SC Express Increases Thermocouple Measurement Accuracy

The NI SC Express family provides high-performance PXI Express data acquisition with integrated signal conditioning for directly measuring sensors. The NI PXIe-4353 high-accuracy thermocouple module features 32 channels for precise thermocouple measurements that can scale to high-channel-count systems. The NI TB-4353 front-mounting terminal block includes screw terminal connectivity and a unique design to minimize thermal gradients.

Figure 1. The NI PXIe-4353 thermocouple module and TB-4353 terminal block can achieve 0.26 °C measurement accuracy.

Thermocouple Measurement Accuracy Considerations

The NI PXIe-4353 thermocouple module and TB-4353 terminal block incorporate many design technologies to minimize error and achieve accuracy as low as 0.26 °C. Thermocouple measurement error can be attributed to a variety of sources including noise, resolution, offset error, and gain error of the device, as well as external noise, thermocouple error, and cold-junction compensation (CJC) error.?

External Noise

Thermocouple output signals are in the millivolt range, making them susceptible to noise. Filtering for high-frequency noise is an important feature to look for, and in particular 50 Hz and 60 Hz noise filtering can be important for eliminating power line noise that is common in laboratory and plant settings. The NI PXIe-4353 module uses a 24-bit delta sigma analog-to-digital converter (ADC), which includes built-in digital noise filtering, with 50 Hz and 60 Hz notch filters when used in the high-resolution mode.

Device Noise and Resolution

The input range of your thermocouple device should also be evaluated to ensure you get the best possible resolution. Because all thermocouple types fall between -10 mV and 78 mV, you should look for devices that match this input range so that the full resolution of the ADC is being used. NI PXIe-4353 has an 80 mV input range to measure all thermocouple types.?

Device Offset and Gain Errors

Offset errors can impact the absolute accuracy of your measurements, and are often a larger impact on the accuracy than gain error because most thermocouple signals are very close to 0 V. NI PXIe-4353 includes an autozero function that automatically measures and accounts for offsets in the input circuit for every measurement. Using this feature reduces the offset errors and drift to almost negligible levels relative to other sources of error.

Thermocouple Errors

These are errors introduced by the thermocouple itself. The voltage generated by the thermocouple is proportional to the temperature difference between the point where the temperature is being measured, and the point where it connects to the device. Temperature gradients across the thermocouple wire can introduce errors due to impurities in the metals, which can be very large relative to most measurement devices. You should refer to the documentation of your thermocouple to understand its accuracy impact to the overall measurement.

Cold-Junction Compensation (CJC) Errors

CJC errors represent the difference between the actual temperature at the point where the thermocouple is connected to the measurement device (the cold-junction temperature), and the adjacent thermistor temperature, which is measured by the CJC channel of the device. The accuracy of this cold-junction temperature measurement will have a direct impact on the accuracy of the overall thermocouple measurement, and is often the largest error in many thermocouple measurement devices. ?

NI PXIe-4353 compensates for the temperature at the cold junction by measuring the thermistor that is in closest proximity to the thermocouple channel. From this reading, the NI-DAQmx driver software automatically determines the proper offset and applies it to the thermocouple measurements. The two primary contributors to the accuracy of the CJC temperature measurement are thermistor error and isothermal error. NI PXIe-4353 with the TB-4353 has a typical CJC error of 0.22 °C, which includes the combined thermistor and isothermal errors, when used at room temperature.

Thermistor Error

In many devices, the error from the temperature sensor is a significant contributor to overall CJC error. Thermocouple devices typically measure the cold-junction temperature with a thermistor because of their high accuracy over the operating temperature range required by most devices. Because the relative change in resistance in thermistors is very high compared to the change in temperature, the accuracy impact from the measurement device is usually low relative to the thermistor accuracy. To minimize this error, the TB-4353 uses multiple high-accuracy thermistors to measure the cold-junction temperature for the thermocouple channels.

Isothermal Error

Isothermal error is defined as the temperature difference between the thermistor and the actual temperature of the cold junction. This temperature difference is impacted by the design of the measurement device, and the system environment. Temperature gradients are created inside the terminal block when the temperature of the system changes, or when there are heat sources adjacent to the terminal block, such as the PXI chassis or other nearby devices.

To reduce susceptibility to temperature gradients and to improve the accuracy of the CJC measurement, the TB-4353 terminal block features eight thermistors that are distributed on the terminal block (shown in Figure 1). This is an advancement over thermocouple measurement devices that include only one thermistor for measuring cold-junction temperatures. In addition, the TB-4353 has a unique design that allows heat to dissipate evenly between the cold junctions and the thermistors. This minimizes temperature gradients across the set of channels associated with each thermistor.?

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Figure 2. The TB-4353 has eight CJC thermistors to reduce susceptibility to temperature gradients across the terminal block. The dashed lines indicate the channel to CJC association.

Improving Isothermal Errors

Because the isothermal error comes from the temperature difference between the cold-junction and the CJC sensor, anything that can introduce a temperature gradient in the thermocouple device will affect this error. The TB-4353 incorporates several design techniques to significantly reduce isothermal errors; however, the error can be further minimized by paying attention to the system setup. Thermal gradients in your system can impact the magnitude of the isothermal error, so follow these guidelines to improve your overall accuracy:

• Use the smallest gauge thermocouple wire suitable for the application. Smaller wire transfers less heat.
• Run thermocouple wiring together near the terminal block to keep the wires at the same temperature.
• Avoid running thermocouple wires near hot or cold objects. If you connect any extension wires to thermocouple wires, use wires made of the same conductive material as the thermocouple wires.
• Minimize adjacent heat sources and air flow across the terminal block.
• Keep the ambient temperature as stable as possible.
• Keep the terminal block, module, and chassis in a stable and consistent orientation.
• Install the PXI Express filler panels in any unused slots.
• When stacking the system, avoid stacking above a heat source as heat rises and can introduce error.
• Allow thermal gradients to settle after temperature change in system power or in ambient temperature. A change in system power can happen when the system powers on. Refer to the warm-up time in the NI PXIe-4353 specifications for more information.
• Replace the fan filters in the PXI Express chassis regularly to ensure proper system cooling.

SC Express Thermocouple Measurement Accuracy

The measurement accuracies for the NI PXIe-4353 and TB-4353 with a typical system setup in a room temperature (23 °C) environment is shown in Table 1. You can achieve thermocouple measurement accuracy less than 0.3 °C for the most common thermocouple types (J, N, K, T, E). See the NI PXIe-4353 specifications for more detailed accuracy specifications, including additional thermocouple types and maximum specifications.?

Table 1. Typical measurement accuracy is less than 0.3 °C for common thermocouple types.

SC Express Achieves Higher Measurement Accuracy

Thermocouple measurement error can be attributed to many sources including noise, resolution, offset error, and gain error of the device, as well as external noise, thermocouple error, and CJC error. The NI PXIe-4353 high-accuracy thermocouple module and TB-4353 isothermal terminal block minimize these errors to achieve high measurement accuracy.

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