11-15-2011 01:58 PM
Thank you, Will,
The S-type thermocouple I am testing should have no problem. I used another handheld thermometer to connect with the thermocouple. The temperature reading from the thermometer showed no problems and increased with the furnace temperature after 1100 oC. Then I used the TC01 to read the temperature and the reading just kept dropping.
Could you tell me what difference is between a handheld therometer and the USB-TC01? Any difference regarding they deal with the signal from the thermocouple?
11-16-2011 11:47 AM
You are very welcome.
There shouldn't be any major differences between the way the TC-01 and your handheld thermometer read, other than the TC-01's software interface. Could you tell me the steps you took to configure the TC-01 for the S-type?
Will H | Applications Engineering | National Instruments
11-18-2011 11:26 AM
Regarding configuring the TC-01 for the S-type, I just chose the thermocouple type in the temperature logger of the TC01.
There is a good news. The reading from the TC01 started to follow the increase of the furnace temperature after I connected the negative input of the TC01 to ground. However, there is still a increasing temperature difference between them, 4 C difference at 1300 C. How can I configure the TC01 to improve?
11-18-2011 04:53 PM
You mentioned the accuracy at 1300C seems to be off by about 4C. Does this difference scale with temperature? For instance, at 750C do you see a 2C difference, or still 4C?
11-30-2011 09:52 AM
I hope you thanksgiving was good.
Yes, the difference scales with temperature. it was about 2 C at 1100C.
Please see attached temperature curve, there exist still abnormal temperature reading from TC01during furnace cooling even though there seems no such problem during heating up after I did the grounding. The cooling rate was supposed to be linear.
11-30-2011 06:24 PM - edited 11-30-2011 06:26 PM
My Thanksgiving was excellent, thank you for asking. I trust that yours was good as well : )
I took a look at the graph you sent and, combined with the other issues you've seen, I believe the overall problem is with shield grounding.
You did mention that you grounded the negative input of the TC-01 to ground, but it's also important to ground the mesh metallic sheath (shield) that surrounds most thermocouples. These shields are quite susceptible to external noise, especially given how small the actual voltage difference is between the two metals in the thermocouple. (It this difference that the TC-01 actually reads to determine the temperature, which you may already know).
I'm assuming your thermocouple has such a shield, and that a lack of ground is causing the behavior you showed. If it does not, please let me know.
As far as the temperature difference, 4 degrees Celsius is not a-typical. Thermocouples aren't incredibly accurate, and the difference is likely a combination of errors and noise from the thermocouple itself, the TC-01, and the rest of the system. Ordinarily I'd suggest using a thermistor for more accurate temperature measurements, but those wouldn't be able to operate in your temperature range.
Finally, be sure that you've included cold junction compensation in your use of the thermocouple. Since the others seem to work fine, I assume you've taken care of this, but it's a good thing to double check since it can throw off your readings.
Hopefully this helps, Xu. As always, please let me know if you have further questions.