Isothermal Errors and Cold-Junction Compensation (CJC) Data Scaling for the NI 9211E and NI 9219EHardware: Reconfigurable I/O (RIO)>>NI Single-Board RIO>>Analog Input>>NI 9211E, Reconfigurable I/O (RIO)>>NI Single-Board RIO>>Analog Input>>NI 9219E
Problem: How do I determine the isothermal error for the NI 9211E and NI 9219E? Solution: The accuracy of a thermocouple measurement largely depends on how accurately the cold-junction temperature is measured. The NI 9211E and NI 9219E circuitry uses a thermistor to measure the cold-junction temperature. The thermistor is next to the front connector and measures the air near the terminals where the cold junctions are made. The difference between the temperature of the thermistor and the cold junction is the isothermal error. When taking thermocouple measurements, it is critical to minimize heat sources and drafts around the thermocouple wires and the thermistor, such as power dissipation from internal power supplies. The NI 9211 and NI 9219 modules include a module enclosure, which helps stabilize the environment surrounding the onboard thermistor and thus minimizes isothermal errors. The NI 9211E and NI 9219E are board-only versions of the modules developed specifically for the NI RIO platform and do not include an enclosure, so you must test isothermal errors in the end product to verify the accuracy of the module. Testing Isothermal Errors and Determining an Isothermal Error Offset Constant National Instruments fully tests the NI 9211 and NI 9219 isothermal errors in the standard module enclosures.NI determines the isothermal errors by measuring the temperature difference between the thermistor and the cold junction of each channel, under all typical user configurations. This includes variations of chassis power, adjacent module power, slot configuration, and module orientation. After compiling all the isothermal errors in different configurations, NI calculates the isothermal offset constant by taking the mean of the minimum and maximum isothermal errors. A positive offset indicates that the cold junction is colder than the thermistor reading. NI includes the maximum isothermal error deviation from this offset constant in the cold-junction compensation sensor accuracy specifications in the module operating instructions. Scaling CJC Data You can calculate the temperature of the thermistor by converting the voltage on the CJC channel (s) to resistance, then converting the resistance to temperature. To determine the voltage on the CJC channel(s), set the Calibration mode to Raw in NI-RIO to receive binary data from the module. Use the following equation to convert the binary value to voltage: VT = (Range ÷2Bits) x Binary where VT = thermistor voltage reading Range = 5 V for both the NI 9211E and NI 9219E Bits = 24 for the NI 9211E and 16 for the NI 9219E Binary = binary number returned from the module for the CJC channel Then use the following equation to convert the voltage to resistance: RT = RS x VT ÷ (VREF - VT) where RT = thermistor resistance reading RS = 10 kΩ VT = thermistor voltage reading VREF = 2.5 V for the NI 9211E and 5 V for the NI 9219E Finally, use the Steinhart-Hart equation to convert the thermistor resistance to temperature. Because in this case the thermistor reading is warmer than the cold junction, you subtract the isothermal offset error from the Steinhart-Hart equation: T = [ 1 ÷ [A + B(ln(RT)) + C(ln(RT))3]]-(273.15 + OffsetConstant) where, for the NI 9211E and NI 9219E: T = thermistor temperature in °C A =1.2873851 x 10-3 B =2.3575235 x 10-4 C =9.4978060 x 10-8 RT = thermistor resistance reading 273.15 = the conversion from Kelvin to Celsius OffsetConstant = isothermal offset constant in °C For example, when used in a CompactRIO chassis, the NI 9211 has an offset constant of +0.7 °C and the NI 9219 has an offset constant of +1.5 °C. Related Links: KnowledgeBase 3R27J3GH: What is CJC? Developer Zone Example: NI 9211E & 9219E CJC to Temp Conversion.vi Attachments:
Report Date: 04/15/2009 Last Updated: 05/26/2009 Document ID: 4WEE00P4 |
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