# Calculating RTD temperatures vs internet calculators

#### BetaTripp

##### New Member
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 And 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

#### rjenkinsgb

##### Well-Known Member
Most Helpful Member
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:

#### KeepItSimpleStupid

##### Well-Known Member
Most Helpful Member
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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