

Getting Grounded: A Thermocouple Rite of Passage - jerryr
http://www.mindtribe.com/2015/01/getting-grounded-a-thermocouple-rite-of-passage

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jeffwass
I've used cryogenic thermometers in my physics research, which IIRC were not
thermocouples but basically calibrated chips of semiconductor, increasing
their resistance at low temperatures. (Our usual temperature range went from
20 K down to 10 mK).

The trick with these puppies is to read the resistance without heating the
thing up. We'd typically use a brief 4-probe DC measurement (4 probes so the
leads don't contribute to the resistance), and inject a few or tens of nA, and
read the voltage (in range of uV to mV).

Sometimes we used a lock-in amplifier, which is a pretty ingenius invention.
You input a small AC signal, and then bandpass the output at the same
frequency. That really cuts down on the noise at other frequencies, and lets
you zero in on tiny signals from tiny currents with minimal heating.

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maho
I once wanted to measure the temperature dependence of some electronics
component. Aha, I thought, I don't need no oven or such fancy things, I will
just do my measurement while cranking up the room temperature, which I can
record with a thermocouple!

I wondered why the thermocouple always returned to the same "room temperature"
after half an hour or so, even though I and everyone else began transpiring
heavily. It took me embarrassingly long to realize that the thermocouple only
gives you the temperature _difference_ between the two points of contact (tip
and inside multimeter), and the multimeter took half an hour or so to warm up.

~~~
cnvogel
There must be a reference temperature sensor near the "cold junction", where
the NiCr-Ni thermocouple wires are terminated.

Remark: The external temperature probe is always called "hot", the reference
junction always "cold", even if you measure liquid nitrogen with your
sensor...

If the "cold junction" is just the banana-plugs, then it's obviously not too
easy to get a reading of the temperature there. Better devices will use these
rectangular thermocouple-sockets (a rectangular block with two small,
dissimilar, slots) and have proper NiCr-Ni cabling inside, just up to the
termination point on the PCB, right adjacent to the voltage amplifier/sensor
for the reference temperature.

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ixtli
s/dark sheep/black sheep/g

~~~
pingswept
Or maybe s/dark sheep/dark horse/g ?

In any case, I think the g is superfluous, as "dark sheep" only appears once.

