Thermocouples and Cold End Compensation in Temperature Measurement

Temperature measurement is a basic and common measurement indicator in any system design. As key data in many end devices, there are many measurement methods and various components to choose from.

RTD, NTC, and thermocouples are all representative choices for temperature measurement. In different system designs, the appropriate temperature sensing method needs to be selected according to the required accuracy and the measured temperature range while also considering various aspects such as linearity, power consumption, and cost.

Temperature Sensing - Thermocouple

Different temperature sensing has different advantages and disadvantages. Generally speaking, the thermocouple is a relatively inexpensive temperature sensing for a wide range. Its small size, extreme speed, and low output impedance are relatively advantageous features, and it can measure extreme temperatures that are not covered by many temperature sensing ranges.

The fast response is due to the thermocouple's low heat capacity, especially when the sensing junction is exposed, and the thermocouple can respond to temperature changes within hundreds of milliseconds. Also, the inherent voltage output of the thermocouple eliminates the need for an excitation power supply, which greatly reduces the self-generated heat of the device itself.

On the other hand, thermocouples have several disadvantages over other temperature sensing, with low-level output, poor sensitivity, and non-linearity being additional concerns when choosing a thermocouple.

Low-level output means that a stable signal conditioning component is required. Otherwise, the accuracy of the entire temperature measurement system will be difficult to meet expectations. Components in a thermocouple system must be connected with great care, and unexpected thermocouple effects (e.g., solder and copper producing a three μV/°C thermocouple) can make it difficult to achieve the desired standard accuracy of the entire "end-to-end" system.

Even if the signal is well adjusted and no additional errors are introduced, the internal inaccuracies cannot be eliminated due to the metallic nature of the thermocouple itself.

In general, thermocouple measurement accuracy can only reach the measurement accuracy of the joint reference temperature, which is about 1°C to 2°C. And when thermocouples measure millivolt-level signal variations, they are susceptible to noise generated by stray electric and magnetic fields.

Thermocouple Cold-End Compensation

Cold-end compensation, when talking about thermocouples that can not get around the point, thermocouples that want to achieve the desired accuracy must be used to provide error correction for the cold-end compensation. Only by knowing the exact measured cold end temperature can the thermocouple measurement end temperature be measured and the accuracy of the standard device be improved.

Although accurate enough, cold-end compensation using the thermostat method is only suitable for laboratory measurements. In most practical applications, placing the thermocouple reference junction in an ice bath is somewhat impractical. Therefore, in most practical applications, the cold-end compensation technique is chosen.

This method requires an additional temperature sensor to measure the reference point temperature, usually an RTD, NTC, or an integrated temperature-independent IC.

Different sensor options are limited; for example, RTD measurement will be very accurate but high in size and cost, while NTC response is very fast but easy to drift.

Using a controlled cooling compensator to electronically simulate an ice bath is also possible. The cold junction compensator circuit does not maintain a constant temperature but tracks the cold junction. This tracking has the same effect as keeping the cold junction at a constant temperature but is simpler to implement. It allows the thermocouple output to be expressed as a slope over the expected cold junction temperature range.

Cold Junction Compensator, ADI

This cold junction compensator IC requires a low supply current to minimize self-heating and to ensure that it can operate at isothermal temperatures with the cold junction. Special curvature correction circuitry within the cold junction compensator matches the "bend" in all thermocouple outputs to maintain accurate cold junction compensation over a wide temperature range.

Thermocouple Related Considerations

Thermocouple lines are often exposed to static and unexpected high voltages and require circuit protection. The added series resistance can be used as a means of protection. Still, the impact of the component on the overall accuracy of the thermocouple needs to be evaluated, especially when high current limiting resistors are used. Similarly, integrated circuit bias currents combined with high-value protection resistors can protect thermocouple circuits well and produce significant measurement errors.

The trade-off between protection and accuracy is a compromise that needs to be considered in conjunction with the actual application.

On the other hand, because thermocouples cover a wide range of temperature extremes, there are applications where thermocouples are exposed to continuous operation at high common-mode voltages, with the attendant serious noise problems, especially in industrial environments. Under these conditions, current isolation of the thermocouple and the signal conditioning circuit is required.

Previous: CR1220 Battery Equivalent, Specification, Application

Next: EMC Design Techniques