Unknown Temperature Probe

[c-swanholm]

I’ve learned some more about my thermistor after examining it more closely. I have a high resistance probe. At room temperature it measures about 28Mohm. I believe it has some capacitive and inductive properties as well which is making it tricky.

I was trying to build a simple voltage divider to measure the voltage drop across a resistor as the resistance of the temperature probe changed and I haven’t been getting any good results.

It’s called a general purpose probe but it doesn’t seem to me to be very simple and definitely not purely resistive. I've been supplying it with a DC voltage and it's creating AC voltage. Furthermore my "voltage drop" i was trying to create was actually increasing the voltage. I was using an equal sized resistor (28Mohm) and a 15VDC source and the voltage across my fixed resistor was 18V... 

http://www.analog.com/media/en/technical-documentation/data-sheets/AC2626.pdf

Do you have ideas how I can measure temperature with this probe using LabVIEW?

I've tried measuring a current output from the probe, I've tried configuring it as a thermocouple and some of the various other options in LabVIEW but i'm new to the software so it's very possible i didn't set it up correctly. 

[GerdW]

Hi c-swanholm,

did you read the PDF you linked in your question?

Did you notice the electronics sketch on the last page? Why don't you replicate this in your lab?

The specsheet clearly says your probe is a current source!

[c-swanholm]

Yeah I kinda tried that… I don’t have an appropriate battery to use. i think ill have to make a voltage divider for that too

[GerdW]

Hi c-swanholm,

all you need is a power supply in the range of +4 to +30V as is written on page one of your spec sheet.

In my lab we have 5V, 12V, 24V as standard power supplies and even some weird 9V or 19V (from old laptop) ones…

What kind of lab do you use?

[c-swanholm]

yeah i have a powersupply im using its not an adjustable one so im making another voltage divider to make the other source

[c-swanholm]

ive got my circuit made...

The diagram is looking for an output voltage with reference to 273mV....

so my next question is what should i expect for results.

it gives me a conversion of 1uA / deg C but im now looking at voltage not current

im measuring voltage with labview and the probe doesn't really respond to changes in temperature but it does respond to being touched.

[c-swanholm]

Heres where im at with this... see attached.

not getting any change in voltage when the temperture changes.

I thought these probes were supposed to plug right into your DAQ modules and read temperature

[c-swanholm]

the 394ohm resistor should be more like 408ohm

[GerdW]

Hi swanholm,

why is your electronics different to the spec sheet? Why do you need 4 resistors instead of just 2? Why do you draw the probe as resistor - it is a current source (as is also shown in the spec sheet)!

Please give the recommended electronics a try!

[c-swanholm]

because i dont have exact value resistors so i used a couple rheostats to compensate. The probe is in series with the other resistors so whats the difference what i drew it as in the picture. I made an equivilent circuit yes? I don't have a power supply that can make a 273.3mV signal so i used a voltage divider does that make sense?