What is the actual temperature of the drum with IBTS?

Can someone explain what is going on with roasters equipped with IBTS that requires preheat temperatures to be set differently than on previous versions of the roaster? If the preheat temperature is set to 300°C, does the drum heat to 300°C? Also, are the drum and bean temperatures reported by the roaster and by the software during the roast the correct ones or are they offset for some reason?

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The correlation, as I understand it, between the new sensor and the old probe are not equal. The new sensor is more accurate and you just need to adapt to different readings and not try to equate it to the original bean probe.

The probe never measured drum temp, what’s being asked here is the difference in drum temp from the old IR sensor and the new IBTS sensor.

I don’t have the answer, but I imagine that the IBTS is designed to report bean temp based on the emmissivity of beans where I conjecture that the old sensor was designed to measure the drum temp. If this resembles what’s going on, the new sensor doesn’t read the drum temp the same because it’s designed to read the bean temp and the emmissivity is different between the drum and beans.

It isn’t so much that I am trying to compare the different versions of the roasters. Instead, I am wanting to understand the temperature settings I am encountering.

For example, in The Coffee Roaster’s Companion, Scott Rao writes that a reasonable charge temperature for “classic drum roasters” ranges from 193°C to 227°C. That seems significantly different from the preheat temperature of 210°C to 310°C that is listed on page 28 of the most current version of the manual. Similarly, just about every profile I have looked at (e.g. on royalcoffee.com) shows charge temperatures of around 200°C.

In fact, even the profile on Aillio’s blog (The Start of Something…. Our New Infrared Bean Temperature… | by Aillio | Medium) for a 1000g IBTS roast appears to show a charge temperature under 200°C.

Given that, why preheat to 300°C?

IR sensors don’t measure temperature, they see color and correlate it to temperature based on the emmissivity of the material. The IBTS is set up to measure bean temp, so an empty drum won’t read the same as the old sensor that was set up to measure the drum temp.

When Scott Rao mention charge temperatures for classic drum roasters these temperatures have been measured with classic temperature sensor technology like Thermocouples or NTC resistors. These sensors will never measure the actual drum temperature but rather sense the so called environmental (air) temperature, ET. And they are typically placed in the upper part of the roasting chamber. So they measure a temperature relative to the drum temperature that is misleading due to time lag and airflow in the chamber… I would argue that the REAL drum temperature of a classic drum roaster is much higher that the charge temperatures mentioned by Mr. Rao.
The IBTS on the other hand is measuring the IR radiation from the inside of the drum (before the bean charge). This measurement is much closer to the actual drum temperature as it is probably adjusted to the emissivity from the seasoned dark drum surface. Then when charging the greens the IR reading get adjusted to the coffee bean emissivity.
@jacob am I right in assuming that you change the emissivity factor?

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I found that for me the suggested charge temperatures were too high. I was getting to yellow really early (4 minutes) and the color change looked really uneven. I now charge my 500g batches at 200C on p7. This gets me to yellow around 5 minutes and 1c around 9.

I’ve also found that. I roast between 300-454g and have lowered my charge temp a LOT. like 300ºF and have gotten a much more even roast. F1 P4-7. Its a longer roast, but all the stages are there and always in a nice progression, so haven’t gotten a baked feel to it at least.

There is more than one way to skin a cat. In our office, I roast with high preheat temperatures. Our other roaster, Justin, tends to roast with very low preheat temperatures. We’ve both roasted some excellent coffee this year.

As for the emissivity discussion upstairs – it’s true that the infrared sensor is tuned differently in v 1.5 and 2.0 Bullets, so the preheat temps are necessarily different.

Thats funny you say that, because all my roasts were with a 401F or more preheat and they seemed to come out fine, but when I mingled with the “pros” with the machine, they were pretty excited with that concept and had me lower it to 300f. so, like you said. more than one way. I Do, however, agree with them, with the concept that with the higher temps, you run a higher risk of tipping and possibly unevenness from the outside to the inside on the smaller batches… Thanks for your reply!