Thermistor Tolerance

Dan_15

I have noticed that my Buderus ODR sensor consistently reports about 4-5 degrees higher than a thermometer placed in the same location. Buderus suggested that the tolerance on these thermistors can be +/- 5 degrees (!) which is surprising to me. They also suggest that wires can sometimes be the cause of the problem, impeding resistence, but frankly mine seem just fine--no staple punctures or anything out of the ordinary. I do not have the equipment to reliably measure resistence of the thermistor and wiring for comparison against the documentation. Have any of you sticklers for accuracy ever encountered this issue with Buderus ODR sensor, and if so, what has been your workaround solution? Have you tried replacing the thermistor or the wiring, or shifting the reset curve to accomodate? Thanks, Dan

jp_2

add a trimming pot

a small potentiometer could be added in series if the thermistors registering too low of resistance.

Dan_15

That's a good idea. I'm not sure why Buderus doesn't incorporate into the control a way to calibrate the thermistors, given that these things have such a wide margin of error. Is this true of all thermistors used in ODR sensors and digital thermostats, or do I just have a bad one?

jp_2

band-aid fix

its a band-aid fix. the internal software should be able to correct this type of small error. I guess its somewhere in the software and few people know it.

tighter tolerance thermistors cost more

Jay_14

less obvious fixes

Most sensor manufacturers have tight tolerances on temperature sensors. I would doubt that the 5 to 6 degree F discrepancy is the actual fault of the thermistor bead.

If the sensor is reading incorrectly there are many things one can do to correct this problem:

1)Use shielded cable. Any high voltage wires in parallel to the sensor wire can induce a voltage. The control on the end of the wire is measuring this voltage and event small changes can throw off the sensor reading by a few degrees.

2) Try insulating behind the outdoor sensor. Whenever the temperature outside is colder than inside, there is always heat transfer even if the walls are insulated.

3) Look for sources of heat such as windows, doors, vents.

4) Wire size and length of wire. If the wire was very small gage and was very long, then the wire's resistance starts to affect the resistance reading.

5) Look for moisture. Moisture can cause wires to corrode and the corrosion can add resistance to the reading.

- Jay