Capacitive manometers

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Jeroen Vriesman
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Re: Capacitive manometers

Post by 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.
Attachments
IMG_20150901_213102.jpg
Doug Reber
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Re: Capacitive manometers

Post by 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
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Joined: Mon Feb 07, 2011 4:08 pm
Real name: Jeroen Vriesman
Location: Netherlands
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Re: Capacitive manometers

Post by 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
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Joined: Wed Aug 12, 2015 11:33 am
Real name: Doug Reber

Re: Capacitive manometers

Post by 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
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