I have a little RC-4 temperature logger which has been a really handy device for ensuring that our freezers are cold enough, but no colder.
This article explores an alternative sensor that could be embedded in equipment of interest. The sensor is a NTC thermistor.
Firstly, I found that none of the 2.5mm TS plugs I had connected to the RC4 properly, but the T-R of a TRS plug worked reliably.
The display was observed with two 0.1% precision resistors and the thermistor characterised.
Above, it looks like nominally a 110k/B=4200 thermistor… which is a little unusual. 100k thermistors with B=3950 and 4200 are fairly easy to obtain though.
Above is a chart of the error in using the two commonly available thermistors. At low temperatures the 100k/4200 isn't too bad, around 60° the 100k/3950 is better.
It is a simple matter in Excel to correct readings made with a different thermistor. Here is a VBA function to perform the conversion.
Function temp2temp(ft, ft0, fr0, fb, tt0, tr0, tb) 'function to correct temperature reading to a different thermistor r = Exp((1 / (ft + 273.15) - 1 / (ft0 + 273.15)) * fb) * fr0 temp2temp = 1 / (1 / (tt0 + 273.15) + Log(r / tr0) / tb) - 273.15 End Function
You could squeeze all this into a cell formula if you wished to avoid using a VBA function.
Above is an example measurement run made with a 100k/3950 sensor (Indicated) and the corrected data in brown.