Why does the IBTS read about 30 degrees F higher than IBTS?
If they are both measuring the beans, shouldn’t it read the same?
I’ll take a try at trying to describe my understanding of this.
The bean temp probe will always lag behind whatever it is that it is measuring. the probe itself is inside of a stainless steel sleeve and that SS sleeve has to heat up before the probe can measure the change in temperature.
The IBTS reads, instantaneously, whatever is in it’s field of view.
During the pre-heat of the bullet the IBTS is measuring the surface temp of the drum.
The Bean temp probe is measuring air temp inside the bullet.
After the beans are added to the drum, the IBTS measures the temp of the beans because they are in front of the IBTS sensor.
The bean temp probe will be immersed in the beans but won’t be able to measure the temperature change until the Stainless steel sleeve warms up.
As the roast progresses, the bean temp probe will slowly warm up and be closer to the actual temperature of the beans.
In theory, if we were to hold the bullet at a steady temperature of (as an example) 300F eventually the bean temp probe would arrive at the 300F and display that temperature.
The reality is that the bean temp probe is not in the bean mass long enough and the actual bean temp is changing too fast for the probe to keep up.
The IBTS is a huge innovation in coffee roasting.
Most coffee roasting machines rely on encapsulated probes to measure the bean temp and the enviremental chamber temp and the exhaust temp and so on.
I had a HotTop prior to the bullet and my temperature readings were almost always 30 degrees lower than what I expected they should have been. In the end it doesn’t matter, you learn that you have to drop at 380 on the hottop rather than 405. I tried to build a thermal probe that wouldn’t take so long to measure the temperature changes by putting a stainless steel sleeve around the acutal probe and only leaving the thermistor probe tip exposed, but it was too fragile and still did not react fast enough to be worth the trouble of the modification.
With the bullet and roasting larger batches, it is not uncommon to see the bean temp probe reading cross the IBTS temp and show higher temperatures than the IBTS. When that happens, I have started using the bean temp probe vaule for my “drop” temp.
This seems to be verified by the aggtron measurement that I am getting. I haven’t heard of anyone else doing this.
In the final analysis, the only thing that is important is how the roast tastes.
I could be all wrong on my perception of what is going on.
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I concur with you on that @billc. In my roasting of 1kg batches vs 1lb batches, I tend to switch to monitoring the BT around FC and beyond. With 1kg batches because there is more bean mass in the drum for the BT to take the reading I think it is more accurate to use BT. BT and IBTS readings do converge around FC (for me at least) and as you pointed out sometimes BT will be above IBTS depending on how dirty the IBTS is.
I would very much like somebody, maybe even someone from Aillio, to explain to me the physics of how the BT temp can exceed the IBTS temp.
This has been a puzzlement for me.
-Gray
Two possibilities come to mind…
- a calibration error in either the IBTS or the BT probe
- the IR sensor lens is dirty
The latter can be difficult to deal with. The recommended cleaning agent is alcohol. But the ‘stuff’ that can cause a measurement error on IBTS is not soluble in alcohol. It does, however, soften a little so repeated passes with a clean cotton swab dampened in alcohol will eventually get it clean. The hardest part is that I have yet to find any evidence I have cleaned anything with the swab- it always looks spotless. Yet it makes a difference.
Odds are very good that, if IBTS and BT swap places toward the end of the roast, the IR sense lens needs cleaning.
An aside- if there is crud on the IR sensor lens, Preheat temp is higher than whatever the Preheat setting.
Bruce
I think the main reason is again back to the BT’s thermometric lag
Based on my experience with the Bullet, during a light roast, most of the time I would lower the power close to or right after first carck at around P3/P2/P1 or increasing the fan speed up to F3/F4…IBTS sees it immediately while BT continue to drive upward due to its thermometric lag…
So, most of us may encountered the symptom of BT/IBTS crossed each other at around FC…
If that cross over happened too early, it may well be the dirt that Bruce explained