Thanks everyone for their inputs into my query. As usual with this ng I end up with far more to think/worry about than I started with ..... but then that's half the fun of spending retirement disgacefully, you can enjoy engineering with having to worry about mission statements, corporate plans, brain-dead CEOs & bean counters (well apart from herself that is) !
I'm curious now: does yours use the standard cell just for calibration, and not for powering the potentiometer? I appreciate what Glen wrote, that the advantage of the simple thermocouple potentiometers he is familiar with is that they don't load the thermocouple. But it would have been easy to provide at least a calibration port to check them against a standard cell and adjust a rheostat to put them on cal before sending out to do measurements -- assuming that the mercury cell power voltage was greater than the standard cell voltage.
Cheers, Tom
It's *only* used for span calibration. You must not draw substantial current for any length of time from a standard cell. It's possible to use one as a reference voltage with a modern op-amp buffer, but we're talking old technology here.
The calibration shifts substantially between uses, probably due to Hg cell temperature and maybe polarization effects. Normally you recalibrate before every use, and every little while during heavy use. They will stay within, say 0.1% of span for a reasonable period of use. Typically all the resistors are extremely stable large loose-wound wirewound types, and there is little room for errors to arise. Also, typically, such instruments are used in a lab environment so temperature changes are minimal.
Best regards, Spehro Pefhany
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