FAQ - Thermocouple gauge tubes

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Richard Hull
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FAQ - Thermocouple gauge tubes

Post by Richard Hull » Mon Mar 26, 2018 5:08 pm

TC gauge tubes are very robust mechanically. You can treat them very roughly with no major issues, as a rule.
They are not electrically robust. The heater can be burned out in a fraction of a second if a excess current is applied to it. This can rarely happen if you have the mated controller designed for the tube with its proper mating cable.

Brand new, excellent TC gauges with controllers can be had as cheaply as $200. If you are serious about you vacuum efforts, you must buy a complete TC gauge system and tube brand new.

Surplus TC gauges - a pig in a poke

If you opt to go surplus you may be throwing good money away or you might luck out. Never count on lucking out.

I have seen flawless TC controllers, (without tube or cable), go for $5.00 to $10.00 each at hamfests and they are all typically all in perfect functional condition! Such bargains often have one issue in common.......The tube demanded to make it work is no longer made....or.....you have no idea what tube a naked controller requires, as the manufacturer has not placed on the meter or its cabinet what the number designation is of the tube it must have. An adroit vacuum enthusiast might readily be able to noodle all this out by looking at the scale on its analog meter or, in concert, measuring the probable tube current in a cobbled up circuit....or... by checking the pins on the TC tube plug to see which are heater and thermocouple pins. Regardless, you will need a TC gauge tube for your $5.00 controller.

A brand new tube will never be found at retail for much less than $50.00 If you guess wrong on the tube, you might burn it out due to too much current on a mismatched controller, or have it not work at all as the heater and TC pins are mismatched. (You chose the wrong tube)

You have found a surplus gauge tube for sale

I have found TC gauge tubes for $1.00 each at hamfests. I have bought them and found the bulk to have burned out heaters or are grossly out of calibration range when attached to a matching controller in my lab.

Those that are grossly off in error can often be restored. Idiots often abound in the world of vacuum or, more often, idiots inherit good used vacuum systems and wreck their pumps or gauges by failing to maintain or operate their systems properly.

The thermocouple in the TC gauge tube is designed to remain "as purchased" when new throughout its useful life. Oil back filling a TC gauge tube, over time, can foul the tiny element and harden to a varnish, throwing the gauge out of calibration.

I have repaired such tubes and restored their calibration by pouring MEK and or acetone into the tube's port and placing my finger over the port hole and vigorously shaking the tube to clean the TC element. It might take several attempts. Alas, if heavily varnished, even this might not help. This is a last resort, of course, to desperately save a surplus tube.

If you must purchase a surplus TC gauge system, make sure it is complete with tube and cable. A good price for a working tube and controller that is warranted is $100.00 or more.

General use of a good TC gauge system

TC gauges are always placed in the foreline between the mechancial pump and the secondary diff or turbo pump. To keep your gauge healthy and in good order, always vent your isolated foreline to air immediately after shutting down the mechanical pump. Make sure the foreline is isolated from the secondary pump before venting!!!! This will help keeping back streaming oil from the TC gauge tube.

When "dead heading" the TC gauge, as when checking the condition of a suspect or surplus pump, and using clean oil, you should start a small venting leak just before stopping the suspect pump so that immediate back streaming will not occur and foul your gauge tube at such short range from the pump's inlet.

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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