Baratron Capacitative Manometer question.

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Larry Upjohn
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Baratron Capacitative Manometer question.

Post by Larry Upjohn »

Can anyone tell what voltage change represents (0-10 volts or 0-6 volts) on a "Baratron"? Does it go up for lower vacuum or is highest for the upper limit of vacuum sensed in the vacuum system? I am building a quick test jig for Baratrons and this question came to mind. Thanks for help and insight, Larry Upjohn

P.S. I have printed out several Baratron information sheets and it gives the range but not what the rate or direction of change represents. LRU>
Larry Upjohn
Larry Upjohn
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Re: Baratron Capacitative Manometer question.

Post by Larry Upjohn »

I hate talking to myself but I have answered my question. I hooked up variable 6VDC supply to my mystery Cap. Manometer readout (Datametrics 1400 now Datametrics/Dresser) and discovered what I needed to know. Thanks for stopping by to wonder. Larry Upjohn.
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rheil
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Re: Baratron Capacitative Manometer question.

Post by rheil »

Larry,

For the Baratrons I am familiar with, those used to measure vacuum, the output voltage goes up with the applied pressure. So a 1 Torr baratron will have an output of full scale (somewhat more than ten volts) at ambient pressure. This output will not begin to drop until the applied pressure (vacuum) drops below about 1 Torr, at which time the voltage will begin to ramp down with reduced pressure until it hits zero at the minimum resolution of the sensor. (Or in my case, goes negative and indicates pressure of less than zero!) I still need to figure out how to adjust the zero setting.

So, unlike some vacuum gauges, the Baratron output does not drop until the applied pressure drops to within its rated operating range. There still remains the need to adjust the zero setting once the applied pressure (vacuum) is low enough to get below the minimum rated output. I am not sure exactly how to accomplish this, since my system (and yours too) can never hit zero pressure. I guess we just have to get as low as possible and then set that to zero?

Bob H.
Larry Upjohn
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Re: Baratron Capacitative Manometer question.

Post by Larry Upjohn »

Bob;
Thanks for your reply! I have just aquired two used 15 Torr units and used one 10 Torr units so your information gives me somewhere to start to make sure they are functional. My fusor efforts right now are on a shoestring so I have been very fortunate to collect some good equipment at bargain basement prices. Thanks again, Larry Upjohn.
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SteveHansen
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Re: Baratron Capacitative Manometer question.

Post by SteveHansen »

Regarding zeroing, the key thing is to get the pressure at least below the minimum useful reading (4 decades below full scale) and preferably 5 decades. Then turn the zero pot.

You should be reading in the upper 3-1/2 decades. That's the recommended reading range for a Baratron (or any other CDG).

So, if you have a 10 Torr full scale you should zero it at 0.1 micron but 1 micron will do as long as you are using the instrument at or above 5 microns although you will have built in a possible 1 micron error due to the zero offset.

NEVER touch the linearity and span pots unless you have an accurate transfer standard to compare against.

It's already been covered but the output of the instrument declines with pressure. So, for the 10 Torr full scale example with an instrument with 10 volts out at full scale:
10 volts = 10 Torr
1 volt = 1 Torr
0.1 volt = 0.1 Torr
0.01 volt = 0.01 Torr
0.001 volt = 0.001 Torr

Commercial readouts are usually only good for 4 decades of resolution.

I recently did an article on "Pressure Measurement Realities" for the July issue of Vacuum Technology and Coating. It's on line at http://www.vtcmag.com/ There's also a gauging primer article in the June issue.

Steve
Larry Upjohn
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Re: Baratron Capacitative Manometer question.

Post by Larry Upjohn »

Steve;
Thanks for your great response! The two articles referenced have been printed out and will be good reading tonight. I really appreciate your taking time to respond from your busy schedule for my part time research endeavors here.

I used one of the 15 TORR Baratrons and have worked out the conversion based on the 0-6 Volt signal range. My old Datametric 1400 unit is merely an older DPM with no direct conversion to TORR/micron reading so using your table I was able to determine that my (old) Welch 1400 is pulling a consistant 25-26 microns on my current test platform. My Hastings DV-6M TC is indicating 9-10 microns which was optimistic at best. My earlier chamber glow discharge was assessed by the group here as being in the 30 micron range inspite of the TC readings so addition of the Baratron has confirmed this case. I have a 2" diffusion pump waiting in the wings to add in to the system (when I get time of course). However working through the preliminaries such as the gauging and plumbing has been quite educational.

Thanks again for your input. More later, Larry Upjohn
Larry Upjohn
SteveHansen
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Re: Baratron Capacitative Manometer question.

Post by SteveHansen »

Your manometer has an interesting range and full scale voltage. Would you send me the full part number? I'd like to look it up.

Steve
Larry Upjohn
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Re: Baratron Capacitative Manometer question.

Post by Larry Upjohn »

Steve;
the 10 Torr unit has +-12Volt supplies and 0-4 Volt range: Type is
122AA-00010DB-SPO71-90(serial # 10647-1B). The two 15 Torr units again have +-12Volt supplies and range is 0-6 Volts. Type number on these two units are122A-11095----S (Serial number on the one not hooked up right now is 94031309A). All three units have a KF16 flange attached. The newest one is hooked up to my demo fusor right now and appears to be more stable than the TC unit I have had for a while. Running them in tandem to learn how comparable they are over extended roughing. Thanks again, Larry Upjohn.
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SteveHansen
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Re: Baratron Capacitative Manometer question.

Post by SteveHansen »

Hi Larry,

When I have a moment I'll look up these two specials in the database. Very unusual specs. 122s (now obsolete) are great units - very stable and reliable. I have several of them from the mid '90s are all are doing well.
Larry Upjohn
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Re: Baratron Capacitative Manometer question.

Post by Larry Upjohn »

Thanks Steve! I am now testing the 10 Torr unit and scored 100% functionality on the two tested so far. The ambient temperature in my garage lab is now getting up to 100F plus or minus daily so I am learning that the TC gauge definitely reflect ambient temp variations but the Baratron is much more responsive and much less sensitive to ambient temperature changes. More later, Larry Upjohn.
Larry Upjohn
SteveHansen
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Re: Baratron Capacitative Manometer question.

Post by SteveHansen »

These units were made for a company that made plasma sterilizers (now part of Johnson and Johnson). They must have purchased enough of them to make the special ranges and power supply voltages worthwhile.

The only other piece I can add is that the specified accuracy is +/- 2 percent of reading.

You are probably measuring about 3 decades down from full scale (i.e. 10-15 mTorr range) so check your zero every now and then at some pressure <1 mTorr, preferable < 0.1 mTorr.

Steve
Larry Upjohn
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Re: Baratron Capacitative Manometer question.

Post by Larry Upjohn »

Steve;
Thanks again! Just plugged in the second 15 Torr Baratron after quick cleanup of the KF16 gasket area. All of them have worked though I will need to zero all of them once I get the diffusion pump online. Best wishes with your mass-spec project. More later, Larry Upjohn.
Larry Upjohn
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