Capacitive manometers

[Richard Hull]

A great system gauge setup for an amateur would be a 1 torr capacitive gauge and a cold cathode / penning gauge. The beauty is that the lower end of the penning gauge could be cal'd to the fading end of capacitive gauge. This would warrant that, regardless of gas in the system, the penning would be relatively accurate over its range. The cold cathode gauge would handle most any conceivable amateur vacuum to 10e-6 torr or better.

UHV freaks (better than 10e-7 torr) would be forced to go to the old baird-alpert setup (never a thrill to use). Fusion has no use for UHV systems. Seems stupid to pump to 10e-8 and pollute it back to 10e-3 with D2. By the same token, it would be nice to hit at least 10e-5 before backfill. (saves D2 over the demanded flowing D2 backfill at 10e-3 with a mechanically pumped system.)

Richard Hull

[SteveHansen]

Good point Richard. I have an inverted magnetron transducer that operates up to about 30 milliTorr hence there is a sizeable overlap with my 1 Torr CDG. As for hot vs cold cathode gauges, the common BA hot cathode tube gets swamped by the x-ray effect in the low 10-7/10-8 range but it's still better than the older Penning cold cathode gauges. The newer inverted magnetron tubes are good into the -10 range since the x-ray effect declines with pressure and is therefore never a real factor. (With the BA tube the x-ray effect stays constant since it's a function of geometry and emission hence it becomes as big as and then bigger than the pressure signal below the -7 range.)

By the way, if you have an MKS flow controller (1159, 1179, etc.) you can make a complete PID pressure control system as long as you also have a gauge that produces a "useful" dc output. No other parts needed, just a wire.

[Tom Dressel]

Larry:

How do you plan to mount the guage in your system?

I have had a VG model CML-11 10 Torr guage laying around in my garage for almost 3 years ( it came with my original hummer V purchase) I had no idea that it is such a nice guage. But since it has no threads on the nipple it looks like another tubulated connection.

Tom Dressel

[Richard Hester]

You need a compression fitting to mount a tubulated gauge to a system. All the major vacuum supply houses sell compression fitting adaptors for the common flange types. If you are a daredevil with steady hands, you could try welding the gauge to a flange. I wouldn't, though. I prefer the VCR connector myself, but as usual, you use what you can get...

[Tom Dressel]

How about a Swagelock to VCR adapter?

Tom Dresssel

[SteveHansen]

Use a compression (o-ring) fitting for a straight tube manometer. Swagelok fittings will be unreliable and they crimp the manometer's tube. You can use the 1/2" Ultra-torr type or Kurt J. Lesker supplies brass and stainless compression adapters. You can even adapt hardware store compression fittings with an o-ring in place of the ferrule. (Usually the o-ring has to be backed up by a washer to keep the nut from distorting or ripping the o-ring.)

Don't weld the tube. You can but it will change the calibration as the welding heat will distort the sensor capsule (not visibly but by enough microns to affect the cal). When the manufacturer puts welded fittings on they place a fairly massive copper heatsink on the tube and do the calibration afterward.

[SteveHansen]

My 1 Torr has a KF16 fitting. It goes on a tee and because of where it's placed it's about 10 degrees off of vertical.

[SteveHansen]

One other note. Unless a manometer has been abused or contaminated, the calibration shift over time will be relatively small in most cases. By small I mean that after a few years it will probably be out of its original 1/2% or so spec but it'll most likely be within a very few percent. Compared to the relatively horrible accuracy specs of TC and Pirani gauges, this is still pretty respectable.

Also, absolute accuracy is generally not needed but rather repeatability. If you find that sweet spot where everything works just right, note the reading and next time return to it. Repeatability of even an out of cal manometer is usually extremely good. Just remember that you are being repeatable, not necessarily accurate.

[Doug Reber]

I realize this discussion is nearly 10 years old, but I would like to put new life in it. I have two Tylan General 80-6B's operating Vacuum General Baratron 1 Torr Vacuum Sensor Capacitance Manometer CMO-01 MKS and Vacuum General Baratron 2 Torr Capacitance Manometer CMO-02SO6. All equipment used and purchased off EBAY. At atmospheric pressure, they both read 6.2 full. I noticed that the +15 and -15 V are actually coming out more like 17V/-17V respectively. My questions are:

1) What should the readout be for ATM on these guages? I would expect 9.999 given the 0-10V FS output.
2) If #1 is true, is the most likely cause the voltage difference or are these manometer set to provide this output when they are bad?
3) What do bad analog old-style manometers like these read if the electronics are good but the diaphragms are coated and bad?

Thanks, Doug

[Richard Hull]

Old thread indeed..

