Another instrument which responds to the mechanical pressure of a gas (as opposed to its thermal or electrical conductivity) is the venerable McLeod gauge.
Has anyone here used one to measure the pressure in a fusor chamber?
In this demo, the pressure in a vacuum manifold is quickly determined to be 5 microns.
http://www.youtube.com/watch?v=7SyrkSuA ... page#t=60s
A McLeod gauge (like a metal ruler, triple-beam balance, or mercury barometer) inherently stays calibrated for life, and has a very small temperature coefficient. Its sensitivity is independent of gas species, except for the partial pressure of vapors which condense when the gauge cycle compresses them. Unfortunately, that includes H2O and up to 2 microns of Hg.
As an analog instrument enthusiast and occasional teaching volunteer, I've long wanted to own a McLeod gauge. Never found one sufficiently inexpensive, until recently. In the space of about 6 weeks, three MG's (no two alike) have come my way for a total of about $75 + S&H.
Re. regulations phasing out devices that contain mercury:
One public health investigator has advocated NOT replacing old fashioned Hg manometers for clinical blood-pressure measurement. He figured that in practice, around the world, Hg-free alternatives would give more erroneous blood-pressure readings. Thus more wrong medical advice and wrong treatments, at a statistical cost greater than that of the mercury spills they eliminated.
Baratron
- Rich Feldman
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Re: Baratron
All models are wrong; some models are useful. -- George Box
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Re: Baratron
You might want to check out the article "Pressure Measurement Realities" that is linked at http://www.belljar.net/vtc_articles.htm
The last page of the article has a discussion of the effects of the temperature coefficients on an ambient temperature gauge. The example is for a 1000 Torr capacitance manometer but the values can be directly scaled. With a 5 degree C excursion the increased absolute error 3 decades below full scale is probably less than the nominal error that you might see with a Pirani gauge.
You would also have to look up MKS' exact values as mine are a bit of a composite (to use a word that's been in the news lately).
There are a couple of other articles on gauge accuracy that you might find interesting.
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Re: Baratron
Thank you, Steve I will definately study the reference. I really appreciate your insight and experience, and willingness to share it! I'm sure I'll have more questions as I learn more.
Thanks,
David H.
Thanks,
David H.