Electronic – self-heating effect of dual PT100 temperature probe in a moving mass of water

analogheatpt100rtd

I'm trying to know how much self-heating should I expect in these conditions:

  • Temperature range: 0-45 C
  • Using a 1050Ohm pullup resistor to feed each PT100 (2-wire configuration) with 5 volt excitation and 5 volt reference.
  • Temperature probe with two PT100 inside
  • Temperature probe will be inserted in a pipe with a moving mass of water using a thermowhell.

According with what I've read in several sites, including this texas instruments reference the self-heating coefficient should be 0.01-0.02 C/mW for a moving mass of water.

with a 5V excitation, 1050 Ohm resistor and max value of 117.47Ohm of the PT100 at 45C I get 2.1546mW so the temperature would raise the negligible amount of 0.043 mK using 0.02 C/mW coefficient. Am I right in my calculations? Also, since both PT100 are fitted in the same probe, may they affect each other in terms of heat in a significant way?

EDIT: Andy corrected my calculation error and introduced the water SH as a variable but Spehro answered my second cuestion and provided more practical info./tips :

  • Increased thermal resistance from poor thermowell coupling.
  • Self heating coefficient being variable depending on material,construction.

Best Answer

You would be better off to get actual self-heating numbers from the probe manufacturer (or make measurements yourself). The coefficient can vary over a range of 10:1 depending on RTD element and probe construction (thin film vs. wound and epoxy vs. alumina packing, for example). The probe material will affect it too. TI is a semiconductor maker, not an expert in probe design. If you have a separate probe assembly loose inside a thermowell, it will be worse again! At least you have flowing water which is the next best thing to flowing molten metal.

I would consider 5mA to be an excessive current for a Pt100, usually we use 1mA, so you are getting 25x as much self heating. That's a pretty heavy price to pay for a less professional design of signal conditioner if you care about accuracy.

Most certainly the two elements will interact, again it will depend on the exact construction.