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FAQ: Mechanical Pumps - Types needed - new and used

Posted: Thu Oct 16, 2008 2:51 pm
by Richard Hull
You need a vacuum pump! This is a must have item as it is called also a "fore-pump" in vacuum parlance.

A mechanical vacuum pump is needed and it must be a dual stage pump!

There are only two types normally used by amateurs and many professionals.

If you are serious about this effort, you must purchase a known good TC gauge, first!!!!

1. The direct drive pump
This pump uses a motor directly hooked via its shaft, inline with the pump, itself. These all have a carrying handle and all but the largest are portable type pumps, (actually, luggable). These pumps are typically rather noisy and rotate at high speed. Virtually all HVAC pumps are of this design type.

2. The belt drive pump
This is of a classical design type with a motor separated from the pump body, driven by a v-belt between pulley wheels on the motor and pump.
These pumps are never portable and are mounted on large steel base plates. This type pump usually runs much quieter than the direct drive and turn at a much lower speed. Generally they are two to three times the weight of a similar direct drive pump.

The Brand names that are best are Precision, Welch - Duo Seal, Edwards, Hyvac, Leybold, etc. These are Scientific pumps. Refrigeration pumps, as used in the HVAC business, are fine if they are two stage, as noted earlier, and are of sufficient pumping capacity. Robinaire and Yellow Jacket are typically good if two stage and not worn out or ruined, if purchased used.

The pumping capacity for any fusor mechanical pump needs to be at least 2 CFM for a demo fusor, but for real fusion in a really normal chamber of 6" or more, you will want something of at least 5 CFM Capacity.

New pumps cost between $300 - $2,000 of the type you need. Good rebuilt and warranted pumps in the 5CFM range can still cost $600 or more if purchased from a reliable seller of such pumps. Used, unknown condition pumps are all over the place from $25.00 to $300.00 in the size range above. Watch e-bay.

Buying a pump...........

Never buy a pump that is untested or un-gauged!! If you are buying locally, take your TC gauge with you. 1. Make sure the pump has oil in it. 2. Make sure it runs. 3. Put your gauge on it. 4. If it won't go to 100 microns rather quickly, you might pass it up or figure the oil is merely fouled. Tell the seller it must go below 50 microns. If still interested, offer half of what they are asking.

If buying off E-bay get a promise of money back if the pump will not go below 50 microns. Any back talk related to unable to test should send you packing. This is a clear sign the seller is a doltish idiot and just wants what he has, turned into money and to be done with you. Let some other poor slob who wants the seller's pump get flim-flammed.

Let us assume you now have a pump in hand...........

You bought it new? You bought it used?

What can you expect? How do you test it?

First, you should already own a good thermocouple gauge. Keep it simple and know that it is reliable and tolerably well calibrated. If you can't feel good about the gauge you will suffer mightily with false impressions about your new pump acquisition. Needless money and time will be spent if your gauge is off. New gauges are great and are money well spent as they can be assumed to be very close to accurate, as received.

If you have a new pump, you might be a bit more in luck, even with a questionable gauge. The assumption being that a new two stage pump will evacuate quickly to about 15 microns or less. If your used gauge says 200 microns and you have really tight connections to the gauge, you will know the gauge sensor is bad or grossly out of calibration.

Getting ready to test.......

The first order of business is to check the oil in the pump, new or old. make sure it is at the proper level.

If your pump is old and used make sure you can turn the shaft by hand. Hopefully, the pump is not seized. (You should have not purchased a seized pump in the first place!) If the oil level is low, do not top it off! Replace it! Drain the oil and put in clean oil or a flushing fluid oil.

If your pump is used, blank off the inlet and run the pump for about 2-4 hours. Dump the hot oil and if nasty or even clean-ish, put in new oil and now attach you gauge to the inlet.

If new, connect your gauge sensor to pump inlet with a very, very short piece of vacuum rated tubing. 2-3 inches is about right.

Testing the pump.........

