Smart money subscribes to the thing. Our recent new member Steve Hansen really does a great job on the publication and I have subscribed since day one. His effort on the vacuum forum alone has swollen this edifice to be the largest forum on this board. We owe him a great debt for his agreeing to put a lot of the better articles on line for us, when he is just, in effect, cutting his own throat in lost reprint issue sales.
In the original article, there was a nice table of rated currents for an array of common tubes with full pinouts for heater input and thermocuple outputs.
99% of all TC gauges use a special 10mv meter movement. (very special and tube-specific scaled). By doing this, the manufacturers obviate a whole bunch o' nasties. (the specifics of the nasties ain't important.) The upshot is that most TC gauges have only one adjustment control. It sets the tube current only.... BUT....Due to the special meter and tube-specific related scale, this control is labeled "ZERO".
When you adjust this pot to zero the meter after pulling the gauge tube down below 1 micron, you are setting the tube current to the proper level!
THUS, with the tube set to the proper current, ANY 10mv meter with the tube specific scale WILL read correctly!
Most manufacturers resistively "corral" the current range/zero pot, so that some dunderhead slamming the pot limit-to-limit won't burn out the ultra delicate heater, thereby, killing a $50.00 gauge tube.
In general, if you have to set the tube current and then the zero on two pots, the manufacturer is using a cheapo standard, off the shelf, meter movement with even more whacked out scale gyrations than a normal 10mv meter. Admittedly, in some gauges these are used with a proper meter and one is often called zero and the other FS (full scale) or current set. This type of TC gauge meter was designed by paranoid engineers who wear both a belt and suspenders. They are trying to compensate for small variations in meters and tube specs. All this to wind up with a +/-20% reading!!!?
I have written this before. The 1518 tube is the best in th' biz!!! It is th' cheapest in th' biz ,too! (~$42.00) With a properly scaled 10mv meter movement, you can make a super TC gauge with the scale opening up near the 10 micron range instead of closing down there with only two other components!
A 1.5 volt dry cell ("D" flashlight battery) A potentiometer (variable resistor)
The tube only demands about 15ma of current and the "D" cell will last for 100's of hours on time. PLUS, the TC gauge is completely portable. (no tedious and bungling line cords.)
I have found a lot of these special TC gauge meters for a number of different tubes amongst canabalized meters in surplus stores and at hamfests. They run between $1.00 and $4.00 each. The manufacturers ask about $100.00 for these as simple replacement parts.
For the beginner in vacuum who is working a medium vacuum or using only a mechanical pump, a TC gauge is a bare minimum to know something about your vacuum and troubleshoot problems. The key thing to remember is that what the meter says is almost never what is there, but what was there 10 seconds ago. Also, stable readings can be in error by large amounts especially at the extremes of the analog meter range.
Richard Hull
Created on Thursday, April 19, 2001 5:11 PM EDT by Richard Hull