How do i calibrate a homemade thermocouple when pressure affects boiling point?

[JustinA]

I’m trying to calibrate a homemade thermocouple using an ice bath and boiling water for a simple experiment, but my boiling point reading is consistently several degrees too low. I suspect my issue might be atmospheric pressure compensation, but I don’t have a barometer to check. Has anyone else run into this when trying to establish a reliable reference point?

[Nora.L]

I've chased this too. In my setup the ice bath wasn't exactly 0 C—depending on how long the ice sat, the amount of water, and the container, it drifted a bit. The boiling reading stayed a few degrees low until I stopped assuming the ice bath point was perfect. With a homemade thermocouple, the cold junction compensation and minor bath drift can magnify small offsets, and altitude around sea level still nudges boiling closer to 99-something C.

[SamuelG]

One snap realization for me was that the ice bath isn't a perfect 0 C reference. The bath can drift as ice melts and the water warms, so the ice point is never a fixed anchor.

[Kyle28]

I also found wiring and junction quality mattered. If the junction is moved or the insulation is loose, the reading wanders by a few degrees.

[HannahIJ]

Maybe I'm chasing the wrong culprit. Are we sure the boiling point is the real problem, or is the issue the curve you’re using for your wire type? I worry the data sheet drift or mixed metals could be doing more harm than pressure.