Start the pump. you should hear a few minutes of the noisey lup-lup-lup pumping as the pressure drops. When the pressure bottoms, initially, the pump should get quiet. If it doesn't quiet down in a minute or so, then you have a leak in you head to gauge connections or an open gas ballast. Closing an open ballast will stop the noise and often let the pump dip another 5 microns or so.

After 4 minutes of pumping what is your pressure?

3-10 microns... Excellent! The pump must be brand new, a newly rebuilt one or you have just received a superb, well cared for, used pump.

10-15 microns... also great, but if a new pump you might check your gauge connections for a very minor leak. If a used pump, you still got a fantastic deal!! This is the range that most good pumps normally work at.

15-25 microns.... typical of a good, but well used pump. If used, take off the gauge and blank off the inlet (seal it off to vacuum tightness). NOTE: never let a running pump take in air from the outside world for any period of time, (Puts atmospheric water into the pump oil).

Let the blanked off pump run for 6 hours with the gas ballast open. Drain the formerly new oil or flushing fluid. Does it look nasty? Replace the oil and let the pump cool. Re-attach the gauge and close the ballast, then pump down and check the reading. If it is the same, your pump is just worn a bit and still serviceable for use with an added oil diffusion pump on the fusor. If the reading is lower, (Below the last reading by at least 5-10 microns), then you are getting the old pump back in shape...Good for you! You might blank off again and run the pump for 5 hours with the ballast open then refill one last time with fresh oil. YOU HAVE A GOOD PUMP. It was just filthy.

NOTE: Never run a pump for many minutes or hours with the gauge tube sensor close to the head!!! The oil vapors from hot oil might get inside the gauge tube and foul the thermocouple. A fouled gauge can be cleaned by dumping many loads of acetone or methyl-ethyl ketone into it, shaking it and dumping and then reloading, shaking and dumping, etc.

If you get 15-25 microns with a brand new pump, either you have a leak in your gauge line or the pump is not a very good scientific one.

25 - 50 microns.... A sad state of affairs. If used, your pump is either well worn or still filthy inside. Blank off as above and run for a few hours, dump the oil and put in fresh to try again. If the pressure is now lower, keep doing this until you can see no improvement. The point at which a pump is serviceable for fusor use with a diffusion pump added would be about 25 microns at the head, but you really want to see at least 20 microns or lower, if possible.

If your pump is new and you get 25-50 microns you might just have a bad pump or a very poor one. Note: Some new, cheaper, small refrigeration, direct drive pumps, (Chinese cheapo types), might only hit 25-50 microns right out of the box!

If you can't hit 50 microns and have a used pump, and have tried multiple oil changes and runs, then the pump is just worn out. Your options are, rebuild or start looking for a better pump.

Again, if you read 50 microns or higher, suspect the couplings to the gauge or the gauge itself. If the gauge is known to be perfect, then suspect the hookup line. If you are positive about the short line hookups, then you have a bad pump, for sure.

Finally, never hook a pump to your system without testing it first. In this manner, if you know your pump can hit 10 microns at the head, and when hooked to your fusor it will not pump the fusor to 100 microns, then you know you have a leak, either real or virtual somewhere in the plumbing.

Always test and adjudicate a pump at the head before use or hooking it into a fusor system.

Always know the state of your gauge. You can spend a lot of money and waste a lot of time rebuilding a perfectly functioning pump all because of a cruded up or defective $40.00 theromocouple gauge tube.


Finally!! you have your pump and it is tested. Now to hook it to your fusor, (demo), or your diff pump/turbo pump, (real fusor). The line or hose from you mechanical foreline pump must be kept short and fat!!! This cannot be over stressed. Ideally, you might not need over a 4 to 6 inch long hose, 1 inch in diameter. Position your chamber or secondary pump's outlet as close to you fore-line pump's inlet as possible within the mechanical structure of your finished vacuum system.

Richard Hull

Re: FAQ: Mechanical Pumps - new and used

Posted: Thu Oct 16, 2008 3:13 pm
by Mike Beauford
Another excellent post Richard!

