water to increase pressure by 1 atm!
Note that the most likely form of a pressure you are familiar with is the reading on the gauge you use when checking or filling your automobile or bicycle tires. However, this is calibrated relative to the surrounding pressure of around 1 atm. Thus, the actual pressure inside a tire will actually be 1 atm + the reading on the gauge. And you thought you had a perfect vacuum inside that flat tire when the reading was 00.0! :-)
Similarly, a vacuum can be measured relative to atmospheric pressure and this is often done for the sort of vacuum you find in an automobile engine intake manifold, vacuum hold down plate, and other familiar applications. However, these readings represent the difference between one large number (local atmospheric pressure) and another large number (your vacuum). Since weather conditions (i.e., high and low barometric pressure) can result in a variation of 1/2 inch of mercury or more, such measurements will fluctuate and aren't very useful when the absolute level of vacuum is what's important - as with gas lasers and most other scientific uses of vacuum.
Here are some units and relationships commonly found in dealing with vacuums:
If you are totally confused at this point, for a wonderful description of these units and their history, see: The Electronic Bell Jar - Pressure Units. Some high vacuum equipment suppliers have conversion charts and other related information. See for example, LDS Vacuum Products Techtips Page.
Or, here is an instant conversion chart. To convert from the units in a row on the left, multiply by the entry in the appropriate column.
Micron Pascal Millibar Torr Inches
----------------------------------------------------------
Micron 1 0.133 0.0013 0.001 1/25,000
Pascal 7.5 1 0.01 0.0075 1/3,387
Millibar 750 100 1 0.75 1/33.87
Torr 1,000 133 1.33 1 1/25.4
Inches 25,400 3,387 33.87 25.4 1
The Vacuum Chart provides a nice instant summary of pump types, gauges, and applications, as a function of the level of vacuum.
The following dividing lines between low, medium, high, and ultra-high vacuums are somewhat arbitrary but will be convenient for discussion:
Mountain climbers have to endure reduced pressure and above about 10,000 feet, require breathing equipment. Anyone who has traveled by air knows the standard speech at the beginning of each flight "....should oxygen be needed, the compartments overhead....". This would also happen above about 10,000 feet.
Astronauts on American spacecraft (at least they used to), breath unaided at a pressure of perhaps 1/5 of an atm because they breath nearly pure oxygen. Since in the normal atmosphere, oxygen is only about 18 percent of the total mixture (most of the rest is nitrogen with a little CO2 and inert gases thrown in), the resulting biological activity (and the flammability of common materials, for that matter) is about the same but there is no need to carry the approximately 80% of useless other gases and the stesses on the spacecraft structure (from the difference between the internal pressure and the vacuum outside) are reduced by 80% as well.
A low vacuum can be obtained by any number of simple mechanical means including fans and centrifugal blowers, piston and rotary pumps, aspirators, siphons, chemical combustion and other reactions (which use up the air), etc. Liquids boil at reduced temperature - often room temperature - in a modest vacuum but minimal or no precautions are needed to prepare surfaces and equipment since any outgassing is small compared to the remaining air.
I did a few very scientific experiments to determine values of two types of vacuums with which everyone is familiar:
So, next time the friendly vacuum cleaner salesperson calls, forget the oatmeal test, let them try a Bourdon tube gauge instead! :)
However, note that normal straw sucking would result in hardly any vacuum at all, only needing to raise a fluid by a few inches; 15 inches of mercury is like drinking through a straw from a container about 16 feet below you!
A medium vacuum can be achieved with a high quality mechanical pump.
A high vacuum usually typically requires a multi-step pumping scheme with a rotary mechanical pump going down to 10-2 to 10-3 Torr followed by a diffusion, turbo-molecular, or other true high vacuum pump.
Even if the final vacuum is modest (e.g., 1 to 10 Torr), being able to pump down to an ultra-high vacuum may be needed to purge the tube or whatever of contaminants with a small number of pump down/backfill cycles and minimal use of expensive gases.
In addition to mechanical and diffusion or turbo pumps, additional means are required to achieve an ultra-high vacuum including exotic ion pumps, cryo pumps, cold traps, and chemical getters. Surfaces exposed to the vacuum must be immaculate - a single fingerprint can mess things up for days!
To put a 10-9 Torr vacuum into perspective: If all of the gas molecules remaining inside a typical 17 inch monitor CRT that had been manufactured at this level of vacuum were rounded up, captured, and returned to normal atmospheric pressure, they would occupy a volume of space less than 25 um on a side - roughly 1/10th the diameter of the dot in the explanation point at the end of this sentence or half the diameter of a human hair! Yet, inside the CRT, there would still be approximately 1,000,000,000,000 gas molecules remaining for unsuspecting electrons to run into!
You may also hear the term 'hard vacuum'. I don't know if there is a precise definition for this either but I would assume that anything with a low enough pressure to behave similarly to a perfect vacuum from the normal experiences point of view would qualify. Also note that in terms of the strength required of a vacuum vessel, the difference between a vacuum of 1 Torr and 10-19 Torr is irrelevant. Why? :)
(Original chart from: Chris Chagaris (pyro@grolen.com).)
