Capacitive manometers

[3l]

Hi Folks:

I just won a capacitive manometer on Ebay.

"This Vacuum General pressure transducer is a model CMLB-11s01, 10 Torr, input +/- 15VDC, output 0-10VDC. This is a clean transducer with a 1/2" tube fitting."
Thanx go to Mike Amman.... Salute!

Ok ...If you have a range of 0-10 Torr how does the voltage vary with pressure?
A silly question BUT....
I am automating the entire process of vacuum into a computer operated affair.
Vacuum creation in the past tho entertaining at first, has become dogent after thousands of pump downs and many hours of twidling. I want to boot my box, enter a command,
do something else without interuption.
Chrystler minivan here I come!

Happy Fusoring!
Larry Leins
Fusor tech

[pfostini]

All that is required is the +/- 15 volt power and its return. The output is 0-10 torr which is 0-10 volts on the output. The Readout is nothing more than a digital voltmeter. I just picked a readout up and calibrated it with an adjustable lab supply.

[3l]

Hi Phil:

I like that.... it should go into Opto 22 fairly easily then.
Most DVM are 3 1/2 digit....how many digits is the manometer good for?
My main concern is the switch over from TC to Penning style gauges. I noticed that in Dunaway's catalog the range covered by the manometer would probably work for the precise running of the diffusion pump and my roughing pump. Wow that will make that work a breeze. Cool!

Thanks Phil

Happy Fusoring!
Larry Leins
Fusor Tech

[Richard Hester]

The newer capacitive manometers are supposed to ge good for about 4 decades, probably with the most accuracy in the first 2-3. I don't know about your old Vacuum General monstrosity, though.

[SteveHansen]

Except for premium instruments, the resolution of a capacitance manometer is 1/10,000. Thus a 10 Torr manometer will resolve to 1 milliTorr. The useful reading range is a bit over 3 decades down from full scale, providing the instrument is properly zeroed. If the manometer is to be used as a control input, 2.5 decades is about it.

To zero a manometer pump it down to a pressure under its resolution, one or two decades is generally advised. Then check the zero often as it can drift due (mostly) to ambient temperature conditions.

If the unit can't be zeroed, it's probably contaminated and there's really nothing that can be done except to have the sensor replaced.

Capacitance manometers are really great instruments for medium vacuum work where you need accuracy and insensitivity to the gas species in the chamber. I use a 1 Torr FS unit on my gas accelerator tube and I can measure/control nicely in the 1 to 50 mTorr range.

[pfostini]

I just sent one of mine to MKS for calibration. They said the cell was bad. They wanted 1500.00 to repair it- Crazy money. I will take my chances on ebay for 80 bucks. I bought a few off ebay and they worked and they are still dead on. I like the 0-1 torr for fusor work.
Larry; once you get yours going, pump it as low as you can go and let it stay on the pumps for a few hours before adjusting it. Only adjust the zero adjust and leave the span and the other adjustments be. If you happen to pick up a readout- short the inputs and adjust that for zero then adjust the sensor for zero. I use several at work and they hold up quite well. Most of them are 10 torr units.

[Richard Hester]

I prefer the 0.1 torr for fusor work, as it will be more accurate at the pressures of interest. The only drawback is that the really low pressure devices are temperature controlled and need to sit for a few hours to stabilize. Combined with this would be a 1000 torr device for telling a drag pump when to switch on.

[Richard Hull]

One thing no one mentioned. Old hands like Steve Hanson tend to forget this anyway as a given associated with capacitive manometers.

Once calibrated and zero'd, especially if you are relying on the 3 and 4th decade, (like we are), the manometer can never be physically moved ever again!!!!!.

I am as serious as a heart attack about this!

Gravity will screw up a cal or zeroing in the 3rd and 4th decade like a truck ran over the thing!! Before you zero....Lock the unit into its final resting place. Just a few degrees off axis will screw the low end of the basement reading.

Finally, capacitive manometers really have no favored orientation in a gravity field, in general. It is just that once in position and zero'd there, they will never read correctly in the lower decades in any other position unless bolted down there and re-zero'd. If you want the accuracy, you pay the price in the form of constant re-zeroing with each position change.

These aren't portable devices the way a rough and ready TC gauge is.

Richard Hull

[SteveHansen]

I'll add a couple of points here. Capacitance Diaphragm Gauges (CDG which is the "official" term) are indeed position sensitive as gravity has an influence on the diaphragm. You can operate one in any position but whenever you do change orientation, do a rezero. Actually it's good practice to rezero whenever you are performing a critical operation just to be sure.

Preferred orientation is port down (stuff has less tendency to fall into the port). Least preferred is port up.

As far as a comparison with a TC goes, the worst CDG you can buy today has an accuracy of 0.5% of reading. Best is 0.05%. Out of the box a TC gauge is around 25% or worse depending upon where in its range you are working.

Also, with regard to use in the lower decades, make sure that the gauge is isolated from any excessive vibration as vibration will show up as noise. Bellows tubing is good or, if you have to have a direct connection, try to orient the instrument such that the vibrational motion is parallel to the diaphragm.

For your particular gauge you can get an idea of how frequently you should zero by pumping to the zeroing range periodically and seeing how far from zero the reading is. It may vary during the day if your workplace temperature changes.

Last, if you are using a CDG of 0.1 Torr full scale or lower the best way to preserve the calibration and zero is to put an isolation valve between the gauge and the chamber to keep the sensor under vacuum. This minimizes any small shifts due to diaphragm hysteresis and also keeps condensables from moistening the innards of the sensor.

[SteveHansen]

Must've been a 370 (long ago obsolete). A sensor replacement on a new model instrument is more like $400-$500 (still not cheap) but you basically get a new gauge.

[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