First time posting here.
An interesting discussion of temperature measurement. I would like to make a couple of points more clear.
1. Thermocouples do not "measure temperature." A thermocouple produces a voltage proportional to the DIFFERENCE in temperature between the junction and the open end. The open end of the thermocouple should be connected to an isothermal junction block. That keeps the ends isolated from each other, and at a known and stable temperature. The temperature of the junction block is measured by a different method, like a thermistor or a platinum thermometer. That way the measuring circuit can determine the temperature difference.
The greater the difference between the measuring junction and the isothermal block, the better the measurement will be.
In a laboratory situation the isothermal block may be replaced by a thermocouple held at the triple point point of water (0.01 °C). This point is one of the 14 fixed point references of the International Temperature Scale, and certainly the easiest for most people to achieve.
(The triple point of water is the temperature at which water can exist as a gas, liquid and solid.)
2. A high quality (expensive) platinum resistance thermometer is the specified reference thermometer for the majority of the International Temperature Scale. Mass-produced commercial platinum thermometers are not as good, but generally still better than thermocouples or thermistors.
My qualification: 30+ years in tthe maintenance, repair and calibration of electronic test and measuring equipment.
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