Hottop charge temperature throwing off probe readings?

Discuss roast levels and profiles for espresso, equipment for roasting coffee.
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EvanOz85
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#1: Post by EvanOz85 »

Hey guys. I've been experimenting with higher charge temps on my 2013 Hottop-B in order to speed up my roasts, and I'm noticing something odd. Please note I'm still using the stock K-type thermocouple.

If I charge 227g at say, 230F, my first crack almost always comes around ~370F. I was usually hitting 300F around 6 minutes, and wanted to decrease that to 6 minutes. So I started charging at temps ranging from 350F to 380F, and I have definitely been able to achieve a shorter drying phase. However I have noticed that as the charge temp increases, so does the first crack temp. The last batch I just charged at 380F didn't reach first till 388F, and charging at 350F usually bring first crack to 380F.

Why is this happening?

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EvanOz85 (original poster)
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#2: Post by EvanOz85 (original poster) »

No ideas?

rgrosz
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#3: Post by rgrosz »

The stock thermocouple may NOT actually measure the bean temperature at first crack. The first mod that most people make is to add separate thermocouples to read ET and BT separately.

At the start of the roast, the stock thermocouple basically measures ET. At 227g charge, the bean mass will be below the thermocouple for most of the roast. As the roast progresses, the beans expand, and eventually start bumping into the stock thermocouple. The point at which this happens differs based on which greens you are roasting, and the charge temperature. At the end of the roast, you should see a valid reading for the bean temperature.

I can't explain the correlation between your charge temperatures and the first crack temperatures.
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EvanOz85 (original poster)
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#4: Post by EvanOz85 (original poster) »

I thought that most of that was only true of the older B models before they upgraded to a K-type probe? I believe the probe was re-positioned a few years ago, and as far as I can see is almost always inside the bean mass.

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JohnB.
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#5: Post by JohnB. »

While the new style temp sensor is a slight improvement over the old one it won't give you an accurate BT reading due to it's placement. Most of the time you will get a misleading ET reading (sensor too low) with a 227g batch. A larger batch size might cover the stock sensor probe better but your best option is to install BT & ET thermocouples in the correct locations.
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rgrosz
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#6: Post by rgrosz »

EvanOz85 wrote:I thought that most of that was only true of the older B models before they upgraded to a K-type probe? I believe the probe was re-positioned a few years ago, and as far as I can see is almost always inside the bean mass.
I just checked some of my roast logs for any charge quantities above 215g.

08/31/2013 roast of 225g - first crack BT was 378 and Hottop B-2K display temp was 386
09/07/2013 roast of 232g - first crack BT was 373 and Hottop B-2K display temp was 386
10/19/2013 roast of 227g - first crack BT was 370 and Hottop B-2K display temp was 386
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EvanOz85 (original poster)
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#7: Post by EvanOz85 (original poster) »

I went ahead and ordered a nice Omega temp logger and two K-type probes. I'm just going to drill holes for them in the bean chute cover for now..maybe I'll upgrade to the HTC interface some day..but not today.

Still no answers as to why higher charge temps result in higher first crack temps. Hmm.

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JohnB.
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#8: Post by JohnB. »

EvanOz85 wrote:Still no answers as to why higher charge temps result in higher first crack temps. Hmm.
Could just be a fluke of the HT temp sensor. Higher charge temps would get you to 1C sooner if you changed nothing else but the 1C temp is pretty much bean dependent. Usually falls within the same range but some beans hit 1c sooner & some hit later.
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