Experiments with Vref Ovens

[thermistor-guy]

What thermistor do you use?

Do "typical" NTC thermistors drift over time?

As others have noted, class-encapsulated thermistors are more stable than the epoxy versions. BIPM recommends
glass encapsulation:
    https://www.bipm.org/documents/20126/41773843/Guide-SecTh-Thermistor-Thermometry.pdf

NTC thermistors do drift a little when operated at 100 deg. C or so, and accelerates at temperatures above that. The drift
tends to be "thermometric", where the whole resistance/temperature curve shifts.

At operating temperatures below 75 deg. C, I'd expect a glass device to be stable within 1 or 2 mK over 1 year, with a good device
having less than that. From BIPM:

The most stable thermistors are bead thermistors encapsulated in
glass. Within the range –20 °C to 60 °C, selected and pre-aged thermistors may be
stable to better than a few tenths of a millikelvin per year.

I only use glass devices, typically in a DO-35 package. You might use an epoxy device in a cost-sensitive application.

[dietert1]

Yes, i am using glass thermistors with good results. They are pretty expensive now.
In my oven setups the thermistor serves to control the oven heater, providing very low temperature noise.
There is another thermometer for the critical device inside the oven, usually a pn junction (-2.1 mV/K) or a Pt resistor. An outer control loop fine-tunes the oven set temperature to bring the DUT to the desired temperature. Need to take a closer look at the temperature drift data.
The outer oven i made for our ADR1399 evaluation board also has a glass thermistor and it includes a SHT35 temperature/humidity sensor. After nearly a year the SHT35 temperature may have drifted +10 mK but that isn't a measurement, more like an upper limit. That oven isn't really hermetic and there has been a large drift of humidity. When talking about sensor drift, you want both constant temperature and humidity.

Regards, Dieter