The Importance of TC Gauge Calibration

Every fusor and fusion system seems to need a vacuum. This area is for detailed discussion of vacuum systems, materials, gauging, etc. related to fusor or fusion research.
Luke Harrill
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The Importance of TC Gauge Calibration

Post by Luke Harrill » Tue Jan 01, 2019 1:58 am

Today I finally changed the oil in my Welch 1402 pump, so that my first steps in vacuum application could be made. I used some wire-reinforced vacuum hose to connect the nipple to a rubber cork with a hole drilled into it. The TC went into the cork and all connections were then clamped with metal rings.

My first setup was with a new Varain 0531 tube connected to a used Varain 810. As soon as I turned the pump on, the pressure started to drop. Within the first 15sec, I was down to ~100 microns. After wating about 20min, I was down to ~65 microns. I ballasted the oil and waited for about half an hour with no marked improvements. Now I wasn't pleased with this measurement, but it was only my first change of oil. I rechecked my connections with no improvements.

I then decided to test my Televac tube and controler. Both were bought seperately, but appeared to be new. After hooking everything up, I powered on the pump. It started dropping quickly again. When I looked up, I was down to ~30 microns! After 20min, I was down to 15! So again I went and rechecked the connections. Once tweaked, I managed to get the gauge to go to 5 microns and then bottom out completely!! Now, I know that the manual states that a Welch 1402 could pull down to 0.1 microns, but even with brand new oil in a used pump, that would be unlikely.

I then reattached the Varian 0531 and tested it agaian. Only 63 microns at best, so that was the true reading.

It would be nice if my second reading was accurate (and both would be okay for the turbo), but that is just wishful thinking. There is way to big of a difference to ignore, since I would like to use one of these for gas measurement. Would any of you that have a standarized meter be willing to help me calibrate these gauges?


(I will attach a photo when I can, but I am getting an error saying that the board attachment quota has been reached???) (Edit: fixed)
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Jerry Biehler
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Re: The Importance of TC Gauge Calibration

Post by Jerry Biehler » Tue Jan 01, 2019 11:23 am

The 531 is a full range tube, 1 mtorr to atm, can't tell what the other tube is but it may have a shorter range which will give better accuracy at the lower end. But are you sure the televac controller and the tube you have matches?

Luke Harrill
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Re: The Importance of TC Gauge Calibration

Post by Luke Harrill » Tue Jan 01, 2019 1:53 pm

The Televac tube (model 2A) has a range of 0-20,000 microns. The controller for the 2A is much more expanded in the low end compared to the Varian. I am positive that the tube, cable, and controller match for it.
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Bruce Meagher
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Re: The Importance of TC Gauge Calibration

Post by Bruce Meagher » Tue Jan 01, 2019 5:49 pm

Luke,

I have a calibrated MKS 901p you can borrow. PM me your address and I'll send it over. You can use it to check the results against your two other gauges.

I also have a similar varian 531 controller as the one pictured above. I have to zero the gauge for each specific 531 TC in order for it to be reasonably accurate at the low end. Pump to 1e-4 Torr and then use the zero pot to set the zero.

Bruce

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Richard Hull
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Re: The Importance of TC Gauge Calibration

Post by Richard Hull » Tue Jan 01, 2019 8:41 pm

Bruce tells the tale. All TC gauges are hyper sensitive to the "current" in the heater portion of the tube. No specific TC gauge tube is exactly like another of the same number. They are all of a very similar manufactured standard, however. Thus, most all such gauges have a zeroing pot which is usually nothing but a current control to the heater within the tube. Needless to say, the current control is critical to the accuracy of the lower pressure end of the range. Such lower priced gauges are 100% electrical with no contained electronics.

Some gauge manufacturers are afraid to put such a pot that the user can fidget with in the heater circuit and possibly burn out the heater. Instead they rigidly control the heater current electronically and put the zero adjust pot in the electronics driving the metering circuit. Most such TC gauges have a digital readout and are a good bit more costly than the Analog metered TC gauges. In general, however, digital TC gauges are not necessarily more accurate than analog ones at the low end.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Luke Harrill
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Re: The Importance of TC Gauge Calibration

Post by Luke Harrill » Wed Jan 02, 2019 3:17 am

Thank you so much, Bruce! With the lower end of that tube I will also be able to calibrate some of my ion gauge scales as well. Does it have the standard kf16 fitting? I'll soon be ordering an iso63 to cf adapter for my turbo (then I will be able to pump down that far).

It is great to hear that about each different tube. That puts away some of my worries about functionality.

-Luke Harrill

Luke Harrill
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Re: The Importance of TC Gauge Calibration

Post by Luke Harrill » Sun Jan 06, 2019 3:50 am

Update: According to Bruce's gauge, my 1402 can pump down to 5.3 millitorr after ~15 min. I'm now waiting for an iso63 adapter to come in so that I can hook up both gauges at once.

-Luke Harrill
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Richard Hull
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Re: The Importance of TC Gauge Calibration

Post by Richard Hull » Sun Jan 06, 2019 5:32 am

That micron number sounds more like the fine Welch 1402 belt drive pump that I have come to know and love. Believe that gauge.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Bruce Meagher
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Re: The Importance of TC Gauge Calibration

Post by Bruce Meagher » Sun Jan 06, 2019 6:15 am

Glad the gauge helped. Finn's sweet vacuum controller sure makes it easy for a quick measurement.

Joe Gayo
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Re: The Importance of TC Gauge Calibration

Post by Joe Gayo » Tue Jan 08, 2019 2:35 am

Bruce,

Were did you purchase that readout display? (I noticed that Finn Hammer is a user on Fusor.net)

Thanks, Joe

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