Morning,

I have been trying to wire a Other Brand 107 thermistor probe into a DT85, the manual (found here: http://s.campbellsci.com/documents/us/manuals/107.pdf) states that the probe is around 100Kohm at 25°C and should be wired as a single ended sensor. With the DT85 only being able to read up to 10 kohm we tried to wire a 10 kohm resistor in parallel to step the resistance down into a readable range but couldn't get it to work. Does anybody have any ideas as to what we could try next (we are unsure if this is a wiring or command issue)?

Thanks in advance.

Good morning b.m.jones,

The parallel resistor value can be calculated from Rp=(10000 x Rmax)/(RMax-10000)

So you need to calculate the resistance of the thermistor at the lowest temperature you need to read and use that as the Rmax value then calculate the Rp value.
Looking at the Temperature resistance curve the resistance at -40 deg C is 4067212 Ohms.

Thus
Rp = (4067212 x 10000)/(4067212 - 10000) = 10,024 Ohms
So yes a 10 kOhm resistor is the correct value.

Looking at the Temperature resistance curves it doesn't match any of the dataTaker standard types so we need to calculate the Steinhart - Hart co-efficients
Picking three points from the Temperature resistance curve the numbers work out to be;

a = 0.000828358
b = 0.000208633
c = 0.000000081

Define a thermistor scaling using the a, b and c values above
Read the resistance of the thermistor using a parallel resistor of the value above.
Calculate the thermistor resistance. Please refer to page 294 of the DT80 manual UM-0085-B7 for an example
Apply the thermistor scaling to calculate the temperature

Cheers,
Roger

Hi Roger,

Thank you for your help thus far; I have followed your advice and seem to be closer to the solution but can't quite get it right, here is the test program:

BEGIN"TEST"

RA5M

T1=8.271111E-4,2.088020E-4,8.059200E-8"K"

3R(3W,=1CV,W)
CALC("R~ohm",=2CV)=(10000*1CV)/(10000-1CV)
2CV(T1,=3CV)
4CV=3CV-273.15

END

LOGON

GA

And receive something similar to:

R 9675.2 ohm
2CV 356.4 K
4CV 83.3 degC

To avoid any confusion, my office is most definitely not 80 degC! This has been lifted mainly from the Thermistor manual and the DT8x manual, however I couldn't get the "&" symbol to work with the DT85 and so replaced it with a channel variable. My thinking is that the scaling is right, as when "2CV=100000" is entered then we get 25.0 back.

The sensor has been wired as:

Black - into
Jumper Cable - From + to
Red - into -
Purple - #
10K resistor - From * to #

Is it possible you have any more ideas where I may be going wrong?

Good morning b.m.jones,

Looks like it's not a straight thermistor.
What series DT80 is it and what version of firmware is it running?

Cheers,
Roger

Good morning Again,

Ok, looking further at the manual this is not just a thermistor element but has two extra resistors (refer fig 6-1 on page 11)

The thermistor also has a 249 k Ohm resistor in series with it so we need to subtract that value from the circuit resistance to find the thermistor resistance.

Place the shunt resistor between the Red (-) and Black (*) wires link # to + and Purple to #
Read the resistance as a 4 wire resistance

BEGIN"TEST"

RA5M

T1=8.271111E-4,2.088020E-4,8.059200E-8"K"

3R(4W,=1CV,W)
2CV=((10000*1CV)/(10000-1CV))-249000
2CV(T1,=3CV)
4CV=3CV-273.15

END

Cheers,
Roger