I’ve read quite a few posts around this and am still left wondering…
Which bean temp reading is the correct reading? Bean Temp or IBTS? Specifically, when reading posts from Sweet Maria’s (or other roasting articles online) that reference bean temp milestones ie, “first crack happens when the bean temp is around XXX degrees,” should I be looking at the Bean Temp number or the IBTS number on my roaster? This is super confusing for me and kinda frustrating.
Looking at my roast profiles, sometimes the IBTS temp and the Bean Temp stay apart, with the IBTS temp higher than the Bean Temp. However, other times, they start apart then they converge and the Bean temp eclipse the IBTS temp. So this goes back to my previous question…which bean temp reading should I be looking at. It’s difficult to feel like I’m roasting with confidence if the way my data reads keeps changing on me.
Just my personal take on the 2 measurements: both are correct; both are wrong. Like the man with the broken watch, he had the correct time twice each day but never knew when that was.
The separation between IBTS & BT shrinks as you approach the 1 kg max batch size; at the other limit of the weight range, 350 gm, you’ll find the 2 measurements are separated by a greater amount. The bean temp probe is sensitive to mass & mass flow, but IBTS is less affected.
Early on I found that IBTS seemed to come very close to the 1C/2C temps Thompson Owen noted in his articles about roasting by sight/sound/smell/temp. Those temps were measured with a thermocouple but I ignored that. What I saw as ‘correlation’ was with a batch size of 550 gm. I’ve stayed with that ever since. If I went to a different batch size I’d have to remember new parameters and I don’t want to bother.
What’s important for 1C/2C is the sound… trying to go by instrumentation in this somewhat subjective process will defeat the objectivity you’re trying to use. For a given bean and batch size, I’ve found IBTS temp at 1C/2C fairly repeatable.
If IBTS & BT are swapping position that can be an indication that the IBTS needs to be cleaned. The images don’t have date info so I can’t tell if there is a significant time between the profiles. If they were done close together (within 3 or so kg of greens) then it may not be related to contamination on the IR sensor lens. But that’s my go-to explanation.
I do see that one roast has a lot more modulation on the IBTS data than the other 2. I don’t have an explanation for the difference that I feel sure of but I think it may be related to the drum/roaster not fully stabilized thermally, i.e. I think I see more modulation in the first roast of the day than I do in later roasts.
This is all just speculation on my part. Others may have a better take on what you’re seeing.
I’m curious as to whether others feel I’m conceptualizing the differencea correctly. My take would be that the IBTS would accurately and instantaneously measure the temperature of the beans within it’s line-of-sight. With a quick-turning drum and reasonable charge amount of beans (sufficient to keep the line-of-site from reaching the drum surface), this should be a very close approximate of the “actual” temperature of the beans. Thermaprobes inherently suffer some degree of lag, and also suffer error from proximity to (or lack-thereof) the bean mass. I had inferred that convection as air circulates with the help of the fan, would also lead to potential inaccuracy of a thermaprobe compared to the IBTS.
BT is inconsistent even with consistent roast size. Just watching BT when you preheat, the relatively stable value once preheated will change with ambient temps.
When you charge the roaster, and you watch BT drop and then rise…the turning point, this demonstrates the inherent lag in the BT reading. The IBTS is instantaneous since it uses the emmisivity coefficient to calculate the temp based on the radiation of heat.
Also, BT will also be impacted by steam being released by the beans. You can see this at 1C, where BT-ROR will spike…everyone assumes its because of an exothernic reaction, but if you watch IBTS-ROR at the same time, it will be stable.
