Glass Fibre Thermocouple - Page 1
Forum section: electronics / beginners

author: bitshift
author_rank: Regular Contributor
timestamp: January 18, 2016, 08:33:15 am
content: Hey guys,I'm building a small reflow oven and I need to choose a thermocouple. I read that one should get a glass fibre thermocouple but I can't find any specific reason why.I've found two suitable thermocouples where one of them is the glass fibre type:http://za.rs-online.com/web/p/thermocouples/8140147/http://za.rs-online.com/web/p/thermocouples/8722795/If both are rated to 350 degrees C, why pick one over the other? Also, I expect the maximum temerature to be around 260 degrees C. Is 90 degrees C enough safety margin?

author: amyk
author_rank: Super Contributor
timestamp: January 18, 2016, 09:11:45 am
content: They're both glass fibre insulated. The reason is for temperature resistance - the thermocouple itself will keep working until it literally melts, but the insulation needs to withstand the temperatures required.

author: bitshift
author_rank: Regular Contributor
timestamp: January 18, 2016, 09:30:16 am
content: Understood, thank youSo by that there would be no difference between buying a thermocouple rated at 1000 degrees C vs a thermocouple rated at 350 degrees as long as you don't exceed 350 degrees?So when comparing two such thermocouples you would go for the one that has the smallest probe diameter?

author: Simon
author_rank: Unknown
timestamp: January 18, 2016, 12:33:02 pm
content: you will often see thermocouples on ebay rated for 1000C but that have pvc insulation

author: electr_peter
author_rank: Unknown
timestamp: January 18, 2016, 06:26:04 pm
content: All same letter (K/J/...) thermocouples are very similar in construction. You can measure ~1000C for small time with the tip of almost any thermocouple if adequate thermal insulation (or heat distribution) is provided. Insulation is limiting factor only for long term measurements.

author: Simon
author_rank: Unknown
timestamp: January 18, 2016, 06:43:16 pm
content: Quote from: electr_peter on January 18, 2016, 06:26:04 pmI don't see a situation where you can take an instantaneous temperature of 1000C without some of that heat getting to the insulation that can take a whopping 90C, it's a one time only thing

author: electr_peter
author_rank: Unknown
timestamp: January 18, 2016, 10:54:26 pm
content: Quote from: Simon on January 18, 2016, 06:43:16 pmOnly 90 C? That's a bit low. Do you mean 90C temperature safety range between measured temperature and insulation rating?Insulation will be gone after 1-5 tries with 1000C but there is nothing to stop you doing this. Thermocouples are cheap anyway. Or get properly rated equipment for this task.After seeing manufacture of glass it seems that some of items that can withstand 1000C or higher temperatures are bricks. Employees have to work in 5-10-20 min, shifts, clothes/glasses/hats are burning, massive cooling is needed. Electrics/electronics are generally not even close to the heat.

author: bitshift
author_rank: Regular Contributor
timestamp: January 19, 2016, 09:59:37 am
content: Quote from: electr_peter on January 18, 2016, 10:54:26 pmNot sure where the 90 degrees was coming from regarding the insulation. I was just wondering if I get a 350 degree C thermocouple and put it inside a 260 degree oven, if 90 degrees is enough safety margin for this.

author: electr_peter
author_rank: Unknown
timestamp: January 19, 2016, 06:57:01 pm
content: 350C rated thermocouple should more than good enough for a reflow oven.I would also strongly suggest to install somethermo-type fuse/switchwhich would protect from overheating/fumes/fire if something goes wrong when you are not looking.

author: bitshift
author_rank: Regular Contributor
timestamp: January 20, 2016, 05:55:20 am
content: Adding a thermal cut out is a great idea. I will definitely connect one. The oven has one already, I haven't checked what temperature though.

author: bitshift
author_rank: Regular Contributor
timestamp: January 20, 2016, 06:52:01 am
content: Another quick question related to this. I'm consideringthisthermocouple. I don't need it to be socketed so I want to cut off the connector and solder the wires directly to a PCB. Would this affect any characteristics of the thermocouple?

author: electr_peter
author_rank: Unknown
timestamp: January 24, 2016, 06:53:07 pm
content: You could solder thermocouple directly to PCB. Accuracy would be affected slightly (crimped connections of known metals are much better than solder from mixture of metals), but it won't be very noticeable. This thermocouple is rated for +/- 1.5C + 0.25% accuracy so it will be more than good enough for an oven.I suggest to measure PCB temperature at the connection point with other thermo-sensor to get compensation value for thermocouple.

author: bitshift
author_rank: Regular Contributor
timestamp: January 26, 2016, 08:11:39 am
content: Quote from: electr_peter on January 24, 2016, 06:53:07 pmThank you, that's good to know.I'm going to be using aMAX31855IC to interface with a micro. It does all the cold junction compensation and gives you a digital value of the temperature. The IC is not cheap but it makes interfacing a thermocouple very easy.