thermocouple Fe-CuNi  and analog multimeter about measuring temps   - Page 1
Forum section: electronics / projects

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 09, 2011, 11:45:01 pm
content: I am translating the German manual of my analog meter ( 80s) ,and I got to the point that I need some help.It says that it can measure temperature by the use of one thermocouple Fe-CuNi .And it will be connected on the analog , by having set the selector to the range of 75mV DC ??How is that possible ?How is possible one thermocouple Fe-CuNi to produce voltage ?So to be measured by the meter ?It sounds like that the thermocouple Fe-CuNi has the ability to convert heat to DC !!From the translation it looks that the what ever temperature measurement ,does not scale in such detail , so its no good for finding differences of just 2-3C , rather than 20 50 150 300 up to 900C .I found only that this element called as Type LAnd an sample of it here ..https://www1.elfa.se/elfa3/elfa/init.do?item=76-689-11&toc=19402&name=mantle_thermocouple_fe-cuni

author: Unknown
author_rank: Unknown
timestamp: April 10, 2011, 12:15:25 am
content: Look up the Seebeck effect, this is also used in modern day thermocouples (eg. the type K ones often shipped with DMMs). Sounds like they might be using a type J thermocouple. Thermocouples can be fairly accurate, down to +/- 1-2 Kelvin or so, but probably not with just a voltmeter. The V-T curve is not completely linear, real thermometers (or DMMs with thermometer function) should compensate for this.

author: Jon Chandler
author_rank: Frequent Contributor
timestamp: April 10, 2011, 01:44:06 am
content: Quote from: Kiriakos-GR on April 09, 2011, 11:45:01 pmAny two dissimilar metals will produce a voltage when heated. It's a small but measurable voltage.Here's a method I usedto measure the (<50 mV) small voltage generated and some background information.

author: Unknown
author_rank: Unknown
timestamp: April 10, 2011, 02:30:30 am
content: And the inverse of the Seebeck effect exploited by thermocouples is the Peltier effect exploited by thin thermoelectric coolers/heat pumps/peltier elements usually found in car chillers, heatsinks and to control the temperature of laser diodes.I had seen a guy powering a laptop with a thermocouple. It was actually a series of Peltier elements with one end on a heatsink and fan and the other end on a gas stove. The temperature difference created electric current.One of Dave's blogs has a cheap 'environmental chamber' that if I remember correctly uses Peltier elements.

author: Simon
author_rank: Unknown
timestamp: April 10, 2011, 06:35:39 am
content: Quote from: Alex on April 10, 2011, 02:30:30 amThere are commercially available peltier cell generators normally used in cold regions of the planet and run off oil

author: Unknown
author_rank: Unknown
timestamp: April 10, 2011, 06:41:51 am
content: I see. Are they more efficient than a diesel generator coupled to a motor?Alex

author: Simon
author_rank: Unknown
timestamp: April 10, 2011, 06:47:02 am
content: not sure, I read an article by a guy that used one and he put the exhaust through a radiator so in effect got virtually 100% use of the power in the oil but I can't remember the actual electric output efficiency. I think the comment was that it was about the same but silent

author: Unknown
author_rank: Unknown
timestamp: April 10, 2011, 07:05:19 am
content: Makes sense, thanks. Possibly a combined motor/peltier would be the best overall but I understand you dont want to make too much noise in Alaska with all the wolves.Alex

author: Simon
author_rank: Unknown
timestamp: April 10, 2011, 07:30:41 am
content: well when you have to run it all night no, plus you can't easily recover the heat from a diesel setup without having it inside. This thing was in the main living room was small and also heated the house, keeping the batteries topped up along with solar panels and what little sun they had in the winter

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 10, 2011, 01:59:02 pm
content: Quote from: alm on April 10, 2011, 12:15:25 amWell I did try to connect an K type one, and it does not work ..And so, if I can say something about it , are that the J type , its not comparable - compatible with K type.

author: Unknown
author_rank: Unknown
timestamp: April 10, 2011, 02:15:18 pm
content: Quote from: Kiriakos-GR on April 10, 2011, 01:59:02 pmSorry, my crystal ball is in the repair shop (again). What did you try, what result did you expect, and what did you get? For small temperature differences, expect changes in the order of hundreds of µV. About 300µV for ~7K temperature change with a type K thermocouple according to my quick test, which works out to 43µV/K, fairly close to the published 41µV/K.Quote from: Kiriakos-GR on April 10, 2011, 01:59:02 pmWhy are they not comparable? Of course they're not compatible, they're different metal junctions, with different parameters. That's why they're different types.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 10, 2011, 08:56:06 pm
content: Well as I said , the manufacturer suggests to set the meter at 75 millivolts range and connect the thermocouple.I did that with an type K , and I got nothing as measurement.I tried and another "way" , by the thermocouple in place , I turn the selector in Ohms X1 ,I did got the inner resistor of the K type thermocouple, but it was not change ( the resistance ) if I was heating the probe with my body temperature.

author: Unknown
author_rank: Unknown
timestamp: April 10, 2011, 08:58:19 pm
content: Cold junction compensation might be needed.

author: Unknown
author_rank: Unknown
timestamp: April 10, 2011, 10:59:15 pm
content: Sure, if you want to measure absolute temperatures (since a thermocouple only measures the temperature difference), you need cold junction compensation. But you should see changes in voltage when you change the temperature. Try something like ice water and boiling water to get some significant temperature differences. With my tests I got about 300µV/7K difference, from -80µV to 220µV or so.An analog VOM wouldn't be my first choice, not sure how sensitive thermocouples are to loading impedance, I wouldn't expect them to deliver a significant amount of current. Seeing 100µV changes on a 75mV scale may also be hard, guess that's why they only suggest using it for large temperature differences. Note that the voltage may be positive or negative.Type J will work similarly to type K, but the voltage will change slightly more per unit of temperature change (difference is around 25% I think).

author: insurgent
author_rank: Regular Contributor
timestamp: April 11, 2011, 08:33:15 pm
content: Unfortunately I think that setup will only let you know how hot your blast furnace isThe specified thermocouple (Fe-CuNi aka iron-constantan aka Type J) won't be 75mV until around 1300 degrees C.Here's a nice example chart of mV vs temp for a Type Jhttp://www.omega.com/temperature/z/pdf/z203.pdfThere are adapters out there that adapt the thermocouple and output 1mV/degree to interface with meters. Fluke has one for $200 or so but I opted for the UEi TA2K ( < $40 on tequipment ) and accepts 2 thermocouples, does C or F and allows for a delta reading between the two probes.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: April 11, 2011, 09:51:55 pm
content: Ok here is the table of the MA4S in German , but you can get the idea ...