What do you mean the +/- 15 is coming out +/-17. It is demanded that you put in a tightly regulated +/- 15! Nothing comes out of those pins. It is assumed you know that three connections are required for +/-15...ground or 0 base voltage and a +15 volt supply lead from one supply and a separate -15 volt supply lead from yet another supply.

In theory, if the gauge is good, it should read 10 volts or very close to it at ATM.

Richard Hull

[Doug Reber]

Hi Richard,
Sorry for the confusion. I meant the +15/-15V coming from the readout. Actually though, I just verified the situation to be :

-15.16 V on wire attached to -15V connection on manometer. Same wire when just this wire is disconnected from manometer -15.15V
15.31V on wire attached to +15V connection on manometer. Same wire when just this wire is disconnected from manometer 15.33V
+13.83V on wire attached to output connection on manometer. Back of manometer output connection when just this wire is disconnected 13.83V.

The pressure at atmosphere reads 6.809 with all wires connected to the readout and manometer and -0.0089 with just the manometer output pin disconnected. So the manometer is putting out an over voltage and this is causing incorrect pressure readings. I don't believe the zero adjust on the manometer combined with those on the readout can correct for the 3.83V over voltage.

What do you make of it and do you know of an easy means to correct it?

Much appreciated, Douglas Reber

[Richard Hull]

The plus and minus 15V seem OK. I think you have something electronic bad in the manometer head or the diaphram in the sensor, itself, is just bad.

Richard Hull

[David Mitchell]

Hello Doug. Have you attempted measurements under vacuum? Ideally, you would use another gauge such as a Pirani to calibrate the capacitive units when setting zero.
The operating manuals or technical data sheets would be useful re expected voltage versus pressure. Although I'm not familiar with this type of sensor, I would expect to see FS voltage at the maximum operating pressure, which is less than atmospheric (2 Torr for one of the units, I believe). It's interesting that both gauges give the same voltage reading at atmosphere, so perhaps the measured voltage is invalid at this pressure.

[Doug Reber]

Hi Richard,
You are probably right but there is the chance that I chose the wrong pin as ground.

Hi David, when I first received the equipment I used a power supply and voltmeter to guess at the correct pinnouts. I just figured the highest voltage pin had to be plus 15, lowest the minus 15, and the had to guess at ground and input/output by trying different combos. Interestingly enough, the gauge shows pressure decrease with vacuum but I only have two rather poor TC gauges to help estimate the level and their gauge is really only hood for telling pressure vaguely. Unfortunately my switching dc supply just died but in hooking up several other 8v and 5 v supplies, the pressure reading at gauge tracks the voltage. I may need to go back and check my guesses on the ground going to the nanometer it might be the wrong line.

Thx

[Doug Reber]

I am going to post on the thread specific to the Vacuum/Tylan General 80-6 Gauge Controller so I don't keep double posting:
viewtopic.php?f=10&t=4145&p=68436#p68436

[Jeroen Vriesman]

We use a "baratron" (capacitive sensor) for our bell jar chamber.

Very simple, a 20VAC output transformer making a symmetrical DC voltage over two caps with two diodes, then 7815 and 7915 to make symmetrical +/- 15V for the baratron.
Our baratron is 1 Torr @ 10V, we connected a standard DVM module with a resistor divider to translate that to Pa (making maximum use of the DVM resolution).

The DVM requires 9V, at first I used a simple 7809 for this, but the input common mode range of the DVM module did not include ground (that was not in the specs... but well, 5 euro module ), so had to make +/- 4.5V with 7809 and zener.

Works fine, no idea how to calibrate it. On the picture it shows 250Pa.

[Doug Reber]

Hi y Jeroen, I posted the procedure I used in the "Vacuum/Tylan General 80-6 Gauge Controller " topic. Basically, on a 1 Torr full-scale manometer, it should read 10V at 1 Torr, 1V at 0.1 Torr, 0.1V at 0.01 Torr and so on. So you can tell if you have the voltage divider set up correctly by attaching a fast acting voltmeter across the manometer's terminals, pull vacuum, introduce gas at various flow rates, let it settle and you know you have the voltage division correct if your readout gauge is reading 1.0 while the manometer's terminal voltage is 10V, and so forth.

Best of luck, Doug

[Jeroen Vriesman]

Hi Doug,

correction, it's 5 torr at 10V, but what you say is exactly what I've done. It's just that I don't have any reference, so besides the fact that the baratron says "10V means 5 torr" (which I translated to Pa with the resistor divider) I don't know how precise that value is.

Is there a way to create a good pressure reference, maybe some boiling point at some temperature, but that is not easy to see. Some other physical process happening at some exact pressure?

[Doug Reber]

Hi Jeroen, what you propose sounds like a good calibration method but I could not entertain them for my application. There are several such options shown if you search "http://www.bing.com/images/search?q=Pre ... ORM=RRIMGE". Please let us know how it goes. Thanks, Doug