Temperature probes PT100 / K type accuracy/calibration

[cowasaki]

I have a couple of cheap Chinese stand alone temperature measuring devices using K types, a K type on my uni-t 60 DMM and I've just bought a 3 wire PT100 which I've wired up as four wires and connected to my Keighley 6100.....

The PT100 reads 21 at room temp & 35 against skin (both about right) but 190 against my soldering iron tip which is supposed to be at 290.

The two Chinese stand alone are about 5% out from each other.

They all read different values.  How do people check them? How can you calibrate them?

I could try for 100 degrees using the kettle but not many other repeatable temps.

[Kleinstein]

The soldering iron temperature is not at all an accurate point. Here it really depends on the thermal contact. +-10 K is considered good for a soldering iron.

Boiling water is a much better reference point.  Meting ice is another.  For the 35-45 C range there are rather good medical thermometers.

The meting point of pure zinc is another (even official) test point - though not really easy to use.

Pt100 sensors often only need a 1 or 2 point check.

[cowasaki]

I'm going to bring the Keithley and the cheapo Chinese K type thermometers into the kitchen tomorrow and check the 100C point with all three.  I wanted the PT100 as accurate as possible as I wanted to use that to effectively calibrate some thermometers for home automation etc....