FAQ - TC tube data chart and their use in circuit

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Richard Hull
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FAQ - TC tube data chart and their use in circuit

Post by Richard Hull »

I have a rather fabulous chart which has several functions. I discovered this chart on an old win98 computer while looking for images

1. It is a cross reference between various TC gauge tubes and manufacturers.
2. Supplies data on some tubes
3. It gives several substitution suggestions for equivalents or near equivalents that might get a gauge controller working from a maker who is long gone.
4. For those who wish to custom roll and just electronic the hell out of a great special "fusor specific" TC gauge system, it gives core data on a few tubes related to normal TC filament currents and voltages all along the tube's dynamic micron sensing range. I hope to be doing an Arduino hack on this concept soon using dynamic interpolation.

The Bell Jar website, managed by Steve Hansen, long ago put out an article in the for pay Journal he ran for some years on building your own TC gauge.
The article was a great one and is on his website and also gives data on some TC gauge tubes along with a simple setup to roll your own TC gauge. Note: this is a full range gauge, similar to one you might buy made by a company. It can be found here....

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

For those with the electronic and microprocessor coding chops this expanded table should open windows. I would imagine a limited gauge I might construct around the Arduino not even indicating any vacuum pressure until 40 or 50 microns and then micro interpolate reading down in the range of fusor deuterium operation pressures read out would be on a simple backlit LCD screen. I will add to this FAQ via replies as I monkey with some of my spare gauge tubes. I am sure some young pup will beat me to the punch on this.... The chart is the key to starting this effort.

The chart is presented below as a PDF file.

Richard Hull
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TC gauge tubes.pdf
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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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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Richard Hull
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Re: FAQ - TC tube data chart

Post by Richard Hull »

As an adjunct to the above post, I will explain how a manufactured analog TC gauge works....

Study the image below and note the components.

1. "D" cell battery (source of heater power only)
2. R1 a simple fixed resistor (for protection against user stupidity)
3. R2 a variable zero set resistor (sets not the voltage but the precise current needed to calibrate meter)
4. Meter - this is a very special meter. (It is made to order and scaled to the manufacturers specific tube type )
5. TC gauge tube - Also a very special construction and meter specific. This tube has two bonded components
A. a simple resistance heater powered by the "D" cell above
B. a thermocouple junction pair of dissimilar metals that when heated supplies a DC potential output to the meter (the meter is powered and deflected only by the thermocouple!!!

Discussion:

When a vacuum is pulled on the tube well below it's lowest metered reading (zero pressure/microns) there is not enough air in the tube to conductively cool the thermocouple. Using the heater circuit, we apply heat to this junction at this time. We turn the heat up using resistor R2 to make the meter read full scale or 0 microns on the scale. The current needed to make this precise heat to "zero" the meter is known and reported by the manufacturer.
The meter is now calibrated and the current to the heater in the tube should be real close to spec assuming a clean heater-thermocouple junction.

With atmosphere in the tube, the junction is convectively cooled by the air and in spite of the heater will not produce any output to the meter, indicating atmospheric pressure, (needle not moved upscale at all.) As we vacuum out the air, there is less and less convective cooling. The thermocouple junction gets hotter and starts to supply voltage to the meter moving the needle up across the meter face and thus down scale in indicated pressure. Finally, we approach the limit of a mechanical pump and the needle is almost fully deflected as the voltage from the now almost totally uncooled thermocouple is putting out so much voltage that it approaches full scale deflection and a zero micron pressure reading.

Special stuff

The meter, its scaling, and its impedance must match that of the thermocouple's response and electrical characteristics. This all fell on the manufacturer. As they work hard at this matching tube-to-tube and meter to meter, no electronics is needed to affect a nearly perfect union in pressure measurement of air pressure. Note I said air pressure. Pumping gases other than air ruin this match made for air as other gases are more or less thermally conductive than the air mixture of nitrogen, oxygen and argon.

Why R1 avoids stupidity by the user:

If a person turns R2 to zero ohms to calibrate, without R1 in the circuit, the battery would immediately destroy the heater!!! The manufacturer puts enough resistance in R1 so that if his tube needs 15ma of current to the heater, zero ohms in R2 might only allow 25ma, a non-destructive current to go to the heater. This allows a range of 10-25ma to be adjusted in by R2. Naturally, 15ma is the ideal for a zero reading on the meter at a sub-micron air pressure.

There you have it. No electronics and ultra simple circuit.

Richard Hull
Attachments
TC diagram.jpg
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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Richard Hull
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Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
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Re: FAQ - TC tube data chart and their use in circuit

Post by Richard Hull »

The above diagram is simplicity on steroids.

The moment you try and make your own TC gauge using a $5.00 surplus gauge tube off e-bay, the issues begin to mount. Without a matching meter with scaling designed for the TC gauge tube and its critical impedance, you are in "get serious country". Electronics and often hand scribed meter scales on a surplus meter is demanded for an analog meter readout. If you want a digital readout display you will need to add a microcontroller that you must program. All of the above is very doable, if you have the chops to move on it.

Take this as an advisory post. Following the bell jar article in the original post, you can with some little effort roll your own TC gauge for analog readout if you have only a known good gauge tube and a suitable analog meter. This involves a simple op amp in electronics and removing the meter face to scribe the new vacuum scale at the various known voltage points where a vacuum level is known.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Richard Hull
Moderator
Posts: 14991
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: FAQ - TC tube data chart and their use in circuit

Post by Richard Hull »

If you are a roll your own type TC gauge guy and use an op-amp to scale a common meter, as in the Bell Jar article, I would highly recommend the 1518 gauge tube as it is the only one that has a measurable span between 5 and 0 microns. Admittedly it is not much and any reliance on this span is a crap shoot as with all TC systems, it is better than all the other tubes at the low end.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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