I've got an NTC thermistor. This one, to be exact. It has a table of resistances corresponding to temperatures. I've used it to derive a Steinhart-Hart equation using the NTC's resistances at 0°C, 20°C, 40°C, and 60°C. I've used the equation and calculator on this website to do so.
When I measure the temperature of the air, it works great. PT1000 thermometer tells me 21.1°C, thermistor tells me 21.1°c. But when I put it under my tongue, it tells me 34.4°C, while the thermometer tells me 36.4°C. A whole 2°C difference!
When I put the thermistor in a cup of water with the thermometer, the thermometer tells me 20°C. The thermistor, 19°C.
I've tried using a different set of coefficients corresponding to resistances at 0°C, 35°C, 70°C, and 110°C, but it makes no difference. I was about to type in the whole table from 0°C to 55°C, but something tells me that's not the way.
I've read some of the posts regarding NTC's on this forum, and came across thermal dissipation constant. My thermistor has a dissipation constant of 1mW/°C. However, for all temperatures of interest to me, 0-55°C, the wattage through the thermistor will be under 0.3mW.
I need it for load cell temperature compensation. I will be applying thermal paste to it and covering it with Kapton tape.
I did get the thermometer off eBay second-hand, but it's been calibrated this year, according to its label, so I trust it. It reports the temperature of ice-water correctly, anyways, and I need to assume I have one correct temperature reading. The thermometer: link.
I don't doubt my ADC, as it's an ADS1115. However, when I try to measure the input voltage (3.3V) on another channel at the same time as the thermistor voltage, the thermistor reading changes. Weird. I suspect it's my code, but I'm leaving it alone for now. 3.3V or 3.27V won't account for the difference I'm seeing, I've tried.