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Calculating RTD temperatures vs internet calculators

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BetaTripp

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Hi guys,

So, I'm trying to work out how to accurately calculate the RTD temperature of a given resistance with a decent number of decimal places.

I have done the maths and calculated the temperature coefficient and so on but don't get a result that matches up with what I expect to satisfy a set specification. The maths doesn't match up with the internet calculators and makes it so my readings look to be way out. Either my calculations are wrong or the item I am calibrating is out of spec. I can't imagine it would be as it is a Beamex calibrator and they're usually pretty spot on. I expected the maths to be more accurate but I am having doubts in how to apply it to a real world application.

The calculations I did were using the formulas

1613896427271.png


And

1613896455808.png


Where equation 1 is calculating the coefficient using the RTD resistance at 0 degs and 100 degs and 2 the resistance temperature.

The result of these calculations do not match up with the internet calcs which is basically what I'm saying.

Thanks

BetaT
 
There are both different types with different temperature coefficients, and the TC is not absolutely linear, with most types - the simple equations give reasonable approximations only.

This seems to be a good page for overall info on them:

Edit - and this covers the calculations needed (transfer functions) get precise measurements from raw values:
 
The internet is always right!

I;ve seen these https://www.thermosensors.com/rtd/definitions_2/ calcs used/

Your using an RTD and not a thermister. They are different.

here is https://web.mst.edu/~cottrell/ME240/Resources/Temperature/RTD table.pdf an RTD curve,

I tried to match up some thermisters and I did pretty well. Those tables use R at 25C

Remember that RTD's will/do require 4-wire type measurements or a special 3 wire technique.

You;ll need to use a kelvin technique (voltage across, current through) to get your resistance use a 4-wire ohmmeter.
 
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