Question about thermocouple resistance - Page 1
Forum section: electronics / projects

author: Electric flower
author_rank: Regular Contributor
timestamp: March 05, 2014, 10:40:12 pm
content: In two weeks im going to hold a presentation in school about thermocouples and measuring temperature with them.I started to do experiments at home with measuring voltage with three multimeters and everything went fine.NOTE: Everything was connected right how it should be !!!So basicaly this is where i got stuck. I was measuring thermocouple resistance at room temperature: If i connect thermocople wrong way i get almost the same reading.Protek 506: 12.4 ohmPANCONTROL PAN51: 12.4 ohmUNI-T UT61E: 12.34 ohmAnd then i heated tip of thermocouple with lighter and these are following results:Protek 506: 50.2 ohmPANCONTROL PAN51: 91.8 ohmUNI-T UT61E: 0.00 ohmIf i connect+of thermocouple to-of UNI-t i get reading of 32.00 ohmsI'm very confused with measurments wile heating, why are results so different and UNI-T being other way around.Bonus info if this may help: Voltage given by thermocouple when heating it.Protek 506: 25.6 mV -21-6 mV if + of thermocouple connected to -PANCONTROL PAN51: 26.7 mV -19.2 mV if + of thermocouple connected to -UNI-T UT61E: 25.9 mV -21.6 mV if + of thermocouple connected to -Thanks for your time and good luck trying to help me

author: IanB
author_rank: Super Contributor
timestamp: March 05, 2014, 10:49:00 pm
content: When you heat up a thermocouple junction it will generate a voltage, which will most likely upset the resistance measurement of a multimeter. The resistance range of a meter is designed to measure passive components like resistors. It cannot measure something that generates its own voltage or current.But what do you gain from trying to measure the resistance of the thermocouple? I don't think this measurement has any practical use.

author: calexanian
author_rank: Super Contributor
timestamp: March 06, 2014, 05:37:21 am
content: Some clarification may be in order. A thermocouple is a junction type device where the voltage drop across that junction stays constant independent of current flowing through it (For the most part, we are ignoring heating and other factors) this voltage drop changes with temperature. Actually a simple diode will act as a thermocouple and often is used as such on a semiconductor die where a traditional welded thermocouple is not practical. Use a meter to measure the forward voltage drop of a 1n4001 or something like that (Any rectifier will do) then hit it with freeze spray or touch the case with a soldering iron and watch the voltage drop change. Another device for measuring temperature is the thermistor. Thermal-Resistor. This device will change its resistance based on them. It is a specially made carbon resistive blob that through internal thermal stress changes resistance. You can hook up an ohm meter and a typical thermistor used all over is 10K at about 70 degrees F. Read it with a meter then stick your finger on it and watch the resistance drop from the heat in your hand.Hope this gets you pointed in the right direction.

author: Richard Head
author_rank: Frequent Contributor
timestamp: March 06, 2014, 06:40:37 am
content: A multimeter set to the ohms range will force a constant current through the test leads and the meter will measure the voltage drop across the unknown resistance at that current. Knowning the voltage drop and the current the meter displays the unknown resistance value.A thermocouple generates a very low VOLTAGE across its junction (actually there are two junctions but we will neglect this!). If you measure the thermocouple resistance the multimeter will read an error resistance that due to the voltage that the thermocouple produces. In fact you should find that if you swop the multimeter test leads around it should read a diffrent value.That's the theory anyway!Dick

author: Nerull
author_rank: Frequent Contributor
timestamp: March 06, 2014, 02:21:12 pm
content: Quote from: calexanian on March 06, 2014, 05:37:21 amA thermocouple generates voltage. Some spacecraft use thermocouples to generate power - Cassini, for example. To use them, you measure the voltage produced by the junction. Thermal diodes use a different mechanism, where you pass a current through a diode and measure the voltage drop.

author: calexanian
author_rank: Super Contributor
timestamp: March 06, 2014, 03:21:41 pm
content: Yes. True. And it works very well. It does however take a temperature differential and many many many junctions to generate any appreciable power. With the energy harvesting tech coming out and ultra low power micros out now I supose it may be getting more practical. Look up the nuclear batteries on the voyager probes. 40 years nearly out and still working!