Answer Posted / carl_ellis
Question: What is output of Thermocouple at 25 deg C?
I'd flunk all of you except B0nz0 because only B0nz0
understands thermocouple thermometry.
The answer is, you don't know because the cold junction
temperature is not specified. The question attempts to
find out if the concepts of a thermal gradient and cold
junction compensation are understood. Which is why, I
assume, the question does not specify the type thermocouple.
B0nz0 is correct; any type thermocouple sitting on a desk
at 25 deg C; hot end = 25 Deg C, cold end = 25 Deg C, will
read 0.0mV because there is no thermal gradient.
The cold junction temperature is the same as the hot end
temperature. Doesn't matter if it's a Type K or J or S.
With the same temperature at both ends, the output is 0mV.
But without specifying what the cold end temperature is, an
answer of 0mV assumes that the cold end is at 25 Deg C.
In the real world, it usually isn't.
The hot end can be at 25 Deg C with the cold end on a
terminal strip in control panel that's at 45 Deg C because
of the heat generated by the electronics in the control
panel. Then you'll read a negative voltage because the
cold end is hotter than the hot end!
The hot end can be at 25 Deg C and the cold end is
connected to handheld temperature calibrator in northern
Canada in the winter where the cold end terminal connection
might be at -30 Deg C.
In each case, it's the cold junction compensation that
corrects the temperature reading by measuring the
temperature of the cold end connection and taking that
temperature into account.
If I were to ask the question, I'd ask, "What is the output
of a thermocouple, both ends of which are at 25 deg C, and
why?". The answer to the first part is 0mV.
Tell 'em Carl sent you.
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