Trying to understand this temperature sensor.

[jerryk]

I have a temperature sensor of a sauna that I want to read via MCP3008 and a raspberry pi.  I am already using 4 LMT86 sensors on the MCP3008 with no issues.  The sauna sensor consists of two unknown type ntc thermistors in parallel as shown in the attached photo.

Temperature/resistance  observations are as follows:

Temp C    Bead    Glass
 7             71K     18K
25            53K     10K
50            31K       6K

I have tried to understand how to use Steinhart Hart to figure this out but I'm a little bit stuck on the equation itself and how the two thermistors are being read by the existing microcontroller.

Edit:  added schematic to show the temp sensor connections to the MC.

Any help or direction would be appreciated.

Jerry

[alexig]

Hi Jerry,

I'm thinking that they might be using 2 different types of thermistors to average the results as one could be suited better for lower temperatures and the other for higher temperatures.

There's a nice site that will calculate the co-efficients for you using the 3 data points you have provided - http://www.thinksrs.com/downloads/programs/Therm%20Calc/NTCCalibrator/NTCcalculator.htm

And then it's just a matter of using the formula (https://www.ametherm.com/thermistor/ntc-thermistors-steinhart-and-hart-equation) and comparing both thermistors to see which one is better suited.

[Wimberleytech]

It appears that one thermister bows downward and the other bows upward.  If you plot the average of the two voltage-divided outputs you get something closer to linear (it appears from just a few data points).
The top graph is the average of the two lower graphs.