New small vacuum system based on eBay turbopump

[Carl Willis]

The parade of small, affordable used turbopumps continues on eBay this very minute (Item 370459163994). The Pfeiffer TPH-055s from this source arrive dirty and beat up, but fully functional. Having purchased four of them, I think I can safely generalize and suggest that others' experience will also be good. Converters for the TPH-055, particularly the TCP-035 control board, show up on eBay fairly regularly also. I have two now, both purchased for under $80. If that kind of deal isn't good enough, there is a homebrew converter described for this pump here: viewtopic.php?f=10&t=4036#p21444 The manual for the TPH-055 is available here: http://labspc200.rutgers.edu/manual/tphu060.pdf . The manual for the controllers is available here: http://www.ptbsales.com/manuals/tcp-015.pdf

I assembled and tested a new vacuum system based on the TPH-055 designed to replace my old miniature diffusion pumped system (viewtopic.php?f=10&t=3655#p21063 ). This new system should be an order of magnitude faster, and considerably cleaner and more convenient. The first order of business was to wash a TPH-055, repair its ISO-63 flange surface, and replace the lower bearing lube wick. The latter component is available for $79 here in Albuquerque (http://www.pchemlabs.com/product.asp?pid=2776). I installed a TCP-015 controller, a 36V/2A switching supply, and a switch to enable 66%-speed "standby" operation of the pump in an aluminum chassis. The high-vacuum side of this system is all metal-sealed 2-3/4" CF hardware, while the foreline is KF-25. Other components include:

-Teledyne #2-2100-263 cold-cathode gauge: reconditioned, brazed into a 2-3/4" CF flange, and calibrated against an Alcatel CF2P gauge
-Hastings DV-6 thermocouple gauge in the foreline
-An electrodeless discharge tube to be used with a handheld Tesla coil for leak testing
-Vibration-isolated platform for turbopump
-100 l/min rotary-vane roughing pump

The little system works like a champ. After running for a day in "standby" mode (66% full speed), the cold-cathode gauge had bottomed out at 1E-7 torr. With the chamber connection blanked off as shown, the pump will exhaust from atmosphere to mid-microtorr range in about two minutes.

-Carl

[Richard Hull]

Superb and clean system. Good work!! An example of how the best type of vacuum system might be installed.

Richard Hull

[Rich Feldman]

Nice work, Carl.
I'd especially like to applaud your schematic diagram, and encourage others to learn and use schematics with standard symbols for vacuum technology.

Here's one reference by the American Vacuum Society.
http://membership.avs.org/avs/rec_prac/ ... s_1967.pdf
Perhaps someone here would like to share a better "favorite reference",
or recommend an alternate standard if one exists,
or point to a source of pre-drawn symbols for your favorite drawing program.

Rich

[edit] Carl, can you clarify a schematic detail? The thing with "DT", is that your electrodeless glass tube for Tesla coil zapping (using glow color to indicate a leak detection gas species)? It appears to be connected only to the turbopump foreline, since there is no connector dot where the lines cross.

[richnormand]

Nice work Carl.

You are making more progress than me. Your system looks a lot cleaner too, impressive!!! Something for me to aspire to in the Mark II design. Same pump and the controller is based on Alex's as in your link to this forum. Never found a proper controller on e-bay to this day but still looking.

I used a 2.5" PVC sewer cap for the top plate as it fits just snug in the pump mouth. Stretched an O ring around the outside and used a plate to squeeze it. The four clamps in the photo are to keep the assembly snug. This is just a test and I did not think I would reach 2 10^-6 torr with this ghetto setup!

I am modifying the controller to remove the 555 timer speed control and replace that with a phase lock with a longish time constant and modify the supply voltage in an analog way to control the speed instead of altering the motor pulses.

Also to note is that sometime the pump will be in a state at startup where one of the MOSFET will be full on (gets HOT) and the other full off. Not a good scenario. A small twist on the pump and all is OK and it speeds up. I'll post the circuit mods when it works to my liking. In the meantime many thanks to Alex. A great way to put these pumps to a good use in good homes!

How do you intend to control the D2 flow vs pump speed?

Cheers.

[Carl Willis]

Rich F., the symbol marked "DT" denotes the discharge tube. Where lines cross in this diagram, a connection is implied. So it is on the foreline, where the pressures might be high enough for it to be of maximum utility. I'm not a stickler for fealty to any particular schematic standard, so long as the communication is effective (good labels, additional photos, whatever is needed). My drawing borrows mostly from the old AVS symbols. More recent ISO symbolism is coming into vogue (the printed Pfeiffer catalog is a good example). Some of these symbols for vacuum instrumentation are here: http://en.wikipedia.org/wiki/Piping_and ... on_diagram .

Rich N., nice results with your sewer pipe cap and all. I'm glad you're working on improvements to the homebrew controller, because that is truly what brings these systems into a price range competitive with diff pumps (discounting some uncertain eBay luck finding the TCP-035 controllers). To answer your question about pressure control, I intend to use the 2-3/4" CF high vac valve as a throttle in the same manner I have in the past. Turbine rotor speed might be a viable method for helping to control chamber pressure. Pumping speed (not compression-ratio-limited) is in theory proportional to rotor speed, but I suspect with this pump that the more useful effect would be attributable to compression ratio. The TPH-055 is already very limited in this regard (compression ratio at full rotor speed is 100 for H2, 600 for He / D2). In theory, the compression ratio can be expressed as an exponential function of rotor velocity

K = exp(v/b)

where b is a constant of the gas species and the turbine geometry. (So if the rotor speed drops to the "standby" speed, the compression ratio drops to ~70 for D2/He or a mere ~12 for H2.) With compression ratio limiting, the effective speed is S = K*Q/Pf with Q being the gas load and Pf being the foreline pressure. I'm not sure if, in practice, this affords a good kind of control, though. The turbine speed takes a long time to change, and the pumping behavior probably gets very complicated once the outlet stages leave molecular flow somewhere in the neighborhood of 0.1 torr.

-Carl

[DaveC]

Just a bit additional on variable rotor speed for a turbo. There MAY be some issues with excessive mechanical resonance of the rotor at certain speeds. You can often hear this as the units spool up.

So this could be a caution flag on variable speed pressure controls.

FWIW....

Dave Cooper