Hi John,
Thermocouples are very robust, and have a very wide operating range.
However they require extremely accurate voltage measurements to get to
sub degree temperature accuracy. On top of that they require a local
temperature sensor to measure the 'reference' temperature (Or an actual
Icepoint bath). Once you have those that a lookup table or up to 14th or
so order polynomials (Depending on thermocouple, and range) is required
to convert the 'reference temperature' and millivolt reading to the
temperature measured.
Look at some of the NIST copies of the ITS-90 thermocouple tables and
coefficients:
https://srdata.nist.gov/its90/download/download.html
https://srdata.nist.gov/its90/download/allcoeff.tab
Overall, a lot of things going on there with errors that all stack up
(including silly things like not enough range in floating point number
routines for polynomial calculations). Don't get me wrong, they make
great sensors. Probably not the right sensor for a small home brew board.
IC Temp sensors, thermistors, or RTD's may all be reasonable options
here. IC Temp sensors for simplicity, RTD's for accuracy, and
thermistors (except where they are in a loop, holding the temp constant)
for cost.
Dan
On 4/5/2018 12:00 PM, [email protected] wrote:
Message: 11
Date: Thu, 5 Apr 2018 09:44:48 -0500
From: John Green<[email protected]>
To:[email protected]
Subject: Re: [time-nuts] Environmental sensor recommendations.
Message-ID:
<cagrb8tlmgxhowoqsgxpg2c2xe8skxnqk1y9_chkmjwmikec...@mail.gmail.com>
Content-Type: text/plain; charset="UTF-8"
Why has no one mentioned thermocouples?
I had some experience with thermistors a few years back designing thermal
attenuators and equalizers for CATV. NTC thermistors can have a large
change of resistance for a unit change in temperature. They aren't linear,
but there are formulas for computing resistance vs temp. PTC thermistors
have a much smaller change per unit change in temp., but are much more
linear. And, they are susceptible to self heating, which makes things
interesting. If I remember correctly, in my research something called an
RTD was supposed to be the king when it came to accuracy and repeatability.
As someone else has stated, the IC devices are supposed to be quite good,
but you have to interface with them.
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