#4 FAQ: vacuum gauges - pressure measurements

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Richard Hull
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#4 FAQ: vacuum gauges - pressure measurements

Post by Richard Hull » Mon Jun 06, 2005 7:24 pm

All who would know specifics in great detail need to fully and completely explore Kurt Leskers website on pressure measurement and look and technical notes and different devices to learn precisely how they work. Go to

http://www.lesker.com/newweb/menu_pressuremeas.cfm

The fusor vacuum environment is special in that it is hung squarely between medium and high vacuum and also at a point where gauges at the lower pressure end of medium vacuum "play out" or are getting inaccurate. Likewise, the fusor functions at the high pressure end of the high vacuum region where instruments made for measuring high vacuums are also inaccurate or just starting to pick up data.

We are in a no man's land more or less. There are two fusor types and these normally demand that different pressure measurement systems be in place.

For demo fusor systems:

1. We need to see our demo systems in the lower end of the thermocouple range which is in the 100 micron - 1micron range.
2. We need to accurately know the running or operating pressures in the .1-10 micron range.

Working fusor- doing fusion:

1. We demand that a good fusor be exhausted well into the high vacuum region and this requires the capability of looking at pressure far into the submicron range. This region is usually in teh 10e-4 torr to 10e-8 torr range.

2. We demand that the deuterium back fill be closely regulated to great accuracy which demands clean measurement back into the micron range of 1-20 microns. The same high vacuum gauge will not work here.

SO...... what do we do?

Units of pressure: What do we use?

There are, seemingly, a million pressure units. The anally retentives and science hard core weenies will use SI units (pascal). No one uses SI units who has been doing vacuum work. (maybe some euro types and paper publishers, but no one else).

The only two units you need fiddle with are the units of yesteryear which are still in nearly universal use.

1. The micron - a very old unit ( one millionth of a meter of pressure) and actually is the simple unit which hovers just right around the fusors operating pressures. This unit is still the unit of choice on all thermocouple gauges. As most of us will only have a TC gauge, microns is still "good-speak"

Atmospheric pressure is 0.760 meters of pressure or 760mm of pressure. Very roughly speaking, a micron is about one millionth of an atmosphere. This is the air pressure at about 60 miles above the earth.

It is considered bad form in the vacuum biz to speak in terms of 0.05 microns. When we talk sub-micron, we must switch units.

2. The Torr - this is the simplest and most useful of all vacuum units and is universally spoken due to its being constantly discussed as powers of ten. 1 torr = 1 mm of pressure. 760 torr= 1 atmosphere. 1 micron =10e-3 torr, etc High vacuum begins roughly at 10e-4 torr = 0.1 micron.

At 130 miles above the earth, where the low orbit of the space shuttle lies and a frictionless orbit is found, we are at a high vacuum level of 10e-6 torr or about one billionth of an atmosphere. Still, even in this deep vacuum, there are over a billion molecules of air in every tiny cubic centimeter of space.

If you use a TC gauge you are just going to have to get used to the micron. As both the torr and micron are metric and are just a power of 10 conversion all can be done in one's head and a good feel for vacuum is readily at hand.

Stupid units that have no relation to useful conceptual stuff are the pascal, bars and millibars, inches of mercury, etc.

We all speak in the common vacuum parlance here and if forced to choose one, the torr would be the best.

Gauge selection:

Thermocouple or TC gauge:

Your very first gauge must be a thermocouple gauge. This is mostly due to its low cost and the fact that you can limp through all fusor phases with just this one gauge. This gauge can be very inaccurate just where we need it the most. It will lie about pressures when different gases are used. Offseting this is the fact that a lucky used buy might land a good TC gauge with tube for $50.00. This gauge relies on the thermal conduction of a gas in the sense that the gauge looks at how fast a heater can lose its heat or cool as detected by an electrical thrmocouple sensor. This is always a function of the number of molecules per unit volume around the heater and sensor. Most common TC gauge tubes and cables use a common old radio tube base. (octal- 8 pin and it has a keyed center post to align the pins. All tubes only use 4 of the pins, two for the thermocouple and two for the heater. All tubes are internally wired differently and the heaters all draw different currents and the thermocouples typically report differing voltages. So they are pretty much controller specific. (make sure you have the tube that the controller calls for.) Most good controllers will have the tube number they work with printed on the meter face or the cabinet's model number plate.

