From: "Spehro Pefhany"
Subject: Re: Reference Junction for Thermocouple
User-Agent: tin/pre-1.4-19990216 ("Styrofoam") (UNIX) (Linux/2.2.14 (i586))
Date: Wed, 30 Oct 2002 13:07:02 GMT
NNTP-Posting-Date: Wed, 30 Oct 2002 08:07:02 EST
The renowned John Larkin wrote:
> Our NMR temperature controller (the one I showed you)
(and a nice design it is, both in absolute terms and for value)
> just has
> soldered TC-to-copper (pcb trace) junctions with a nearby LM45
> temperature sensor. The uP ADC/mux digitizes one side of the
> thermocouple, then the other, then the LM45 ambient temp. The LM45
> signal gets converted into temperature, then is run 'backwards'
> through a temperature-to-voltage type 'T' lookup table, the result of
> which is the reference junction compensation voltage. That is added to
> (or subtracted from, can't remember, too much Pinot Grigio and cognac)
> the TC difference voltage. That final, compensated voltage is run
> through a 'foreward' lookup table with interpolation to give the
> actual temperature. Add a little software lowpass filtering and stuff
> and you get an impressively impressive hi-res temperature display.
Yes, the way it should be done. A simple LUT often isn't in the cards in
a resource-starved environment like the ones I have to deal with (BOM in
the $10-$20 range) for example, especially where many T/C types must be
Given speed to burn on the compensator, the reverse function can be found
with a forward function and a bit of searching (usually speed isn't an
issue with the compensator), of course. The acid test is when you allow
for different types of T/Cs and the reading doesn't change when you change
T/C type, by even a few microvolts worth of temperature.
> All in assembly, of course.
Of course! ;-)
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