Thermocouple Measurement

[tlantz]

I have thermistor that reads 10k at room temperature, and as the temperature increases, the resistance decreases. I know I can build a small circuit and measure either current or voltage, and calculate the corresponding resistance, and then the temperature, but I was wondering there was a simpler way to do this -- such as within the measuremend and automation explorer.

Thanks,
Travis

[cktears]

I doubt there is a way to do this without any additional hardware. Measurement and automation explorer is just a software to administrate those hardwares. You need at least a DAQ or DMM.

[mikeporter]

Well if you can (for example) comeup with a lookup table that maps a raw input reading to engineering units, you can program that into MAX. MAX can also map value ranges like 0-5V is 50-100 deg--that sort of thing.

Finally, when dealing with thermistors be very careful how you drive them. They are after all still resistors, and resistors disipate voltage as heat--which in a thermistor is an error signal.

Mike...

[tlantz]

I have a DAQ -- but within Measurement and Automation Explorer, when you are working with your input channel information, there are about 7 different options for what type of thermocouple you have, and I was wondering what the difference was.

Thanks,
Travis

[mikeporter]

The difference between various thermocouples is that they use different linearization algorithms. But which do you have a thermocouple (as your message header says) or a thermistor (like your message said)? They are completely different technologies. Thermistors are basically resistors that change value depending on their temperature--as a side note most resistors do, these are built to enhance the effect. Due to this structure, thermistors are passive devices that you have to "excite" to get data from them.

Thermocouples are basically two different kinds of metal spotwelded together. This junction will generate varying amounts of voltage as the temperature changes. Hence, they are active devices that you simply measure. Note that any sort
of contact between dissimilar metals will produce this effect in varying degrees (so to speak 🙂 )--even copper wires and terminals.

Mike...