Brand new TC gauges with tube might cost as little as $250.00. Good used ones are in the $100.00 with tube. A lucky find might snag one with tube for $50.00

Capacitive manometer gauge:

This is just the opposite of the TC gauge and is certainly the best gauge on earth at this time for use with a fusor. A .1 torr capacitive diaphram gauge with controller will give precision pressure readings from 100 microns = 0.1 torr down to about .01 microns=10e-5 torr. This is the only gauge that can do this in one gauge and would be the most perfect of all fusor vacuum pressure gauges to own. Be prepared to see a list price of gauge and controller in the $2000.00 range for a new gauge. Used sensors can be found on e-bay for $100.00 to $500.00 and controllers in the same range. This gauge will give accurate readings regardless of gas type unlike all other gauge types which require conversion factors or re-calibration for specific gas types. New gauges of this type are $1000.00 without any controller. But controllers are easy to make with only a little electrical experience.

Ion gauge:

A pretty worthless gauge as it doesn't even work well even near the medium vacuum end of pressures. This gauge is much more common in high vacuum systems where 10e-4 to 10e -9 torr pressures are common. Most ion gauges really shine in the 10e-5 to 10e-7 torr range. Don't consider this gauge unless you get a freebie.

Discharge, Penning or cold cathode gauges:

These gauges are rather rare now, but, oddly, are actually useful for fusor work especially if used with a TC gauge. This gauge is useful from 10e-2 to 10e-6 torr. Its low end is OK for micron reading and its high end is suitable for full neutron fusor evacuation.

This is basically a magnetically enhanced air glow discharge device where the current at a given voltage needed to sustain an glow-discharge in the gas is a function of pressure based on the number of ionic current carriers in the discharge.

This gauge and its controller is sometimes found rather inexpensively at hamfests or surplus dealers.

Piranni gauge:

This is a wheatstone bridged gauge that often includes a reference resistance in one of its bridge arms. It is really just a stabilized and refenced TC gauge system. Some of the best piranni gauges can respond down to 10e-4 torr, gaining a full decade over the TC gauge. The gauges are typically far more expensive that the TC gauge, but used deals can be found in the $100-$200 range. Still not my choice but better than a simple TC gauge.

As you can see, no gauge that is cheap or inexpensive can handle the full pressure range of a working fusor and be considered to be very accurate.

Combinational systems:

It is now fashionable to place two of the above sensors in one pressure gauge tube and design a highly intelligent controller which selects the appropriate sensor within the tube to use at its appropriate pressure range. This overlaped, dual sensor system may now be claimed to cover many decades of pressure. Such systems demand the specific controller designed for the sensor. If one sensor goes bad you must throw the good sensor away and buy a complete new head. Such systems are very expensive, even at surplus prices, due to the novelty and newness of these devices.

The ultimate choice of gauge is a function of your desire for accuracy versus the depth of your purse. Lucky purchases not withstanding, you will spend between $200.00 and $2000.00 for your new vacuum gauge.

Thanks to John Hendron for the following links.

Wanna' roll your own TC gauge around a common gauge tube?..... checkout.

http://www.belljar.net/tcgauge.htm

Want still more general info on gauges and measuremnt units?...... go to

http://www.belljar.net/articles.htm

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: FAQ: vacuum gauges - pressure measurements

Post by zexelon » Thu Sep 22, 2005 3:26 am

This has to be the most helpfull FAQ here.

Thanks Richard

Sincerely
Melvin

bevan
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Re: FAQ: vacuum gauges - pressure measurements

Post by bevan » Thu Nov 29, 2007 11:42 pm

It may be worth you looking at the wide range combination gauges at Leskers

http://www.lesker.com/newweb/Gauges/wid ... LC_910.cfm

Can cover a wide range (1.0 x 10-5 to 1,500 Torr (1.3 x 10-5 to 2,000 mbar)), digital versions of the traditional gauge technology allows it to be controled via a PC and software, meaning you don't have the expense of the control boxes -also are more accurate.

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