Re: FAQ: Mechanical Pumps - new and used

Posted: Thu Oct 16, 2008 3:33 pm
by David Rosignoli

As always, a great FAQ for the record.
Are you also suggesting leaving the pump inlet blanked off when not in use?

I had hooked up a thermocouple gauge to a Welch 1402 that I thought was in pretty good shape. (The pump certainly can pull some vacuum as I had run it on a plastic bell jar and a glass bell jar.) There were no other fittings on the pump. As I let the motor run for a few seconds, the motor slowed down more and more until it almost stopped completely. (I turned it off before letting this happen.) I did not get more than a blip on the T/C gauge. Not sure why I am getting these results. The motor seizing disappered when opening the inlet to the air.

Also, what if you have a t/c gauge (or 10 even) and want to verify its accuracy. What to do? You can compare against the others, use a reference tube, take the pressure down further below the range measured on the t/c gauge, or use a lower pressured gauge that has a higher known accuracy (e.g., capacitance manometer). Then, zero out the t/c gauge.


Re: FAQ: Mechanical Pumps - new and used

Posted: Fri Oct 17, 2008 4:39 am
by Larry Upjohn
Your FAQ is most timely! I have just completed a seal changeout on my 1400 with the result that my readout was just about the same before cleanout and new seals. However there is no puddle of oil on the floor. Your above instructions on the cleaning and maintenance of TC senders inspired me to rinse my brand new but contaminated (with flushing mineral oil) DV-6M with acetone. Low and behold droplets of MO showed up on shake out. Repeated several times and the change is documented below. My baseline 12-15 microns has returned. Photo proof attached. This reading has held now for 1 plus hours as well so I think I accomplished two goal. Now on to first light. Photo #1 is pre-seal and TC cleanout reading, Photo #2 clean reading now and Photo #3 Clean and sealed pump minus paint that chipped off during cleanup. To quote the former mayor of Carmel, "You made my day!"

Re: FAQ: Mechanical Pumps - new and used

Posted: Fri Oct 17, 2008 1:42 pm
by Richard Hull
Larry - I am glad you received value in the FAQ post and now have your system back in order. The vacuum pump is just part of the system. A vacuum gauge that reads badly can easily make a great pump look terrible. You have proved a point. Thanks for the kudos. Much of this sort of information is found on the Bell Jar site maintained by Steve Hansen.

Many are the T/C gauge tubes that could use a thorough triple or quadruple slosh-and-shake with acetone. This is especially true of neophyte operated T/C gauges and surplus, used, gauge tubes.

Dave - I have always preferred T/C gauges where you are forced to set the thermistor heater current to a fixed value. The oldest Hastings gauges had this feature as well as those of many other manufacturers. Provisions were made with a "Set Current" switch to adjust the current control pot. Assuming your T/C tube is brand new or at least sparkling clean, setting this current properly will absolutely avoid way out of range readings and can substitute for a precise calibration effort.

To fully understand T/C gauge tubes and the manner in which they work, there is a downloadable article on the Bell Jar site regarding making your own TC gauge that gives all the data for a number of tubes.

My absolute favorite tube is the 1518 tube. I have a gauge that is portable and uses this tube and a single "D" cell for power. The recomended heater current is very low and the tube has a slightly expanded range under 20 microns.

You are correct in your calibration suggestion. The ideal way to calibrate a T/C gauge is to take it to 10E-4 torr or lower and then set the gauge zero adjust pot (current setting) to zero microns.

As regards the inlet when not in use: A formal blank off when not in use is just fine but, for storage, (I have a lot of pumps in storage), I tend to just put a triple folded over thickness of saran wrap, ("cling film" to our Brit friends), tightly over the inlet and held down and in place with a heavy rubber band. It is only when the pump is running, exposed to outside air that the inrushing air is compressed and expaned to such a degree that condensation occurs, thus, fouling the oil rapidly with water.

Richard Hull

Re: FAQ: Mechanical Pumps - Types needed - new and used

Posted: Wed Nov 21, 2018 5:22 am
by Richard Hull
I have updated this FAQ to a more suitable first pass post on obtaining and conditioning a mechanical pump.
Richard Hull