Inches of Hg Torr or
Rel to 1 atm Absolute mm of Hg PSI % Vacuum Microns
----------------------------------------------------------------------
0.0 29.92 760 14.696 0.0 -
0.40 29.52 750 14.5 1.3 -
10.24 19.68 500 9.7 34.0 -
18.11 11.81 300 5.8 61.0 -
25.98 3.94 100 1.93 87.0 -
27.95 1.97 50 0.97 93.5 -
28.92 1.00 25.4 0.4912 96.6 -
29.52 0.40 10.0 0.193 98.7 -
29.88 0.04 1.0 0.0193 99.9 1000.0
29.916 3.94*10-3 10-1 1.93*10-3 99.99 100.0
29.9196 3.94*10-4 10-2 1.93*10-4 99.999 10.0
29.91996 3.94*10-5 10-3 1.93*10-5 99.9999 1.0
29.919996 3.94*10-6 10-4 1.93*10-6 99.99999 0.1
29.9199996 3.94*10-7 10-5 1.93*10-7 99.999999 0.01
Unless you have worked with a decent vacuum system in the past, own a HVAC service business, or just happened to pick up something that looked like a pump of some kind at a garage sale (but you weren't really sure and got lucky), you don't have the needed equipment! However, an adequate 'medium' vacuum system can be put together for less than $400 - possibly a lot less if you are determined and somewhat resourceful.
Note that it is the difference between atmospheric pressure and that of your vacuum that determines the stress on the container - whether you are pumping down to 10 Torr or 10-14 Torr is for all practical purposes irrelevant with respect to implosion risk!
If you insist on trying these bombs, cover the whole affair with some sort of shatter-proof outer cover like a thick solidly constructed Plexiglas shield but I don't recommend this in any case.
See Vacuum Pumps Suitable for Various Home-Built Lasers for diagrams of the types of vacuum pumps described below that are relevant for our purposes. See the links in the section: Vacuum Systems References, Links, Forums for more complete information on the operating principles of the pumps described below (as well as many other topics).
There are many types of mechanical pumps but they are usually based on one of two basic principles: positive displacement (perhaps these should be called negative displacement in dealing with vacuums!) and turbo-molecular:
The rotary vane type is the most common 'roughing' or 'backing' pump for use with high vacuum systems. (These terms indicate that there is another pump or pumps for actually going from a pressure of a few milliTorr to a pressure of 10-6 Torr or lower.) This is due to their relative simplicity and their ability to achieve decent performance with reasonable tolerances in the machining of their component parts. A shaft and rotor mounted off-center in a cylindrical pump chamber rotates a set of spring-loaded sliding vanes (usually 2 but possibly 4 or more). The vanes sweep a variable size volume which increases on the vacuum (intake) side and decreases on the exhaust (outlet) side. The entire mechanism is totally immersed in low vapor pressure oil. While the oil is essential for lubrication and cooling, its most critical function is to seal the spaces between the moving parts - there are no other seals between the rotor, vanes, pump chamber, and end-plates. Unlike a piston pump in which the ultimate vacuum is determined by the dead space above the piston and in the valves when the piston is at the top of its stroke, a well constructed rotary vane pump's performance is limited mainly by the back-streaming of gases from the oil. In most cases where such a pump fails to meet expectations with respect to ultimate vacuum, the cause is contamination of the oil (or the use of improper oil with a high vapor pressure) rather than a problem with the pump mechanism.
For more details and an animated graphic, see Lesker's Rotary Vane Mechanical Pump Technical Notes. (Local copy should this link decay: Copy of Lesker's Rotary Vane Mechanical Pump Technical Notes.)
A diffusion pump has no moving parts (at least at the macroscopic level). An electric heating element in its base boils a small quantity of a special 'diffusion pump oil' inside a sort of tower or percolator structure which has vents to direct the jets of oil vapor downward toward the higher pressure region (to the mechanical pump) where it condenses on the cool surfaces of the pump housing and is recycled. In in the process, air and other gas molecules are dragged along with the oil vapor. When the oil vapor condenses near the bottom (higher pressure) part of the diffusion pump, the trapped molecules are released and sucked up by the roughing pump. The actual pressure differential between the top and bottom is miniscule - only a fraction of a Torr (and the diffusion pump cannot be fired up until the roughing pump has brought the vacuum down to this level). But this is adequate to suck out most of the remaining air or other gas molecules and once it gets going, the pumping speed of a diffusion pump is quite impressive despite its passing resemblance to a coffee percolator. I know what you're thinking. :)
Diffusion pumps require cooling of their own. This is usually tap water through a coil wrapped around their exterior though some use some sort of air cooling.
For the cyclotron at my high school (right, how many high schools have atom smashers - but that is another story), we had an air-cooled glass oil diffusion pump (probably because no one else wanted it or even knew what it was). Somehow, this fragile glass structure survived all sorts of catastrophies despite being located under the main vacuum chamber situated between the pole pieces of a magnet weighing several tons and joined by a clamp-type glass and O-ring seal.......... OK, I know you're curious. See the section: The Central High School Cyclotron.
An alternative to a diffusion pump that appears distinctly low-tech (but no doubt requires very high-tech manufacturing) is the turbo-molecular pump (or "turbo pump" for short). Like the diffusion pump, the turbo pump is placed in-line with the roughing pump. A multistage turbine running at very high speed generates a compression ratio of 108 or more for outlet pressures in the range of 50 microns to 20 Torr (depending on model). These represent an alternative to diffusion and ion pumps for going 'the last mile'. Thus, an entirely mechanical high vacuum system is possible (and quite common). You probably won't see one of these at your local hamfest but one may be gathering dust in the deep dark recesses of one of your University's physics labs. They also appear on eBay from time-to-time but the final bid prices are usually quite high - typically around $1,000 for a small one - and that may not even include the controller! See the section: A Small High Vacuum System.
Turbo pumps can achieve a very clean ultra-high vacuum but due to their high speed (up to 90,000 rpm or more!), are not very tolerant of abuse or debris entering their intake port. And, at these speeds, mechanical bearings represent a maintenance item. Some use a super high quality ceramic ball bearing with special oil lubrication at one end and a passive magnetic bearing at the other end (just a couple of ceramic ring magnets, one partially inside the other). The best type use magnetic bearings throughout with the lower one usually being active feedback controlled.
