I've been roasting on a North 500g Mill City roaster for about 4 years, and for about 6-8 mos before that on a Hottop. The North 500g came with a 5mm TC which just failed. I use Artisan on both roasters and all during this time my beans have consistently hit FC about 360F give or take.
So Mill City Roasters does not sell the 5mm TC now, only a 3mm (X 35mm recommended length) so I ordered and installed one.
The problem is the temperatures at FC are now reading in the range of 390F which I just don't understand. If the beans have been at FC at 360F and now at 390F which TC was reporting the actual correct temperature? The beans haven't changed.
Thanks for your help with this.

Both temperatures are "wrong"
A thermocouple only tells you the temp of the thermocouple.
The 5mm has more mass to heat, so it's reading lagged more than the 3mm.
An experiment I've done is to roast much slower than usual. (18min vs 9min). The first crack temp was quite different, and the color on the longer roast was way darker for a given end temp. I have a 3mm probe, and this showed me there's significant lag even with that size.

You will also see your turns are now sooner because of less lag. I have a near bare wire BT TC and my turns are always 35 to 40 secs.

BillF wrote:<SNIPPAGE>
The problem is the temperatures at FC are now reading in the range of 390F which I just don't understand. If the beans have been at FC at 360F and now at 390F which TC was reporting the actual correct temperature? The beans haven't changed.
Thanks for your help with this.

Hi Bill, Assuming the setting for the type of thermocouple you installed is correct, maybe the old thermocouple had some build-up from 4 years of roasting and would act as insulation. You'd see that as a lag when the temperature is changing. HTH, -LN

Also a good article on what to expect from different thermocouple sizes here:

http://dailycoffeenews.com/2019/07/09/o ... -readings/

It doesn't really matter what temperature first crack occurs, within reason. That being said your 360F is very much on the lower end and 390F more in the range I'm used to (195C-205C) 383F-401F.

To answer the question, I'd say your first thermocouple was reading abnormal, and replacement normal. Success!

More on the probe size at 13:25.

Hi Bill,
It's a good idea to check the accuracy of your thermocouples by inserting the probe into boiling water and comparing your artisan reading to a trusted kitchen thermometer, or another reference thermometer. You can check the accuracy at temperatures closer to the roasting range by heating sand on your kitchen stove and going as hot as your reference thermometer will allow.

If you are using Artisan, recall you can dial in a temperature offset to compensate for probe errors. If you set up an offset for the OLD probe you may need to update Artisan with the NEW probe's offset. You aren't using an extension wire on the new probes I assume. That can be tricky and cause substantial errors.

TCs drift over time due to oxidation so it's good to replace them every few years. Placement and probe length are a huge factor too. Are your new probes deeper into the bean mass? The PID TC on my North 500g went off, and I replaced it with a shorter TC and it reads much lower now.

These temperature numbers don't have to be spot on, but if you want to compare your phase temperatures with other roasters' it's important to be close to accurate. Then you can make allowances for probe design and placement as best you can. My beans hit FC at about 368F to 372F.

I changed over to RTD probes last year and they are much faster than the 5mm TCs that shipped with our roasters. RTDs are generally more accurate and don't require an offset and don't degrade over time, but they are made with micro-wire and can break if banged around. Converting the 500g was a pain and required drilling out the old mount holes and buying new Phidget devices.
It took me several months to get the hang of the new probes...
John

hankua wrote:It doesn't really matter what temperature first crack occurs, within reason. That being said your 360F is very much on the lower end and 390F more in the range I'm used to (195C-205C) 383F-401F.
To answer the question, I'd say your first thermocouple was reading abnormal, and replacement normal. Success!

I agree with your argument. If we take the point of view that roast temperature measurements should be close to the actual bean temperatures during the roast (conducted at atmospheric pressure) 360F is way below where 1st crack should occur, and clearly the OP's first thermocouple was not even close to bean temperature. I presume both thermocouples are located in the same position within the roast chamber. If so, and unless the OP's roasts are very fast, I'd guess the first thermocouple is just not recording temperature correctly and the second one is. This can be checked by seeing what temperatures they record when immersed in icewater, room temperature water, and boiling water. Both should read nearly the same. The 5mm one may take a bit longer to stabilize, but that shouldn't make a difference in their temperature measurements during the timeframe of a typical roast.

Pressino wrote:360F is way below where 1st crack should occur, and clearly the OP's first thermocouple was not even close to bean temperature.

Before I added a fan powered exhaust / chaff collector, my drum roaster was operating at very low velocity of vent air. At that time, I was getting 1st crack right around 360 dF every time. Now that I roast with more vent air flow, 1st crack is around 380 dF consistently. My guess on this is that the air circulation within the roasting chamber affects the temperature probe's sensitivity.