> > I don't know if thermistors will survive at 250C (No experience). I know
> > RTD's will go that high - but they are usually a pain to
> interface. I like
> > thermocouples myself.
>
> Why are rtd's hard to interface ? It's just a handy valued resistor (100
> ohms at 25C is common). All you need to read it is an ohmmeter
> and a table
> (supplied). Your trusty dvm will work fine if you don't need wire
> compensation, and unlike thermocouples they do not need special chips and
> outstanding high temperature insulated wiring skills (you can use the
> chassis/oven wall etc as return).
>

Hello Peter,

I only meant that RTDs are a pain to interface in comparison to a 1-chip
solution. Recall the original poster was looking for a solution that was
easy to interface to a PIC. I have interfaced both RTD's and thermocouples
in industrial products. I have used the MAX6675 - it does the job and is
only 1 part between the thermocouple and the processor (PIC in my case).
When using RTD's we always end up using many more parts (more board space,
more costly to assemble, more opportunities for assembly mistakes). For me
the MAX6675 is an elegant solution.

If you are aware of a simple way (1 chip) to interface RTD's to the SPI bus
(including open/shorted RTD indication) please tell me what it is. I am in
the design phase right now of another product which has both RTD and K
thermocouple interfaces (customer picks which one they want to use). Right
now the RTD interface we use has op-amps and generates a voltage that is
read on a analog input of the PIC. Not very elegant if you ask me... but it
works. If you know of a better way (less parts, direct digital to SPI etc.)
lay it on me - I'm all ears.

>
> Were the wing temperature readings on the doomed shuttle coming from
> thermocouples ? They went off scale *low* as the wing exceeded
> temperature
> probably cooking the wiring and shorting the signals.
>

Don't know. Interesting to think about though. Maybe they use thermocouples.
However, I don't think that shorting together thermocouple wires will cause
a low reading. Maybe if the wires shorted to something else funny things
will happen. But if you short the two wires of a thermocouple together you
just get another thermocouple which measures the temperature at the point of
the short - don't you?

-Mark

Doesn't matter what your business card says... We're all in the results
business.


> Peter
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