For more details on turbo-molecular and another related type, the molecular drag pump, see Lesker's Turbo-Molecular and Drag Pumps Technical Notes. (Local copy should this link decay: Copy of Lesker's Turbo-Molecular and Drag Pumps Technical Notes.)
And, if you're not too squeamish, see LesioQ's Edwards Turbo Pump Disassembly Page for a dissection of one that must have lost a battle with some debris.
Though no self respecting high vacuum system would be without at least one of these high vacuum pumps, this is not really essential for most of the gas lasers under discussion especially if you have a well maintained 2 stage (or better) mechanical pump. However, if you come across a small diffusion or turbo pump (almost any size would be adequate for laser tubes) in good condition at a decent price, grab it. You can never tell when your interests might wonder in directions where a true high vacuum system would be needed.
While not generally thought of as pumps, the following perform related functions helping to rid the system of moisture and other unwanted volatile materials:
In addition to helping to achieve a high vacuum, a dryers and cold traps may also help to prevent contamination to the oil in the vacuum pumps.
(From: Steve Roberts (osteven@akrobiz.com).)
You really want an ion pump or a getter sublimation pump a.k.a. Titanium Pump (TP), or if you're really lucky, a turbo pump, although they don't hold up to abuse too well. (A debris screen should be installed on the turbo pump inlet and even with that, extreme care must be taken to assure nothing other than gas gets to the high speed turbine blades.)
Diffusion pumps are old news and outdated, they run hot, and some are huge! Depending on what is being pumped, they may need to be torn down and cleaned frequently. Small ion pumps are about $1,200 rebuilt. maybe $250 for the pump and $1,000 for the controller after rebuild, which means they are dirt cheap if you find a used one. I've seen a few diffusion pumps given away in the past few years but they really were clunkers and sensitive to contamination. Ion pumps do run out as they have a consumable material inside them that reacts with remaining gas, but they also get you a cleaner vacuum then a diffusion pump.
Ion pumps also act as a gauge by themselves, although not as accurate as a dedicated gauge. What you want is an appendage ion pump. They are small and fast for your purposes, and small enough you can lift them.
TPs use a hot titanium filament to bury gas molecules under a thin film of metal. The only thing they use up are rods which are presently about $160 for a lifetime supply. TP are more suited for large vacuum chambers, but they create a fast clean vacuum and are resistant to an amateur's mistakes.
The gauges are only about $250 for a new digital one, which is almost cheaper then buying or scrounging a used ThermoCouple (TC) gauge and these do wear out. You're better off looking for a digital capacitance manometer then an old beat up TC. (Of course I've acquired several complete TC gauges for less than $50 each complete and put one together for around $25 - the cost of the sensor - using junk parts. --- Sam)
A well maintained two-stage rotary mechanical pump (the sort of thing you find in high school physics departments. OK, perhaps except for the 'well maintained' part!) can achieve a vacuum of less than 5 milliTorr (5 microns) if conditions are favorable. However, with use and age which results mostly in oil contamination, even 100 milliTorr may be optimistic. So, if you find one of these at a garage sale (as I have), it may be necessary to at least totally drain the old oil; flush, run, and drain again; and then fill with fresh vacuum pump oil of the proper type. Just an oil change without flushing isn't quite as good but may achieve adequate performance for a pump that hasn't been abused. Proper rotary vane vacuum pump oil is available from vacuum, scientific, and laboratory supply houses as well as from refrigeration service companies. No, motor oil or 3-In-One won't do! Refrigeration service pump oil isn't what you want for a true high vacuum but is good as a flush and is certainly adequate for the CO2 and N2 lasers. It may even be available from a place like Pep Boys at all of $10 for a gallon! (But real vacuum pump oil is also available for about $10 to $15 per gallon from some suppliers like Duniway Supply.) Flushing the pump and replacing its oil may be the single most important thing to achieve acceptable performance. Unless the pump was abused, it will probably be all that is needed.
See the section: Rotary Vane Vacuum Pump Mainenance for more details on reviving a pump that doesn't pump very well. :)
Moisture is also a killer of oil, so using such a pump as a wet-dry vac isn't a good idea either! A "gas ballast" capability is a feature common to many rotary vane vacuum pumps. (The actual technical term is "vented exhaust valve" but "gas ballast" is what is most often found in vacuum pump descriptions.) Whatever it's called, the purpose is to introduce atmospheric pressure air into the outlet gas stream to dilute any water vapor and reduce the amount that gets dissolved back into the oil. However, the gas ballasting usually increases the base (lowest) pressure attainable by up to a factor of 10 or more depending on design so where there is an option (usually an adjustable valve), it should only be used near the beginning of a pump-down cycle and then disabled (closed). The gas ballast can also be used to help purge the oil of dissolved vapors since air bubbling through the oil will tend to pick up dissolved stuff. For most home-built lasers, this isn't an issue since there shouldn't be much water vapor to evacuate and having no gas ballast will be just fine. However, some pumps (usually HVAC types) don't provide the option of disabling the gas ballast (though there are usually ways around this). A gas ballast that is always enabled may limit high vacuum performance since as noted it can increase the lowest attainable pressure.
Small high quality rotary vane pumps may cost $1,000 or more new, perhaps $500 used but it's quite possible to find them even cheaper from auctions and other sources. See the section: Sources for Vacuum Equipment and Supplies.
CAUTION: Many of these pumps DO NOT provide an automatic means of preventing pump oil from being sucked back into the evacuated part of the system after the pump is shut off. Unless the instructions for your pump explicitly state that a "safety valve" or "automatic vent valve" or something similar is provided - and it is known to be operating correctly - the system must be isolated or vented manually to prevent the possibility of oil entering EVERYTHING including the vacuum gauge sensor heads. This will likely require a complete tear-down and cleaning to restore reasonable performance. The vacuum gauges should definitely NOT be powered up if their sensors are contaminated with oil as they may be damaged resulting in altered calibration or total destruction. Pfeiffer rotary vane pumps provide an electrically or hydraulically actuated safety valve; Welch two-stage pumps are supposed to be immune but Welch single stage pumps are not (both unconfirmed); and Precision Scientific pumps may allow much of their oil to enter the system. Don't ask me how I found out. :(
If you do pick up one of these used with original oil, replacing the oil should greatly improve its performance as all sorts of contamination can be sucked in when used to evacuate refrigeration systems and there was probably little or no maintenance ever performed on the pump itself!
