[time-nuts] Environmental sensor recommendations.

Dan Kemppainen dan at irtelemetrics.com
Thu Apr 5 13:42:12 EDT 2018


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, time-nuts-request at febo.com wrote:
> Message: 11
> Date: Thu, 5 Apr 2018 09:44:48 -0500
> From: John Green<wpxs472 at gmail.com>
> To:time-nuts at febo.com
> Subject: Re: [time-nuts] Environmental sensor recommendations.
> Message-ID:
> 	<CAGRb8tLmGXhOWOqSgxpG2c2xE8sKXnqk1y9_cHkmjwMiKEc4tg at 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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