Thermocouple Gauges VS Vacuum Gauges

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Lance Newman
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Thermocouple Gauges VS Vacuum Gauges

Post by Lance Newman » Sun Apr 01, 2018 10:38 pm

I noticed a difference in vacuum measuring equipment - most people use thermocouple gauges. This may sound like a stupid question, but what's the difference? Can a regular vacuum gauge be used in a fusor? Or is it not suitable in such conditions?

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Re: Thermocouple Gauges VS Vacuum Gauges

Post by ian_krase » Sun Apr 01, 2018 10:58 pm

Vacuum gauge is an extremely general term.

Thermocouple gauges are what you mostly want to use however pirani gauges can also be used as an alternative to thermocouple gauges however they're less likely to be calibrated if you get them used. Also pirani gauge equipment is more expensive and more specific to vendors.

Michael Bretti
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Re: Thermocouple Gauges VS Vacuum Gauges

Post by Michael Bretti » Mon Apr 02, 2018 12:12 am

Thermocouple gauges are a type of vacuum gauge. It all depends on what range vacuum you are looking to read - each gauge type can operate at different ranges specific to the gauge. You can also get combination gauges that can cover a wide range of vacuum levels. For a fusor, generally operating in the single millitorr to several tens of milltorr range, a thermocouple gauge will work fine. If you want to go deeper than 10^-3 torr (such as for beam on target systems or high vacuum conditioning of the chamber), other types of gauges are required, such as pirani, ion gauges, etc. I have found combination pirani-Bayard Alpert gauges, which can work from atmosphere to 10^-9 Torr for the gauge I got, in practically new and unused condition off eBay for only $40.00. I bought several of them, and will be using them for a couple of systems I am building. Hot filament ion gauges, both nude and enclosed types, are pretty common on eBay for very cheap as well, in both new and used condition. You just have to look around and be patient - they are always popping up on eBay.

Here is a pretty extensive and detailed PDF on the fundamentals of vacuum technology. There is a whole section dedicated to vacuum gauges (Section 3, starting on page 76). I can't attach and upload the file since it is too large, but here is the link:

https://www3.nd.edu/~wzech/Resources_LE ... ENTALS.pdf

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Re: Thermocouple Gauges VS Vacuum Gauges

Post by Andrew Seltzman » Mon Apr 02, 2018 12:55 am

I'd recommend an MKS 901p, it's a combination piezo/pirani gauge that will function between atmospheric and 1e-5torr
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Rich Feldman
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Re: Thermocouple Gauges VS Vacuum Gauges

Post by Rich Feldman » Mon Apr 02, 2018 2:53 am

I think the responses so far have missed the gist of question in OP. Assuming that when Lance says "regular vacuum gauge", he means the kind sold at hardware stores and auto parts stores, I bet even in Panama. It doesn't help that they are also seen on demo fusors, such as the one featured in Make magazine.
vac2.jpg
vac2.jpg (12.09 KiB) Viewed 868 times
Let's ignore the presence of vacuum tutorials in the forum FAQs.

A typical "regular" gauge uses a Bourdon tube mechanism, just as in typical mechanical pressure gauges, to indicate the difference between pressure under test and ambient pressure. Sitting unconnected, it reads zero. Connected to a perfect vacuum, or a working demo fusor, it will read about -1 bar or -30 inches Hg; different words for -1 atmosphere. Plus or minus a couple percent for gauge error, another couple percent according to local weather conditions, and minus a lot if the laboratory elevation is far above sea level.

How useful is that, if we are interested in absolute pressures much less than 1% of an atmosphere?

The thermocouple and Pirani gauges of which we speak measure absolute pressure, by sensing thermal conductivity of rarefied gas.
A typical design, if exposed to a 99% vacuum (absolute pressure 0.01 atm = 7.6 torr),
will have its readout still pegged at full scale maximum pressure indication.

As your pump runs and the absolute chamber pressure falls, the "regular" gauge will bottom out (indistinguishable from complete vacuum)
around the time the absolute pressure gauge starts moving down from the high end of its scale.

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Re: Thermocouple Gauges VS Vacuum Gauges

Post by Michael Bretti » Mon Apr 02, 2018 3:34 am

I think the term "regular" can be pretty subjective here. It all depends on the range you are after and the application. A "regular" vacuum gauge for typical refrigeration work could be the common Bourdon type gauge. A "regular" vacuum gauge in the ultra-high vacuum or physics field could mean an ion gauge. In general terms of vacuum technology and engineering, I don't think one could claim there is a definitive, universal regular gauge. The average person might assume the Bourdon type is a regular vacuum gauge, but for someone who works in the high vacuum field it is just one of many types of gauges.

Here is a simple and handy chart that gives the average measuring range of all the above mentioned gauges:

https://www.lesker.com/newweb/gauges/ga ... uide_1.cfm

Also some additional introductory material on the difference between these types of vacuum gauges:

Introduction to vacuum gauges.pdf
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Re: Thermocouple Gauges VS Vacuum Gauges

Post by Richard Hull » Mon Apr 02, 2018 3:41 am

To give a bit of finality to it all...........If you are talking about the gauge depicted above........It is absolutely of no value whatsoever for fusor work.

There is no such thing, nor will there ever be such a thing, as a "regular" vacuum gauge.

In this case a picture was worth a thousand words.

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Lance Newman
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Re: Thermocouple Gauges VS Vacuum Gauges

Post by Lance Newman » Tue Jun 12, 2018 11:52 am

Sorry about the lack of articulation. I meant ones that were like in the picture.
Also, a digital micron gauge wouldn't work either, or would it?

https://www.amazon.com/CPS-VG200-Vacuum ... cron+gauge

I saw one of those on the Kuba and Noah Youtube video a while back. They did fusion with the gauge hooked up to the main chamber.

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Rich Feldman
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Re: Thermocouple Gauges VS Vacuum Gauges

Post by Rich Feldman » Tue Jun 12, 2018 3:25 pm

Good to see you back, Lance.
That "micron gauge", sold for refrigeration service, AFAIK is a thermocouple or Pirani gauge with integrated, battery-powered digital readout. Maybe Kuba or Noah, or someone else with a users manual, can check for us. It would be handy to have a gauge with no wiring to clutter the apparatus.
vacrometer.jpg
vacrometer.jpg (7.09 KiB) Viewed 189 times
Does the label really say vacrometer?
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