G'Day,

I have an indoor/outdoor thermometer which is no longer working, the probe however is fine. I was wondering if it would be possible to connect this probe up to my DT80s2 & get temperature readings from this sensor? If so, how might I go about connecting this to my logger & code?

I have tried many things from the manual in the 2-wire RTD section, so far without success.

Thanks,
Grant

Good morning Grant,

You need to know the type of temperature sensor it is and how it works.
With a multimeter try reading the resistance of the two wires.

If the resistance is very low, just a few Ohms, then it is probably a thermocouple.
If the resistance is around 100 Ohms then it might be a RTD, but RTD's are generally not 2 wire but more usually 3 or 4 wire.
If the resistance is over 100 Ohms then it is probably a Thermistor.

Once you work out what group of sensors it belongs to you then need to work out the exact type.\
This will require you to do a calibration then fit the measured results to the sensor type.

Cheers,
Roger

G'Day Roger,

The resistance of the two wires on my multimeter jumps around quite a bit, but when it settles it reads 32.3kΩ. Does that sound about right?

If it's any help, the sensor is from an earlier version of this type of thermometer.

Cheers,
Grant

Good afternoon Grant,

Then is is most likely a thermistor.
Looks to be a H type with a resistance of 30 kOhms at 25 deg C
A resistance of 32 kOhms would put your room temperature at about 23 deg C which also means it an NTC (Negative temperature curve.)

This doesn't fall into the predefined dataTaker thermistor types by using a parallel resistor and applying the Steinhart-Hart coefficient in the thermistor scaling type we should be able to read it.

We can use this as an exercise in how to read non-standard thermistors so the first thing we need to know is what is the lowest temperature do you want to measure?

Cheers,
Roger

G'Day Roger,

I intend to place the sensor inside my industrial enclosure (where my logger is located - in an outdoor, full sun location). I would expect a temperature range of roughly 0-55°C. The temperatures logged using my logger have so far ranged from 5-54°C (over about the last 12 months).

The temperature range on the sensor's box states -50 - 70°C.

Cheers,
Grant

Good morning Grant,

I am going to make some assumptions here just to get us started.
At this stage we are just trying to get a resistance reading from the sensor, then we can try a reading a few resistances at different temperatures and refine the calculations.

IF the Thermistor is from MEAS then it might be a 44108 http://www.meas-spec.com/downloads/44108.pdf
Looking at the data sheet we see the resistance at 0 deg C is 94,980 Ohms. This is much greater than the maximum 10,000 Ohms the DT80 can read so we need to use a parallel resistor

The calculation for a parallel resistor is on page 295 of the DT80 manual (UM-0085-B7)

Rp = (10000 x 94980)/(94980 - 10000) = 11,176 Ohms
So that would be a 12 KOhm resistor of 0.0% or better.

Wire this up as per Figure R6 on page 294 and use the calculation shown to return the resistance value of the Thermistor.
When we can read resistances we can then look at a more exact temperature resistance curve and calculate the Steinhart-Hart coefficients

Cheers,
Roger