Note that for no justifiable reason, HVAC service pumps tend to have really narrow inlet fittings and these should be replaced to achieve best performance for general vacuum work. This is easily accomplished if they use standard pipe threads. If not, some improvisation will be needed. I replaced the fitting on my Precision Scientific D25 look-alike which had been used for HVAC service and had a very narrow screw-on hose connector. While the original fitting was very convenient since no tools were required to change hoses, the hoses were also very narrow - about 3/16" ID - and were so contaminated with who-knows-what or leaked or something that pulling any sort of decent vacuum wasn't possible anyhow. The replacement was a brass 5/8" HB x 1/2" MPT hose barb - about $2 at the local hardware store. (For most home-built lasers, this modification really isn't essential since the required flow rates are generally not that great but it won't hurt and then you'll be all set for the future.)
The Electronic Bell Jar has a detailed article on refrigeration service pumps.
The specifications below are for Welch's belt-drive ("DuoSeal") pumps. Newer ones will always have a belt guard (some OSHA requirement I assume) but many of the older pumps do not. (Adding a belt guard is definitely advised in any case unless the pump can be permanently positioned to place the pulley-side against a wall out of harm's way - yours and the pump's.) For a given pumping speed, belt-driven pumps run slower and cooler (which increases reliability and life expectancy of both the pump and oil) and tend to be quieter than similar performance direct-drive pumps, but they are larger, heavier, and likely to vibrate more. However, should the motor need replacing for some reason on a belt-drive pump, a standard model from any number of sources or your junk corner will do. The motor in direct-drive pumps (which Welch also manufactures) may only be available from the original supplier, if at all and installation can be a real treat. I also don't think direct-drive pumps have quite the same aesthetic appeal as belt-driven pumps. :) A testament to Welch's pumps longevity is that these same models have been around for at least 50 years and probably a lot longer. And, those 50 year old pumps are still serviceable, requiring at most a cleaning and relatively inexpensive rebuild kit to meet original factory specifications. A photo contact sheet with many of these pumps is shown in Some Welch Belt-Drive Rotary Vane Vacuum Pumps. These photos are prettier than the ones on the Welch Web site and I believe the captions match the pump which wasn't the case the last time I checked there. :)
Here is a chart of the most important specs for a variety of Welch belt-drive rotary vane vacuum pumps. More detailed specifications for these as well as Welch's direct drive pumps may be found on the Welch Vacuum Web site. They also have repair parts and kits for all their pumps as well as exploded diagrams (with repair manuals and more coming soon) should your acquisition turn out to need a bit of maintenance or a miracle. :) (See: Basics of Vacuum Pump Repair for a really quick summary of an overhaul.) These pumps and pumps from other manufacturers can also be found in the catalogs or on the Web sites of vacuum equipment suppliers like Duniway Supply or Lesker Vacuum Systems and Components.
Ultimate Pumping Motor
Number Vacuum Speed Power
Model of Stages (Torr) cf/m l/m HP
--------------------------------------------------------
1399 x+ 1 1.5x10-2 1.2 35 1/3
1404 1 1.5x10-2 1.2 35 1/3
1406 1 1.5x10-2 2.1? 60? 1/2?
1403 1 1.5x10-2 3.5 100 1/2
1380 x+ 1 1.5x10-2 5.6 160 1/2
1373 x 1 1.5x10-2 10.6? 300? 1?
1400 x+ 2 1x10-4 0.9 25 1/3
1405 x+ 2 1x10-4 2.1 60 1/2
1402 x+ 2 1x10-4 5.6 160 1/2
1376 x+ 2 1x10-4 10.6 300 1
1397 x+ 2 1x10-4 17.7 500 1
1374 x+ 2 1x10-4 23.0 650 1-1/2
1375 x 2 1x10-4 35.4 1000 2 *
1398 x 2 1x10-4 53.1 1500 3 *
1395 x 2 1x10-4 71.0 2000 5 *
1396 x 2 1x10-4 99.1 2800 7-1/2 *
x Exploded parts diagram and parts list available on Welch's Web site.
+ Listed on Welch's Web site as being in current production.
* Motor requires three-phase power.
Notes
Under the motor wiring cover, there will be a 4 position main terminal block and possibly a separate single position terminal block (T and X, respectively, my designations). Each of the wires will be labeled. On existing complete systems, the thermal cutout inside the motor may be wired into the controller to provide an indication of pump shutdown due to overheating. In this case, five wires (including Earth Ground) will probably attach the motor to the controller.
Where the pump is to be used by itself, the terminal blocks should be wired as shown below. A 3 wire grounded line cord (with or without a switch) can be used as long as a properly rated fuse or circuit breaker is provided.
AC Motor Motor Run Thermal
Position Line Winding Capacitor Cutout Jumper
------------------------------------------------------------
T1 L1/H U1 Brown
T2 U2/Z1 X1
T3 Z2 Blue
T4 L2/N 21
X1 20
+----------------+
H | Blue N |
| | | | |
+---------------+ +---+
T | 1 | 2 | 3 | 4 | X | 1 |
+---------------+ +---+
| | | | |
U1 U2 Z2 21 20
Brown Z1
U1 and U2: main motor winding, Z1 and Z2: phase motor winding, Blue and Brown: motor run capacitor, and 20 and 21: overtemperature switch inside the motor.
