Identify temperature sensor - Page 1
Forum section: electronics / projects

author: Deckert
author_rank: Regular Contributor
timestamp: November 19, 2013, 11:20:07 pm
content: Hi guys,I've got this little dual-temperature clock with an internal sensor and an external sensor. Issue is, the external sensor and lead wire got chewed off by my lovely Labrador and it's now missing. I opened it up and the internal senor looks like this:The external sensor seems to be the exact same thing (I desoldered the internal sensor and connected it to the external socket to check). However, I have no idea what it is. A thermistor? J-type junction? Something else? Does the colour of the bead mean anything?--deckert

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 20, 2013, 12:35:33 am
content: Found some that look the same:https://www.google.com/search?q=black+thermistor&tbm=ischThermistors increase in resistance with increasing temperature. Thermocouples generate increasing voltage with increasing temperature. I'm guessing it's a thermistor, but you can check it yourself.

author: rolycat
author_rank: Super Contributor
timestamp: November 20, 2013, 12:57:23 am
content: Quote from: tsmith35 on November 20, 2013, 12:35:33 amThe type used in thermometers is more likely to be an NTC or negative temperature coefficient thermistor; the resistance of thesedecreaseswith increasing temperature.As well as getting the right type, you will also need to match resistance at room temperature and beta, a parameter which determines the resistance variation with temperature (which will not be linear). It's probably going to be easier to buy a new thermometer.

author: tsmith35
author_rank: Frequent Contributor
timestamp: November 20, 2013, 01:18:32 am
content: Quote from: rolycat on November 20, 2013, 12:57:23 amOops.. yes, that's what I meant.Typing too fast for my brain (as usual).

author: Deckert
author_rank: Regular Contributor
timestamp: December 30, 2013, 11:10:19 pm
content: Quote from: rolycat on November 20, 2013, 12:57:23 amWho said anything about making it easier?I justhadto learn more about thermistors...In any event, I hooked up a 10kohm 10-turn pot to the external sensor input and adjusted it down in steps of 1kohm, each time cross-checking the resistance with my DMM. The thermometer updates its readings every 20 seconds, so it was quick to plot all the temperature values and work out the R/T characteristics.I found the correct thermistor (10kohm at 25 degrees C) over here at RS for a fraction of what the thermometer cost me:http://za.rs-online.com/web/p/thermistors/5288536/By measuring the voltage over the multi-turn pot with a 'scope I could also see how the thermometer took its measurements. Every 20 seconds, it would send out a train of pulses at 13kHz for a period of 360ms. That gives 4680 pulses per measurement interval - nice averaging by the thermometer!My thermometer is working again and I'm much the wiser on thermistors. Thanks for all the replies!--deckert

author: SeanB
author_rank: Super Contributor
timestamp: December 31, 2013, 07:24:29 am
content: If you want the thermistor back you need to put on gloves and search. Dogs love dangling cables for some reason. Daxie loves plastic bottles and eats them, you can tell what it was just by looking at the plastic traces in the dump.Those thermistors are almost all the same type on the common probes, nice to see RS has them, though I can buy the whole unit for just R10 more than RS, and it comes with a battery as well.

author: Deckert
author_rank: Regular Contributor
timestamp: January 01, 2014, 01:25:30 am
content: Quote from: SeanB on December 31, 2013, 07:24:29 amIndeed - no forensic analyst needed with Lexi either.QuoteWhere can you buy them that cheaply?Okay, I'll add that this thermo has a clock and stores the times+dates of the min+max temp as well. Got it for R200 at Makro. It currently sits on my bench to measure the ambient temperature of the lab with the internal sensor, but the external sensor is very hand to quickly check the rough temp of a component or fan outlet. If I need 0.05% accuracy I'll grab my T502KJ, which I got as a present from Tyton.--deckert

author: SeanB
author_rank: Super Contributor
timestamp: January 01, 2014, 05:32:07 am
content: Which Makro was that? I haven't seen them at my my local one in Springfield Park. I was looking at the simple cheap refrigeration ones. The min/max with no datalogging are around R80.I use a cheap IR thermometer from Ardendorff to do quick temp measuring, works reasonably well for around 1C accuracy. Not often I need a more accurate or higher resolution sensing, and I use it often for AC and teating elements.

author: Deckert
author_rank: Regular Contributor
timestamp: January 01, 2014, 01:48:22 pm
content: Quote from: SeanB on January 01, 2014, 05:32:07 amCenturion, but was a while ago (as in, a year or so). Can't find anything similar on their online site right now.Quote from: SeanB on January 01, 2014, 05:32:07 amNice. External sensor or just internal?Quote from: SeanB on January 01, 2014, 05:32:07 amAn IR one should probably be next on my list - the Flir E4 is fabulous, but way out of my hobby budget. Agreed that 1C is more than enough for most situations. Basically just want to see if the device really needs a bigger heat sink under a load of X.--deckert