Thermistor question

[Bethw]

Possibly simple question: I have a bunch of 10k @ 77f type 2 thermistors for hvac duct sensing. Some of these are 20 yrs old. Some appear to be not reading correctly. If I go to the controller input, and put a 10k resister across the input, I should get 77F reading on the building system readout, correct? or?. The inputs are configured for 10k type 2. What I am seeing is 52F, 46F, and other numbers when the 10k resister is connected. I have found only 1 that reads 77F. I think all thermistors, type 1,2 etc should read 77F@ 10k, correct?

 

[Martaine2005]

No, at 77 degrees F the thermistor will /should read 10K ohms.
According to the the 10K-2 Thermistor output table, at 77 degrees F or 25 degrees C should see 10K ohms.
But if the Thermistors are NOT at 77 degrees, obviously the reading will differ.
Placing a 10K resistor across the thermistor will also change the reading.
Perhaps try heating your old thermistors with a hairdryer and trying them again in the control unit. They may have just drifted out of tolerence after 20 years or the the control unit needs calibrating or both.

Martin

 

[Bethw]

Thank you. I guess the way I stated the problem may have hazed over the actual question I need answered...question is: if I put a 10k resistor (all by itself) across the input of the controller that is configured for a 10k type 2, will I see a 77 deg. readout? (I am not).

 

[Martaine2005]

I have no idea what controller it is or what it does for that matter.
But IF it SHOULD read 77f and it DOESN'T, then there is clearly a problem with the controller side.
Why not tell us all the info of the controller so that someone here, who has time to read up on it can help you.

Martin

 

[Bethw]

These are all Distech controllers. Open source software. Nothing unusual here. Inputs are for supply air, return air, mixed, etc. my biggest issue is that the sensors, which were installed with the controllers years ago are now being re commissioned. Some are not reading at all (sensors or inputs ? one of the reasons for this thread). I started to check the inputs with a 10k 1% resister, and would get readings of 47, 56, etc. It wouldn’t seem that it could be off that much. When checking the resistance of the thermistor in place, it “seems” close to what I would expect.

 

[hevans1944]

Clearly the so-called controllers are (1) not working properly or (2) seriously un-calibrated or (3) both of these. Why not scrap the controllers and start afresh with something off-the-shelf that actually works and has a manufacturer's warranty? Surely your time is more valuable than wasting it finding out that a 10 kΩ 1% tolerance resistor produces many different and unrelated "temperature readings" when substituted for a thermistor? How many controllers are you responsible for making them work again? What would it cost to replace all those that fail your 10 kΩ thermistor simulation test? Is there a calibration procedure to bring the faulty readings back to a nominal 77 F "temperature" calibration? Quit spinning your wheels with this and move on to solving real problems. Don't confuse the issue by paralleling your "calibration" resistor with a thermistor. Use just the "calibration" resistor as a go/no-go test of each controller.