Thermistor VS spring thermometers, Anyone know anything about digital Thermostats? Accuracy? Lux brand? @%#

[Nefrodite]

i'm itching to hook it up, but this is bugging me. I've been trying to replace the cheesy old round one we have. bought two lux brand digital programmable ones, each reads about 5-8 degrees higher then both my old honeywell thermostat and a separate themometer. Whats up with that? i'm not sure how the digital one measures temperature, probably thermistor, but the old ones i have have that spring thing, which is more accurate? i hope someone on here knows, cuz web searches definetly yielded nothing.

 

[Superdoopercooper]

I'm no expert on this matter, but it just seems like you have a calibration problem.

So... the problem probably is this: the new digital ones use a Thermistor (or even a temperature diode)... Unless there is some sort of calibration done at the factory... any two given Thermometers could be X degrees apart. And, it may just happen that your separate digital thermometer and your Thermostat match up very well. I'm sure there is process variation in those too (although, for some reason, I bet those are either just inherently more accurate, or they are more likely to be calibrated at the factory because its easier to calibrate a mechanical assembly).

Therefore, I think you really don't have a good avenue for comparing apples to apples regarding the temp in your house. What you need to do is:

1) forget the temperatures of old
2) install the new thermostats, and adjust them until the temp in the room feels like what it used to. This new setting may be 5 or 8 degrees off of your old one.

See... exact temp really doesn't matter... it's just relativity that matters. If your old thermostate was comfortable at 72 degrees... fine. As long as you when you move it to 74, it actually causes the temp in the room to go up ~2 degrees. Same thing with the new digital thermostate... if 77 is now the comfortable temp... who cares... as long as the temp still changes by ~2 degrees when you change it to 79.

Since most people rarely/never change thermostats... I'm sure manufacturers were never really that concerned with ABSOLUTE accuracy. They just cared about what I wrote about in the above paragraph.

If you are really concerned about it and/or you feel like tinkering with the thing... I'm sure there is a way (probably fairly easy) of adding circuitry that will give you a more exact reading of the TRUE temp in the room. You'd need a calibrated or accurated temperature transistor/diode/thermistor... for a reference... and you could replace the one in the thermostat with it... as long as the circuitry that is already there will support a change like that. Depending on the component used, it may take some tinkering with the circuitry to get the measurement to work correctly (example... replacing the factory thermistor with a diode... diodes have different electrical characteristics, so you'd need to modify the circuit to support that).

The thermistors the factory puts in are CHEAP. That is the only way they can make those things somehwat affordable while keeping their profit margins up.

If you have a circuit diagram of the thermostat, I could help you figure out how to make it more accurate, if you wish. My recommendation though is to just blow off what temperature NUMBERS you are used to, and go with the new numbers on the new piece of equipment.

whew... lots of words. I'm too verbose sometimes. Hope this helps.

 

[Demon-Xanth]

I agree, for that application the exact value isn't required. A thermistor is only as accurate as the support circuitry. (as is thermocouples and RTDs). The spring thermometer probably is useless for accuracy anyways.

 

[Nefrodite]

thanks guys guess i'll keep the darn thing after all