Assuming there are no external leaks, the short list of causes for poor vacuum performance are old/dirty/contaminated oil, low or no oil, open or defective gas ballast valve, sticking internal parts due to buildup of insoluble varnish/gum on pump chamber or valves, and then finally, damaged internal parts.
CAUTION: Make sure that your pump has the correct amount of oil before attempting to run it! Else it may only be good for a boat anchor. There are some so-called "dry" vacuum pumps but these are not that common.
Here is a basic guide to the "processing" of a new old pump:
Shaft seal replacement on some belt-driven pumps (like Welch) can be performed without major disassembly but on others (like Precision Scientific), it requires removal of the pump assembly from the case. Shaft seal replacement on direct-drive pumps is almost always more involved and may require total disassembly.
A thermocouple, thermistor, Pirani, or other vacuum gauge which covers the operating range of your vacuum pump(s) will be highly desirable to determine the performance once the basic condition of the pump has been evaluated. Of the Hastings TC gauge tubes, the DV-3 or DV-6, and DV-5 or DV-8 are perfect for testing one and two stage mechanical vacuum pumps, respectively. The lower limits of their "best sensitivity ranges" go down to the pressures listed in the specifications for these pumps and the most accurate portion of the gauge response curves are where the realistic operating points will be located. Connect the gauge tube to the pump with a short length of new vacuum hose or metal plumbing - old tubing may be so full of trapped gases that it will take forever to get a decent vacuum.
If your pump didn't come with an exhaust oil mist separator (a thing that looks like a small engine air filter) attached to the outlet port, consider obtaining or building one, especially if you won't be pumping against a high vacuum with the gas ballast valve (if present) closed most of the time (the gas passing through pump is the major cause of misting). An alternative is to simply run the exhaust through a hose to the outside. Oil mist will eventually get over everything, is potentially flammable, and may be a health hazard. However, from my experience, oil misting from Welch belt-driven pumps is minimal - just having the plastic cover from an aerosol spray can covering the outlet port (or that dome thing that comes with Welch pumps) really traps most of it.
The first thing to do is isolate the problem to the pump or everything else. Connect the vacuum gauge sensor directly to the pump with a short length of vacuum-rated hose and see how far down it will go. (Many types of hose are really not vacuum-tight even though they may be fine for liquids or air under pressure. I had some rubber hose that appeared to be quite good but when stretched or even when axle grease was smeared on the outside - volatile solvents? - the pressure would go up.) Threaded connections are also a potential source of internal leaks from between the threads even if they seal perfectly. As long as the pressure is generally decreasing, be patient and give the pump time to do its work.
Where the pressure bottoms out and then gradually increases significantly as the pump gets hot, the cause is usually outgassing of the pump oil indicating the need for an oil change. Running for several hours with the gas ballast open for part of that time may clear dissolved gases from the old oil and result in a significant improvement or at least confirm that dirty oil is part of the problem. This is even useful after an oil change since there is never any way to get rid of all the old oil unless the pump is totally disassembled. Let the pump run hot (don't add any cooling fan that isn't part of the pump) as this will help the oil to outgas. Make sure the exhaust port is not obstructed so the gases can escape - remove any oil mist filter or extra pipe. The vapor pressure of all pump oils increases with temperature but might result in a pressure rise of at most 1 or 2 microns, not 25 microns! A small fan blowing on the pump case to cool the oil will also help but shouldn't make that much difference once there has been an oil change (or several) with the proper oil and the new oil has had a chance to outgas while running hot. If there's still a large pressure increase as the pump heats up, change the oil again!
The following chart shows the results of replacing old tired oil with new oil for several pumps:
<- Base Pressure (microns) ->
Original Oil After Process
Pump Mfg/Model Drive/Stages Process Hot Cooled Hot Cooled
---------------------------------------------------------------------------
Welch 1399 Belt/1 F R 50 -- 45 35
Welch 1402 #1 Belt/2 R 10 4 3 2
Welch 1402 #2 Belt/2 10 6 -- --
Prec. Sci. D25 Belt/2 F R 30 15 8 3
Pfeiffer Duo 1.5A #1 Direct/2 F R F R R 10 -- 5 4
Pfeiffer Duo 1.5A #2 Direct/2 6 4 -- --
Ulvac G-50D Direct/2 F R R 10 4 1.5 0.3
Notes:
After running with flushing fluid for a few hours, this was replaced with with new belt-drive oil (Precision Plus Plus Duo) and run for a few hours. This was then replaced with new direct-drive pump oil (Precision Plus Plus 19). The incredibly low base pressure - less than one half of the spec'd value - is not a typo. Before the second oil change, the base pressure (cooled) was 1.4 microns. The only reason I used belt drive oil for the first oil change was that it is cheaper. :) I don't think most of the difference in vacuum performance was due to the type of oil but rather the second oil change eliminated more of the original contaminated oil residue. Most of the other pumps should benefit from at least one additional oil change as well. And, of course, flushing for the 1402s.
Problems due to damaged internal parts are fairly far down on the list though as I've found out, reed valve reeds can break due to unknown causes, corrosion possibly from water-logged oil being one. Of course, if the pump has ingested enough debris, the vanes could be nicked or chipped along with scoring of the cylinder walls. It would be possible to replace or regrind the vanes and rebore the cylinder but that's for the advanced course. Pumps used in high vacuum (scientific and industrial) applications are much less likely to suffer from such damage. On the other hand, they tend to be run continuously, possibly for years, and thus may be worn out in other ways. However, note that one common fault with older pumps - a leaking shaft seal - may make a mess but will generally not affect vacuum performance.
