Thermocouple Measurement
The NI PXI-2527 and the NI TB-2627 can measure thermocouples.
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Tip NI software can convert a thermocouple voltage to the thermocouple temperature. For example code, visit ni.com/zone, click the Example Code link in the navigation bar, and enter PXI-2527 in the Search field. |
When measuring thermocouples, be sure to account for error in the measurements. The total error in thermocouple measurement is the sum of the system error (determined by the thermal EMF of the NI PXI-2527 and the CJC temperature of the NI TB-2627) and the thermocouple error (determined by the type of thermocouple used).
Determining the System Error
To determine the system error for the NI PXI-2527/TB-2627, first calculate the thermal EMF error of the NI PXI-2527 using the following equation.
| EEMF = [(T+1 – T) / (V+1 – V)] × VEMF | (1) | |
| where | EEMF represents the thermal EMF error of the NI PXI-2527 | |
| T is the temperature being measured, in degrees Celsius | ||
| T+1 is (T + 1 °C) | ||
| V is the voltage that corresponds to T | ||
| V+1 is the voltage that corresponds to T+1 | ||
| VEMF represents the thermal EMF of the NI PXI-2527 |
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Notes In thermocouple reference tables, T and T+1 are known values used to calculate the slope of the thermocouple Temperature vs. Voltage graph. Refer to a thermocouple reference table to determine the values of V and V+1 that correspond to T and T+1, respectively. Refer to the NI PXI-2527 Specifications to determine the thermal EMF value of the NI PXI-2527. For optimal thermocouple measurement performance (VEMF = 2.5 µV), power down the latching relays of the NI PXI-2527. For more information about powering down latching relays, refer to the Power Down Latching Relays After Debounce property in NI-SWITCH or the Power Down Latching Relays After Settling property in NI-DAQmx. |
After you have determined the thermal EMF error using Equation 1, calculate the system error using the following equation.
| ES = EEMF + ECJC | (2) | |
| where | ES represents the system error of the NI PXI-2527/TB-2627 | |
| EEMF represents the error due to thermal EMF of the NI PXI-2527 | ||
| ECJC represents the error due to CJC temperature sensor of the NI TB-2627 |
Example
Measuring a K-type thermocouple at 200 °C with a CJC temperature of 25 °C, the system error of the NI PXI-2527/TB-2627 is calculated in the following example.
Assuming typical thermal EMF (2.5 µV), first calculate the error due to thermal EMF using Equation 1.
| EEMF | = | [(201 °C – 200 °C)/(8.178 µV – 8.138 µV)] × 0.0025 µV |
| = | 0.063 °C |
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Note In this example, the values of V and V+1 are found in the thermocouple reference tables of Omega Engineerings The Temperature Handbook. Vol. 29. Stamford, CT: Omega Engineering Inc, 1995. |
To determine the system error, add the error due to thermal EMF to the error due to the CJC temperature sensor using Equation 2.
| ES | = | 0.063 °C + 0.5 °C |
| = | 0.563 °C |
Determining the Thermocouple Error
Independent of the NI PXI-2527/TB-2627 system, thermocouple error is the greater of the following values: ± a temperature range or ± a percent of the measurement.
In the example, a standard grade K-type thermocouple is used to measure 200 °C. The Temperature Handbook lists the error for a standard grade K-type thermocouple as ±2.2 °C or ±0.75% of the measurement temperature. Because ±0.75% of 200 °C (±1.5 °C) is less than ±2.2 °C, the error of a standard grade K-type thermocouple is ±2.2 °C.
Determining the Total Error
The total error in thermocouple measurement is the sum of the system error and the thermocouple error. Use the following equation to determine the total error in thermocouple measurement.
| ET = ES + ETh | (3) | |
| where | ET represents the total error in thermocouple measurement | |
| ES represents the system error | ||
| ETh represents the thermocouple error |
To determine the total error in thermocouple measurement in the example, add the thermocouple error to the system error using Equation 3, as illustrated in the following calculation.
| ET | = | 0.56 °C + 2.2 °C |
| = | 2.76 °C |
Assuming typical thermal EMF, the total error in thermocouple measurement at 200 °C for the NI PXI-2527/TB-2627 with a K-type thermocouple is ±2.76 °C.