Thermistor Resting Value Incorrect

[aodhan]

My thermistors read 27C when at room temperature. It's currently 62F = 16.7C in my house... This has to be throwing off all of my temperatures that I'm printing at... Anyone have any information on how I go about calibrating this?

Thanks,

-Matt

[Xenocrates]

So, Thermistors aren't particularly linear. If adjusted to be nearly correct at room temperatures, the beta value throws it way off at printing temps, as there's only so much of a basket where they are accurate (Using the equipment that is available at reasonable prices). If you insist on perfectly accurate temps, I suggest a PT100 sensor (although, again, it can be inaccurate at some values, due to the smallish relative resistance at 0C (100 Ohms, to be precise), meaning signal wiring matters more than a 100K thermistor).

My suggestion if it bugs you. Get a Duet board, as they have high and low end adjustment factors for the beta value, to allow you to tune it to read reasonably well at room at printing temps.

[dc42]

Even better, get a Duet WiFi/Duet Ethernet (OK, I am biased) with the PT100 daughter board AND a PT100 sensor, and use a 4-wire connection between the PT100 and the daughter board. See https://miscsolutions.wordpress.com/201 ... d-silence/. The use of a 4-wire connection eliminates the effect of resistance in the connecting wires.

[aodhan]

Thanks for the input. I am using a handheld gun-style thermometer to read the temperatures off of my bed and I've never seen it to be accurate. I suppose this could be caused by trying to read through the clear glass, but if the temperature I'm seeing off of the thermometer is accurate I then wonder how I calibrate at the printing temps I'm using. I'm expecting there to be two different calibration values for the two different thermistor locations since the bed should be calibrated around 80C and the head should be calibrated around 240C for my ABS prints.

Thanks again.

[dc42]

It's not uncommon to see large temperature errors from thermistors at room temperature in 3D printers. Assuming you get about the same error on both thermistors, it is caused by ADC gain error. In RepRapFirmware we provide a parameter to tune it out.

The good news is that the temperature reading error from this cause will decrease at higher temperatures. Your bed thermistor reading is probably only a few degrees out, although if you are using a glass bed plate then there will be a temperature drop of 5 to 10C across the glass. The hot end reading will be less accurate because of thermistor tolerance and ADC offset error.

The reality is that you need to find what temperature readings work for you.

[geneb]

IR thermometers are pretty ineffective in these cases. Use a thermocouple.

g.