What do you pay (approx.) for a platimun RTD? And where do you get
them.
The thin film ones are cheap and plentiful these days- even Digikey
has them for a couple dollars. They don't have much hysteresis (their
big failing) if you keep them around room temperature. Wirewound ones
are relatively expensive (and slow).
What about a 'regular' ~10k thermistor?
(Yeah I know, the 1 degree ones cost more.)
The slope is more important than the absolute accuracy, because you
want to avoid having to test each instance of the product in a thermal
chamber- too slow and expensive. Platinum RTDs are pretty good.
Cheapie "accurate" thermistors seem to be trimmed for the zero, but
have rather poorly controlled slope.
Or just a diode, the slope is (pretty much) known it's the 'offset'
that is different piece to piece. So maybe a single point 'zero'
where you stick the TC onto the cold junction would work.
George H.
Same thing applies to the RTD, in spades. Just zero it out in an ice
bath and Robert's your uncle. You don't need really high precision
parts.
If you really want to use a junction, a transistor-connected diode is
better than a diode, and transistor-connected diode measured at two or
three currents is even better (but the signal level goes down, so the
measurements have to be better).
For most food and paint baking and most plastics machinery, few care
about a few degrees C. They're good enough, and the whole BOM
including power supply, case and output stage has to come in at less
than those boutique chips cost. It's hard to make the device that
isothermal without adding cost and subtracting ease of replacement and
multi-sensor type ability, so the accuracy of measurement is a moot
point in many cases.
We used a small coil of copper wire as well as special resistors as
RTDs. Worked pretty well. Platinum for the rich customers. These days,
they just have to be well-heeled.