Once the pump is eliminated as the source of the problem, there must be either leaks or contamination in the rest of the system:
Running for a few hours or days will eventually clear out volatile contaminants but proper cleaning in the first place is much preferred. Proper cleaning means using appropriate solvents in a well ventilated area that evaporate without residue and without mixing them with the oil. Else, it will take forever to get a decent vacuum after reassembly. DON'T use WD40 or similar products for anything remotely involved in a high vacuum system! They leave residue behind by design. :)
And before going to extremes in attempting to achieve a perfect vacuum, consider your application. If it's for the N2 or CO2 laser, a base pressure of 1 Torr is really more than just fine. Almost any pump in decent mechanical condition will achieve 1 Torr - possibly even if it is filled with used salad oil. :)
But a comment about using automotive and other non-vacuum pump oil: It's not so much that the ultimate vacuum may be mediocre. It may in fact be quite adequate for some of the home-built lasers. However, there can be issues of eventual damage to the pump if the viscosity is too low or too high or the oil contains additives. The oil may also deteriorate rather quickly. Proper vacuum pump oil isn't expensive - go for it! :)
Pumps with no large oil case (like the older version of the Welch 1405 and similar models where the outer surface of the pump chambers is exposed) have very small oil capacities. Oil contamination and degradation with use will occur much more quickly compared to similar capacity pumps with huge oil reservoirs, especially when run hot and long and with substances that dissolve in the oil. Thus, frequent oil changes are essential with these pumps both to maintain performance and to achieve long life. At least the amount of oil needed is small. :)
I won't mention the time back in high school when we would top off the Welch pump for our cyclotron with refrigeraton compressor oil that the chiller service people had left behind. Actually, it worked remarkably well. Cyclotron in high school you say? :) See the section: The Central High School Cyclotron.
With a combination of its original oil (what remained after the rest was lost due to a leaky shaft seal which has since been replaced) and old oil from my Precision Scientific D25 look-alike, my first Welch 1402 would reach 3.5 microns when cold but then creep up past 10 microns after running for awhile as the oil got hot and the vapor pressure of dissolved crud increased. I don't know if simply running it for a few hours or days would have helped but decided to replace the oil rather than waiting. After the oil change (but without flushing), the 1402 goes down to about 2 microns and reaches only 3 microns when hot. With a small fan blowing on the pump housing for cooling, it holds down under 2 microns indefinitely. Flushing and another oil change would probably get it below 1 micron.
(From: John De Armond (johngd@bellsouth.net).)
Go to Duniway Stockroom and buy a couple of gallons of flushing oil and a gallon of vacuum oil. Drain all the old oil out, fill with flushing oil and run the pump for a couple of hours deadheaded (e.g., with the vacuum inlet capped) until it's good and hot. Drain. Repeat until the flushing oil comes out clear. Fill with regular pump oil and run for several hours deadheaded until the oil is good and hot. If you have a thermocouple vacuum gauge, check the deadheaded vacuum. A used 1402 or 1405 should achieve 3 to 5 microns after the oil has completely outgassed. Outgassing might take a day or more.
If several flushings won't clear the oil or you can't achieve that vacuum, the pump chambers may need cleaning.
I also suggest buying Duniway's shaft seal kit. This kit replaces the original crappy seal with a modern tensioned rubber seal. It completely stops shaft weeping.
These pumps are so rugged that I believe that the very first one, which provided vacuum on the Mayflower, is still running. :-) Everything is rebuildable.
(From: Sam.)
Note that I would NOT recommend attempting a complete overhaul of a vacuum pump unless (1) the performance really isn't acceptable even after several flushings as described above, and (2) you have experience rebuilding other similar equipment or at least lawn mower engines with successful outcomes! It the pump pulls a vacuum you can live with, leave well enough alone. :) However, where the outer cover can be removed without disturbing the shaft seal, it may be worth the risk (mainly of damaging its gasket) to be able to clean out the sump from the inside - even repeated oil changes may not dislodge the very dirty icky oily crud that collects there (though a couple of flushings should do an adequate job in most cases). I do highly recommend having a replacement gasket on hand as the old one is likely brittle with little resilience so oil leaks are almost a certainty when everything is reassembled. If an "official" gasket isn't available (or isn't available at reasonable cost), make one from a piece of cardboard stock (1/32" or so) cut to shape with scissors and a utility knife. That's what they may sell you in any case and it works well! In fact, the cardboard variety is easier to deal with than the rubber foam type which tends to droop and has to be coaxed into position.
CAUTION: On pumps where the shaft seal is internal like some Precision Scientific models, just removing the pump assembly from the case risks destroying an old shaft seal whose parts have stuck to the shaft. Replacement may be expensive or challenging. There is no guaranteed way to prevent this type of failure from occurring though running the pump until it's nice and hot before disassembly might soften things up enough to become unstuck. :) See the section: Rehabilitating a Precision Scientific D25 Rotary Vane Vacuum Pump.
(From: Ed Phillips (evp@pacbell.net).)
I recently replaced the seal in a NIB but never used Welch 1400 pump which had been sitting in my attic since I bought it new (for about $150 delivered) back in 1960. The new seal was around $50 delivered. Nice little pump.
I have a couple of 1405's (or is it 1402? Anyhow, next size up from the 1400) here which have a note painted on the outside indicating that they were overhauled back in May, 1954. They sat around in the back of a CRT rebuilding shop for many years before I picked them up a few years back. They still had a full charge of (very dirty) oil in them. Fired one up, flushed it out a couple of times, and refilled with DuoSeal oil. Using my little old McLeod I indicate a pressure of around 1 micron after a few hours of running. (Gauge connected to the pump through a short section of 1/2" ID rubber tube.) They must have many thousands of hours on them since overhaul. By the way, the shaft seals don't leak on either pump.
