I need to find a replacement for the defective temperature probe of some device (laboratory incubator), which is basically a NTC thermistor. The only information about the defective unit is the following:

  1. Nominal resistance 2252 Ohm @ 25 deg. C
  2. operating range -50 deg. C to +150 deg. C.
  3. Tolerance 0 to 50 deg. C +/- 0.2%.

This is the only information I was able to get from the manufacturer (the part is obsolete). So, I don't know the beta or Steihart-Hart coefficients for this part. However, I have the new thermistor of another manufacturer with exactly the same parameters and with the known beta. How can I determine whether the new thermistor could be used as a replacement? The device has the possibility of automatic temp. calibration.


If you know the temperature characteristics of the new thermistor, then attach the instrument to a known resistance chosen to be in the operating range of the incubator and confirm that the indicated temperature is correct according to the nominal resistance of the new thermistor at that temperature. It should also read accurately at 25°C if the new thermistor matches the old one.

You can make up the test resistance with individual resistors and/or a pot if you don't have a resistance box on hand. Thermistors change resistance a lot with temperature so you don't even really need precision resistors, just measure the resistance with a reasonably accurate DMM to confirm.

I would be reluctant to invoke this 'automatic temperature calibration' unless I understood exactly what it is using for a reference and what it is adjusting.

  • 1
    \$\begingroup\$ This is smart idea! with varied resistant we can back calculation what beta value that mechine use to calculate temp. \$\endgroup\$ – M lab Oct 20 '20 at 19:47

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