Using Thermocouples

[ddrop]

Hey all,

I'm using the NI USB 6259 with type E thermocouples and LabVIEW Signal Express to measure temp. avgs over a period of time. There seems to be a slow rate of response between the thermocouple and the board -- for example, if I pinch the thermocouple, and then release it, the temperature will keep rising for a solid 3-4 seconds after. Also, when the temperature final caps off, it falls at a much slower rate than my Fluke thermometer. Any suggestions?

[Beutlich]

The thermocouple response time can be effected by the exposure of the thermocouple junction and the quality of the medium that causes the temperature change. If you submerged the sensor into cold or hot water, for instance, I suspect there would be a faster response than just pinching the device with your fingers.

Addressing particularly the speed of the device (since we are comparing it to another faster sensor), the speed can be effected by whether the junction is exposed or sheathed, and if it is sheathed whether the junction is grounded to the sheath. An exposed junction gives the best response time, but then the junction is open to damage. Ungrounded thermocouples (junction not attached to sheath ground) are slower than grounded thermocouples, but they are more resistive to noise and electrical phenomena (ground loops etc).

I would recommend getting a different (faster) thermocouple that is designed to be exposed, but if you are confident in do-it-yourself tinker projects I suppose you could try hacking at the sheath to make the junction exposed. You will greatly shorten the life of the sensor, but they or often inexpensive (although, that means getting a new one is easy too).

[ddrop]

Thanks! I figured out my problem, but it didn't have anything to do with the junction. Something was wrong with the way I was taking the mean