Order the appropriate shaft seal from Duniway Stockroom. It's about $25 including shipping which is more than you might think it should cost but the machined cover into which the actual seal is pressed is probably a custom made part. And, this is about 1/3rd of the price direct from Welch! However, if you can machine your own cover or an adapter for the old cover, a $2 seal of the appropriate diameter from an auto-parts store should work just fine. But you better find an actual part number for a suitable oil seal from an auto service manual or seal catalog - just asking for a 3/4" diameter oil seal (or whatever) probably won't be very productive. :) Or, just the seal can be purchased from other vacuum suppliers like Precision Plus. They also have complete seal kits for most Welch pumps at slightly higher prices than Duniway.
The item listed as "Shaft Seal" from Duniway actually includes the seal, gasket, three (3) screws, and instructions, but they are not really as comprehensive as what follows. :)
Before I started collecting thermocouple vacuum gauges, the only tests I could actually perform were (1) seeing how well it would suck a finger, (2) checking the vacuum on a Bourdon tube gauge, and (3) testing the discharge color with my flyback RF source. It passed the first two of these with flying colors but the discharge color test indicated a pressure somewhat below 1 Torr but possibly not by much. The sound it made was also abnormal - quite loud and throbbing even when pumping its capped inlet. This sound was the main reason I decided not to be happy with just sub-Torr performance and do more than let it rot. :) In fact, I didn't really even know whether it was a single stage or two stage pump until comparing photos of several Precision Scientific pumps since no model number was ever legible. Only after writing this did I go look again at the information on the nameplate - almost totally illegible but I could at least make out "gas ballasted two-stage pump". I still haven't figured out how the gas ballast works since there is no gas ballast valve like the one on the genuine D25 nor anything internal in its place that could introduce atmospheric pressure air into the exhaust stream. However, it might have used that gap between the reeds. In that case, it's just as well the pump doesn't have it anymore to limit the base pressure. :)
Now that I have a couple of thermocouple gauges that are reasonably well calibrated, I decided to test this pump to see how well or terrible it performed. The verdict was: somewhere in between with a base pressure of about 150 milliTorr (150 microns). This would be respectable for a single stage pump with filthy oil - which this has. But for a two stage pump, it should do much better. My suspicion at this point was that something was wrong with the second stage.
The first thing I did was to check the exhaust port by unscrewing the large silver-colored cover. With the pump running and the inlet capped, there shouldn't be any airflow from the exhaust port. Imagine my surprise when placing a finger over it resulted in suction! Valves may not be perfect but at best this should be neutral. But there was very definite suction. In reality, it probably wasn't actually sucking air in through the outlet but was probably moving it both ways and my finger was acting as the one-way valve. Next, I removed the cover plate (4 screws) and then made a big mistake: I started the pump and got a face full of oil! :(
But, there was something very strange about what I found under the cover plate: half a reed valve! I probably looked at this 10 years ago and didn't think anything of it but now a problem was clearly evident. The valve is made of a single piece of spring steel and should have had two "fingers" but one of them was missing. The valve is supposed to prevent back-flow of air into the second stage outlet tube. With the valve being non-functional, the oil was being very seriously churned resulting in lots of bubbles - the oil had a nice head which were perhaps the main cause of the pump's inability to go below 150 microns. With the reed valves submerged in oil, the oil really does the sealing so the reeds don't need to make gas-tight contact but they need to be present! To construct a replacement reed, at least as a test, I took a piece of thin polyethylene plastic (a liner from a package of smoked salmon if you must know!), cut it to fit, and installed it held in place under what's left of the remaining reed and its screw. With this addition, the pump sounded a lot better - basically normal.
However, I made the mistake of wanting to retrieve the broken piece of reed. Leaving potential time bombs inside equipment always bothers me. In this case, the chance of any damage from it would have been minimal since it was clearly minding its business at the bottom of the sump but that never stopped me from ripping things apart. :) Six cap screws hold the actual pump assembly to the outer casting (these are at the periphery around the label but not visible in the photo. After draining the oil (I did remember to do that!), taking off the pulleys and belt, and removing the six cap screws, the guts could be pulled free. Wow! Look at all the muck at the bottom of the sump! I retrieved the broken reed and cleaned out as much of the disgusting black grainy oil as I could. This is one reason why using flushing oil is important - no matter how many plain oil changes are performed, they won't dislodge the crud at the bottom of the sump. Flushing oil might have a chance. With this pump, the drained oil always looked reasonably clean and I never suspected what lurked hidden in the sump. The pump chambers probably also need cleaning but I'm not that determined to go inside!
Unfortunately, one additional problem developed that was unexpected: The main shaft seal consists of a machined metal surface on the inside of the outer casting against which a flat plastic ring rotates with the shaft. This forms the rotating part of the seal. The ring is inside a metal cup against the flared end of a molded rubber piece which fits snugly on the shaft. This seals the shaft to the ring. Normally, a strong spring maintains pressure on the rotating part of the seal with the rubber sleeve sliding on the shaft to adjust as the pump assembly is slid in place. On this old pump, the rubber had gotten stuck to the shaft so when the spring was released, it ripped the flared end from the rest of the rubber piece. (I noticed later that the rubber was a bit cracked in places so it was destined to fail eventually anyhow.) Initially, I didn't realize that there was anything wrong with the seal and put the pump back together. It pumped fine but leaked all over the table. After disassembling it for the second time and finding the problem, I unstuck the rubber from the shaft, lined up the broken pieces as best I could, and added an additional spring of my own to keep them together - hopefully.
