# ET & BT probes



## Mr Karlsen (Mar 29, 2019)

Hi,

So I have question about my ET and BT probes - as you can see in the photo, they have very similar readings. The ET probe is located at the start of the exhaust pipe, and BT in well, the beans.
So I've been struggling with why then, they are so similar.
Could it be because my BT probe is not correctly placed? But why then, does the ET take the same initial plunge as the BT?

Do you have any thoughts, or I am reading too much into this and should I just work with what I got?

Cheers,
- Karlsen


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## Rob1 (Apr 9, 2015)

What roaster is it?


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## Hasi (Dec 27, 2017)

Both probes will be affected by cool(er) beans entering your roaster.

In roasting coffee we're looking at three different types of heat transfer: contact, radiation and air (which is a form of contact as well, scientifically speaking).

BT obviously drop as probe gets in direct contact with cool beans, but also because radiant energy from hot machine parts cannot reach probe anymore. As well, air that's being dragged past hot machine parts will be cooled by bean mass at first.

While ET excludes bean mass, it still is dependant on the other two factors which change during bean drop.
Changing air flow may drastically influence your reading, depending on roaster. Some roasters feature ET probes in other locations, have multiple probes or none at all.

Readings will start making sense only once TP has been reached.


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## Mr Karlsen (Mar 29, 2019)

Here is the roaster:
I believe it is quite similar to the North roasters.

Thanks for the thorough response Hasi.
I guess that I am just slightly confused, as even after TP, BT is lower and then becomes higher than ET - in all readings/examples etc., it seems that ET is quite a bit higher than BT and then during the roast that gap is lowered.


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## Hasi (Dec 27, 2017)

don't worry about the actual numbers.

When we had our probes being calibrated, we saw readings had been off as much as 25C in a certain temp range...

Most importantly, you'll learn how to dance with your machine in relation to temp graphs through experience and cuppings!


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