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Homeroasters - Home Roasting Coffee Community » BUILDING A ROASTER » Building a Coffee Roaster

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thermocouple issue
elkayem	#1 Posted on 03/23/2019 6:55 PM
Newbie Posts: 43 Joined: March 09, 2018	I'm having an odd issue with one of the thermocouple measurements on my home built roaster. The measurement seems to be discretely jumping back and forth from the correct temperature to ambient. Jiggling the wire near the hot junction seems to cause these jumps to occur, though I'm not 100% certain about that. I have a replacement thermocouple on order to resolve the issue. I am curious if anyone else has encountered behavior like this, or if anyone has any ideas on what might be going on here. If the new thermocouple fixes the issue, my question may be purely academic. However, I'm genuinely curious what may have happened here.


marcov	#2 Posted on 03/24/2019 5:12 AM
1/4 Pounder Posts: 61 Joined: January 21, 2019	hey, I had similar issues. See here for the problems: https://forum.homeroasters.org/forum/viewthread.php?thread_id=5816#post_68053 And here for the solution: https://forum.homeroasters.org/forum/viewthread.php?thread_id=5816#post_68112 hope this helps :)


renatoa	#3 Posted on 03/24/2019 7:53 AM
Administrator Posts: 2511 Joined: September 30, 2016	I am surprised to hear so many TC issues... mine are rock solid stable, for example 30 seconds sample I did to verify before posting shows a spread of only 0.02 C degrees, the 30 values are only in the 19.15-19.17 C degrees range. The whole box of electronics setup was been placed near the laptop (no wifi, no HDD, just SSD), no brushed motors around. Within 20 cm was a cell phone, with wifi active. Unshielded TC.


elkayem	#4 Posted on 03/24/2019 9:06 AM
Newbie Posts: 43 Joined: March 09, 2018	I didn?t explain the symptoms well enough. My issue isn?t noise. My readings are rock solid too, except that one of my two probes will suddenly jump to ambient temperature and then back, For example, if I am controlling to 200 F, the probe will read 200, then suddenly jump down to 70 F (the temp in my garage), then back to 200. I?ve had a day to ponder this, and my current thought is that there is an intermittent short between the two thermocouple wires somewhere inside the cable. I?ve been roasting with this sensor for nearly a year so it has had a lot of abuse.


renatoa	#5 Posted on 03/24/2019 12:25 PM
Administrator Posts: 2511 Joined: September 30, 2016	If you are talking about the duo TC4-Artisan. I suspect rather a software/comm issue... It happens here too, on LCD I have the right value, in Artisan drops to zero for some seconds, then recover. The Artisan serial log shows as below: 55 18:05:48.118 ... Rx = [u'0.0', u'163.3', u'52.9', u'40', u'0', u'37.81']... 54 18:05:47.120 ... Rx = [u'0.0', u'164.2', u'52.7', u'40', u'0', u'37.37']... 53 18:05:46.044 ... Rx = [u'0.0', u'-0.1', u'52.2', u'40', u'0', u'36.48']... 52 18:05:45.030 ... Rx = [u'0.0', u'-0.1', u'51.9', u'40', u'0', u'36.04']... 51 18:05:44.016 ... Rx = [u'0.0', u'-0.1', u'51.6', u'40', u'0', u'35.60']... 50 18:05:43.002 ... Rx = [u'0.0', u'-0.1', u'51.3', u'40', u'0', u'35.16']... 49 18:05:41.988 ... Rx = [u'0.0', u'-0.1', u'51.0', u'40', u'0', u'34.72']... 48 18:05:40.974 ... Rx = [u'0.0', u'171.1', u'50.7', u'40', u'0', u'34.28']... 47 18:05:39.960 ... Rx = [u'0.0', u'172.2', u'50.4', u'40', u'0', u'33.83']... Happens for ET channel only. During this drop the TC4 LCD shows fine, right values.


elkayem	#6 Posted on 03/24/2019 1:56 PM
Newbie Posts: 43 Joined: March 09, 2018	I have figured out the issue. (As an aside since the TC4 was mention in the previous post, I'm not running on a TC4 but instead using my own set of electronics.) It turns out that some of the bare thermocouple wire was folded back on itself at the screw terminal on my electronics board. In retrospect, this was an obvious place to look. It was easy to overlook, since the braided insulation shield was a little frayed at the end and concealing the problem. The intermittent short near the board cause the thermocouple to occasionally read the ambient temperature near the board rather than at the end of the probe. Renatoa, the snippet of data you are showing in your post looks very similar to the symptom I saw. You might look at your ET channel and see if you have a similar problem.

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