Incorrect Thermocouple Measurements at Low Temperatures Using NI-DAQmx, NI-DAQmx Base, or LabVIEW Primary Software: Driver Software>>NI-DAQmxPrimary Software Version: 8.10 Primary Software Fixed Version: N/A Secondary Software: N/A Hardware: Multifunction DAQ (MIO), SCXI
Problem: My thermocouple readings are inaccurate when they drop below a certain temperature. A B-type thermocouple should support a minimum temperature of 0 degrees C, but I see inaccurate measurements when the temperature drops below 250 degrees C. E-type, K-type, N-type, and T-type thermocouples support temperatures as low as -270 degrees C, but measurements become inaccurate below -200 degrees C. Why am I receiving these incorrect measurements? Solution: NI-DAQmx 8.1 and earlier, NI-DAQmx Base, and LabVIEW use polynomial scaling (voltage to temperature) for thermocouple measurements, but the temperature range that the software supports is not consistent with the minimum accurate range of the inverse polynomial coefficients used for several thermocouple types. The attached VI can replace the Convert Thermocouple Readings VI in LabVIEW. With this example, you can use table scaling instead of polynomial scaling for the thermocouple voltage, which is more accurate and supports the full temperature range of the thermocouple. Related Links: KnowledgeBase 0H78N438: Thermocouple Temperature Conversion Is Inaccurate At High Temperatures KnowledgeBase 3NAFKU9N: Error -200077 (0xFFFCF273) During Thermocouple Measurements with SCB-68 or SCXI KnowledgeBase 3EEG56IT: Thermocouple Temperature Reading Drops When Measuring Higher Temperatures KnowledgeBase 465HI1BH: How Accurate are Thermocouples? Attachments:
Report Date: 05/09/2006 Last Updated: 11/05/2007 Document ID: 3X8DOKHF |
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