Electronics > Metrology

Pt100 sensor precision / accuracy question , batch sample selection .

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Overspeed:
Hello

I have a question regarding Pt100 as the cost vary from few Euro / USD to $$$$$$

Does the Pt100 are sorted to select the best one in a batch ( linearity ?) or does there is other requirements ? I am not an expert in variable resistor

My goal is to built some '' accurate '' probes ( I have no problem with the mechanical parts ) 4 wires to connect to my ASL thermometers but also to my HP 34970A data loger

I have equipment as a triple point water , calibration furnace , calibrator ...

I plan to sort some for temp from 0 to 100 C

Regards
OS

EC8010:
Yes, PT100 vary from affordable to expensive. I've used them a lot. Measuring temperature is much harder than it seems because there's always a thermal resistance between the part whose temperature you want to measure and your sensor. Combined with the thermal mass of your sensor, that makes things tricky. I use the smallest PT100 I've been able to find from Farnell: 219-1840 and I glue them with a thin film of epoxy to whatever I need to measure, so they have to be regarded as disposable. I think the manufacturing tolerance is +/-1C, which is often good enough. Usually, what is required is repeatability and resolution. But it sounds as though you're capable of calibrating your probes if necessary.

You can buy all sorts of expensive probes and they all have higher thermal mass, increasing errors. They often add thermal resistance between the Device Under Test (DUT) and their PT100. I eventually considered them to be a waste of money compared to little sensors glued to the DUT. If you're measuring the temperature of a DUT that is exposed to the environment, you may want to make a little foam house over the PT100 to prevent a thermal potential divider forming from the thermal resistance from DUT to PT100 and from PT100 to environment.

You can look up the equations for determining temperature from PT100 resistance, but many 6 digit DMMs will implement them for you. All they need to know is the PT100's resistance at 0C. Adherence to the equations is assumed.

edpalmer42:
Decent quality PT100 probes will specify the accuracy.  I've seen reference to simple +- degree C, math formulas, DIN standards, and Class designations (which may refer back to DIN - not sure).  As expected, the tighter the tolerance, the higher the cost.

Just be aware that PT100 probes are not interchangeable.  They come with different temperature coefficients (generally called 'alpha' values).  If your probe has a different alpha value from your meter, your measurements will be total garbage.  Your meter should specify the alpha value it's expecting.  A common value is 385 ppm/K aka .000385, etc.

Ed

MK:
Another gotcha with PT100 probes is that the fine wires involved can slip at grain boundaries when shocked/dropped, so the sensor undergoes an unknown amount of change to its resistance for every shock.

Overspeed:

--- Quote from: EC8010 on June 13, 2024, 06:06:16 pm ---Yes, PT100 vary from affordable to expensive. I've used them a lot. Measuring temperature is much harder than it seems because there's always a thermal resistance between the part whose temperature you want to measure and your sensor. Combined with the thermal mass of your sensor, that makes things tricky. I use the smallest PT100 I've been able to find from Farnell: 219-1840 and I glue them with a thin film of epoxy to whatever I need to measure, so they have to be regarded as disposable. I think the manufacturing tolerance is +/-1C, which is often good enough. Usually, what is required is repeatability and resolution. But it sounds as though you're capable of calibrating your probes if necessary.

You can buy all sorts of expensive probes and they all have higher thermal mass, increasing errors. They often add thermal resistance between the Device Under Test (DUT) and their PT100. I eventually considered them to be a waste of money compared to little sensors glued to the DUT. If you're measuring the temperature of a DUT that is exposed to the environment, you may want to make a little foam house over the PT100 to prevent a thermal potential divider forming from the thermal resistance from DUT to PT100 and from PT100 to environment.

You can look up the equations for determining temperature from PT100 resistance, but many 6 digit DMMs will implement them for you. All they need to know is the PT100's resistance at 0C. Adherence to the equations is assumed.

--- End quote ---

Hello

Thanks for your answer
I work on a differential measure system as I need a high precision delta temp measurement ( in the 0.01 Dg C ) between two parts
First problem the rigidity as a P100 is a ''foil '' and if there is strain that generate resistance variation
Connection need to be perfect as possible some supplier use Cu Ag wire

For reading / system I have no real problem I am more interested to know what procedure to use as ?:
I ''cook '' them at least at the maximum temp I plan to use , so 10 Deg C for 100 max reading and pass over Indium melting point
I ''cook '' them with a 2 or 5 mA current ?? so over the 1 mA of the measuring circuit
I install them without strain
I install them with a proven method as same soldering same wire same contacts ...
I run calibration at 0 Deg C as I have a water triple point
I run calibration by comparison at 100 Deg C ( I have no Indium triple point )
I try to trace a linearization curve
.......

Let me know
Regards
OS

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