Thermocouple cold comp sensor placement? - Page 1
Forum section: electronics / beginners

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: April 29, 2020, 11:01:55 pm
content: So I've been building a custom t12 station, mostly because my old atten 936 took a crap and no stations i can buy work the exact way I want. I really need to finish this up because the mains ac Weller I'm using is a pos that will burn its tip up if I don't unplug it between making joints, and it's just way to hot in general for most things.Anyway i decided to use a max6675 mostly because the temp control will be digital pid and I didn't wanna deal with thermsistors or any of that. What I didn't know before buying parts was that if I connect the t12s thermocouple directly to the handle wire and read the thermocouple from inside the station I'm introducing error in the reading because the copper extension wires effectively become part of the thermocouple. So I guess my question is how much error would the copper wire from the base station to the handle create? I thought about just loading the max6675 inside the handle but I cant seem to find any suitable silicon sheethethed cable with 8 #20 to #26 gauge cables, which is what I would need in order to run all the max pins and the switched 30VDC from station to handle, have plenty of 8 pin aviation connectors though.

author: magic
author_rank: Super Contributor
timestamp: April 30, 2020, 05:38:49 am
content: You get as much error as there is temperature difference between the internal PCB in the handle and the inside of the station, because this thermal gradient is invisible to the station as the two wires in the cable are identical and generate no differential thermoelectric voltage / their individual thermoelectric voltages are equal and cancel out.I have a station with such construction and the handle does get a fair bit warm during operation (particularly at high set temperature) and the tip temperature does slowly increase up as the handle warms up. I don't remember how much (and my tests weren't very accurate anyway), but I guess perhaps 20~30°C.Yes, those cheap stations aren't very accurate

author: wizard69
author_rank: Super Contributor
timestamp: May 01, 2020, 05:35:59 pm
content: In general your reference junction temperature measurement must be as close to the actual joints in question. Every change in metal produces a thermocouple. So if you connect copper wires to an iron-Constantan thermocouple you create additional thermocouples. The point of reference junction compensation is to null out these voltages. Here is some specific information:https://www.omega.co.uk/temperature/Z/pdf/z019-020.pdfThat link is part of a lareger manual at Omega that apparently doesn't have a direct link. You can find info at the National Bureau of Standards and other places on the net.Quote from: rwgast_lowlevellogicdesin on April 29, 2020, 11:01:55 pmIt might help to put that iron on a variac.QuoteThis is hard to answer because you need to know what the thermocouple is in the iron. If you know what that is and have the temperature tables for the TC joints created, you can then use the change in temperatures to calculate the temperature shift. If the TC is a separate devices in the iron you can consider bringing the signal back to the controllers socket with TC extension wire. However there is a gotcha there where many TC types have very stiff and un-flexible conductors compared to copper.So what we know is that maximum accuracy requires TC compensation and theat compensation requires knowing what those junction temperatures are to high accuracy. That requires measurement very close to the joints as most instruments using random thermocouples do. That doesn't mean we can't use alternative methods. One thing you can do is to assume the reference junction temperature. Alternative methods though can really suffer accuracy wise.

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: May 01, 2020, 09:59:41 pm
content: Hey guys thanks for the input, I didn't even think about the handle heating up when the CC causes wrong measurements from in the enclosure, or due to any inaccuracies with thermo couple wiring.As I said to get the max6675 in the handle I need an 8 conductor cable. I spent a lot of time looking for 8 conductor ~24awg ruberized silicon cable. I searched everything from Digikey and custom cable houses to ebay and Alibaba and couldn't find anyone offering more than 5 conductor which I have tons of. How ever i did find silicon cable heat shrink on ebay from China (not a single US seller of this stuff), with a range from .8 to 55mm diameter I'm just not sure if that's the ID I'm gonna assume I need 4mm of ID preshrink, but not sure if I pick the 4mm option that is what I need and then I have to wait on the stuff, guess I could use an old computer cable till the silicon arrived.At this point I'm thinking I may just be better off saving the max chips for something else and stuffing a to92 1 wire bus silicon temp sensor in the handle and then using some instermentation amp and a 12 bit adc chip in the station. This definitely seems like a more of a hassle, but maybe less of a hassle and quicker than waiting on Chinese shipping and hoping the heat shrink actually fits and is as nice as an actual silicon cable.

author: magic
author_rank: Super Contributor
timestamp: May 02, 2020, 06:30:37 am
content: Alternatively, ignore this error as the Chinese do or upgrade to a 907F handle which uses the A1321 heater with thermistor sensing and comes with a silicone cable out of the box.