With the pump attached to my home-built thermocouple gauge, the pressure reached 30 microns before the shaft seal started gushing oil. OK, it didn't actually gush but was in the process of forming a nice puddle. :) The pressure was still going down slowly but how far it could go will have to wait until a new seal can be obtained or devised. An alternative to buying the Precision Scientific shaft seal kit at some ridiculous price (cheapest I've found so far is $41 - all I really need is that rubber piece) might be to obtain a $2 1/2" oil seal from an auto parts store and simply glue it in place outside the pump (possibly mounted on a large plate) - there is plenty of room behind the pulley. Either RTV Silicone or Epoxy would be satisfactory. There is no air pressure (positive or negative) between the inside and outside of the outer casting so it shouldn't be difficult to maintain a decent seal. However, one potential problem with this approach is that since the pump assembly is mounted from the other end of the outer casting, there would be some very slight lateral movement of the shaft when the belt is installed and during operation and this might be too much for a typical oil seal to tolerate.
The improvised reed valve was still working and as far as I can tell, was doing its job as well as the original but would probably fail at some point not being made of the proper material. Since it's just a piece of spring steel, a new one could be cut from sheet steel and then tempered. Or perhaps just a thicker piece of polyethylene will survive for the amount of use I will give this pump.
Anyhow, back to the oil seal problem. I went to an auto parts store and asked about oil seals. They looked at me like I was from Mars and weren't going to be bothered trying to find something as inconsequential (to their bottom line) as an oil seal without a specific part number. Therefore, I went to Plan B. (Plans C and D available upon request). :) I found a rubber washer with a center hole just under 1/2" (the diameter of the pump shaft), trimmed it to fit inside the metal cup thing and slid everything onto the shaft. Although not quite as robust as the original custom rubber piece, the spring pressure should maintain a good seal - again hopefully.
I have run the pump for about an hour so far still using the old crappy oil. It levels off at about 20 or 25 microns without cooling. With cooling to minimize the vapor pressure of the oil, it reaches about 15 microns. If my RF source is turned on for a few seconds and then turned off, the pressure drops still further but creeps back to its previous reading. This would seem to indicate that back-streaming of gases dissolved in the oil is probably still the limit, not the pump itself. In any case, new oil should allow it to maintain 15 microns or maybe even slightly better without cooling which isn't bad for an old refrigeration service pump.
The shaft seal (and reed valve) are holding up just fine. There is a slight oil seepage around the main case gasket which isn't surprising since I've taken the thing apart a half dozen times without replacing the gasket. I've added a bead of "Form-A-Gasket 2" which should take care of that problem until I can buy or make a proper replacement. It's ugly but works fine.
One other minor quirk appeared and I don't know whether it is inherent to this pump or due to my reed valve which may work too well: If left under vacuum when switched off, after awhile the pump chambers fill with oil. Apparently there is no relief valve between the first and second stages - none is visible but it's also possible it is hidden inside and just stuck or clogged. So when attempting to start up the pump, the first 2 or 3 rotations of the pulley have tough spots as the incompressible oil is forced between small gaps between rotating parts. (This behavior is common with some Welch belt-driven pumps as well.) At first I thought the slight gap between the reeds in the original reed valve was there to allow venting after shutdown as it would drain the small amount of oil in the reed valve compartment and then suck atmospheric pressure air into the pump chambers rather than oil. So, I turned my polyethylene reed over - it has smooth and rough sides - to use its rough side in the hope that would reduce the seal and act as a substitute. That didn't help so I drilled a 2 mm hole in the polyethylene (I have plenty more!) and that didn't help either. With the hole, the pump still appeared to be working properly but made a more pronounced throbbing sound which I didn't like so I installed a new polyethylene reed. My conclusion is that the behavior is a quirk of the pump since it's intended for HVAC service and typically wouldn't be left connected to a vacuum, or there is an internal relief valve and it is isn't working. Adding a vacuum vent valve to the system should take care of it for the future! As a side note, absolutely no evidence of wear was visible anywhere on the polyethylene sheet that was removed - just a slight dimple at the location of the output port hole. Although the run time of a couple of hours is no indication of long term reliability this is encouraging.
After draining the dreadful oil and running the pump with a load of flushing oil for about an hour, then replacing that with Precision Plus "Plus Duo" oil (similar to Welch Duo Seal oil at much less cost), it goes down to about 8 microns without cooling and under 3 microns with a small fan blowing on the pump casing. This base pressure is actually quite impressive considering the pump's likely previous life. It was still going down, though slowly, when I called it quites. And the smoked salmon liner reed valve replacement still seems to be in pristine condition after several hours of use! Finally, replacing the old gasket (carboard type from Precision Plus) eliminated the need for the ugly sealing job. :)
CAUTION: This procedure may not apply even in the generalities to Welch or other rotary vane pumps as many of these require special procedures for rotor centering. Pfeiffer pumps use precision steel dowels to align everything and are about as simple as is possible in terms of the number of individual components and the ease of servicing.
The reason I decided to go inside was mainly an unexplained noise best described as a sputtering or stuttering even at high vacuum. The base pressure of 4 to 5 microns wasn't terrible but is higher than the spec in the manual - 0.66 micron (though I was told by a Pfeiffer repair technician that 2 or 3 microns is really what to expect if everything is perfect. And that is the base pressure specification on current model Pfeiffer Duo pumps). The real reason was the noise. If you like to skip to the end of the mystery novel, I eliminated some of the noise by plugging the noise damper leak nozzle totally, though I'm not sure the new type of noise is an actual improvement.
I have since acquired a second Duo 1.5A and while this one had a somewhat similar noise audible from the exhaust port, it is lower intensity and virtually undetectable when covered. So, there is something different about the two pumps but I have not determined what it is.
Here is the step-by-step disassembly and inspection procedure of everything readily accessible externally or within the oil case. Descriptions assume looking toward the pump from the pump-side of the entire assembly. (What I call the "bulkhead" is the vertical piece with the vacuum and exhaust connections separating the motor from the pump itself.) A set of metric hex wrenches, a medium size flat blade screwdriver, an adjustable wrench, needlenose pliers, and a few other common hand tools are required. Also have an ample supply of lint-free rags available to deal with the unavoidable oil
