Hi,
A thermocouple measurement is a "relative" measurement method.
It measures the temperature difference between the thermocouple "sensor point" and the "cold junction point".
To get an absolute temperature reading you need a sensor for "absolute temperature" at the cold junction point.
Example:
If you have a handheld thermocouple type "K" (about 41uV/°C near room temperature) measurement device. Let's say the temperature of the device is 25°C.
And you want to measure the temperature of an object with 26°C...
Then the voltage at cold junction is about 25°C x 41uV = 1025 uV (you can't measure this voltage)
The voltage at the sensor point is 26°C x 41uV = 1066uV ( you can't measure this also)
You measure the difference: 1066uV - 1025uV = 41uV.
Now you could say "why not simply take the difference of the temperatures... and multiply them with 41uV?
My answer is: Because your concern is "precision" and the 41uV are only true near room temperature. If you want to measure very high or very low temperature you need to take care about this. You need to use different factors for cold junction and measurement point.
Back to room temperatures:
You urgently need type K wires if you want to extend the sensor wires. Exactely to the cold junction point.
For high precision around room temperature you need a cold junction block, maybe aluminum, around the cold junction points and the absolute temperature sensor.
Then you need an amplifer with very low offset voltage (chopper stabilized) and a good layout to be sure not to introduce additiinal offset errors.
I'd say the practical limit will be around 0.1°C temperature difference.
And for sure you need an absolute temperature sensor with the same precision: 0.1°C.
The key to succes is a good schematic, (noise shielding), high quality thermocouple sensors, high quality extension wires, a good pcb layout and good amplifiers...a lot of things to take care.
Klaus
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Added:
You can increase the sensitivity arbitrarily by putting 10 or 100 thermocouples in series. You of course lose on the response time.
You can not simply connect thermocouples in series: you need to wire each thermocouple back to the cold junction point and there connect one thermocouple in series.
Klaus