How do I use my thermometer?

[J.Leon_Szesny]

I got a K type rod on this,

So I set my thermometer to type K

But I read that type K and type J both work to measure high temps.

 

Problem is that if I switch the type, the displayed temperature drastically changes.

 

So I'm a bit uncertain which is the true reading, if that makes sense...

 

[Joël Mercier]

It is because the J type has a different slope. 

 

I'm just gonna state the obvious and say use K type for your K type probe. 

[J.Leon_Szesny]

 

So we're going to assume the K-type temperature readout, is the correct temperature, yes?

[Alan Longmire]

Yes, that is how they work.  Type J uses a different bimetallic series, and thus if you try to read a type K thermocouple with a meter set to read type J you will get incorrect values.  For example, at 375.5C, a type K probe produces a voltage of 15.35mV, but a type J produces 20.5mV.  This is why your meter shows different readings.

 

You can test this with a glass of ice water.  Once the temperature of the ice water has stabilized, the meter set to type K using a type K thermocouple will read 0.5 C or close to it.  

[Dan Hertzson]

Not to be overly pedantic, but for accurate temperature readings using a thermocouple, not only does the indicator have to be matched to the thermocouple, the wires between the indicator and thermocouple have to match as well.