Converting Output Signals from Temperature Measurements

Temperature measurement sensors generate output signals that require conditioning to transform them into a form that can be used for additional processing.

Signal conditioning comprises of:

  • Amplification
  • Signal Isolation
  • Error Compensation
  • Linearization
  • Excitation

While conditioning is necessary for measurement precision, accuracy also relies on factors such as signal transmission and sensor construction. The transmission distance can affect the signal quality, and the impurities in the metal of sensing devices can lead to temperature gradients that introduce error. Moreover, the method used for transmission and the attributes of the measurement sensor can influence the signal characteristics.

Nonlinearity of Temperature Sensor Devices

Thin Film Detector - Flat RTD Element

Thin Film Detector - Flat RTD Element

A degree of nonlinearity is exhibited by most temperature sensing devices. Each has its own unique signal conditioning requirements and a different mode of operation. Thermocouples operate on the Seebeck Effect. When two dissimilar metals are joined at one end and remain open at the other, creating a voltage at the open circuit, it leads to the Seebeck effect.

The voltage is a direct function of the temperature difference between the point measured on the metals and the junction of the metals. The Seebeck voltage relies on the composition of the thermocouple. The outputs are nonlinear to the temperature measurements, and each type of thermocouple has its own distinctive nonlinearity. From the calibration curves, it can be seen that the nonlinearity of thermocouples result in increased error over a broader temperature range.

Metals such as platinum or copper are used to construct an RTD, in order to increase resistance with increasing temperature. The RTDs may be either wire wound or thin film. Wire wound RTDs comprise of wire coiled around a cylindrical glass or ceramic insulator, while thin film sensors contain a film of material applied to a ceramic insulator which is trimmed until the desired value of resistance is achieved. The resistance vs. temperature curve of an RTD is nonlinear. The nonlinearity could be ignored in cases where the measurement range is narrow. RTDs have an accuracy of ±0.5 to 1 °C, over a range from 0 to 1000 °C.

Made of metal oxides, thermistors may have either a positive or negative temperature coefficient. Positive temperature coefficient thermistors exhibit a linear, increase in resistance with increasing temperatures, while negative temperature coefficient thermistors exhibit a nonlinear, decrease in resistance with increasing temperatures. Thermistors display a much greater signal response and sensitivity to temperature changes than RTDs or thermocouples and therefore can achieve higher accuracies. Nevertheless, the operating temperature range of thermistors is much narrower.

Temperature is measured by infrared temperature sensors by focusing the amount of infrared radiation emitted from an object onto sensors, which transform it to an electrical signal. Also, the amount of infrared energy emitted from an object is directly proportional to its temperature. As the sensor is not in contact with the process being measured, infrared sensors can be effectively used in very high temperature applications where other types of sensors cannot live, or for moving processes such as food cooking on a conveyor belt.

44000 Series

44000 Series

The Impact of Signal Transmission on Conditioning

USB Infrared Temperature Sensor

USB Infrared Temperature Sensor

A continuous signal is used by analog transmission which changes with amplitude to transmit information. Usually, it is mainly used with standard process signals such as 0 to 1 V, 0 to 10 V and 4 to 20 mA. The 4 to 20 mA can travel the longest distance without degradation and hence it is the most commonly used range; in addition it is relatively immune to external noise signals and is often used as a process variable for temperature sensor output.

As soon as the transmitter receives the native output from the sensor, it linearizes the signal depending on the calibration curve for the particular sensor type and subsequently converts the linearized voltage to the 4 to 20 mA current signal. A controller or recording device is used to further process the signal. RTDs and thermocouples generate low millivolt signals that are prone to interference. Besides being more robust, the 4 to 20 mA signal can also be transmitted over a long distance without interference from noise. Additionally, by using a 4 mA variable for the lowest value, transmitter failure can be easily distinguished from a legitimate signal.

An alternative form of differential high speed serial transmission is the Ethernet. Up to 1 GB/second transmission is supported by the Ethernet, which generally requires a dedicated controller and is widely used for home, commercial and industrial applications and lays the foundation for today’s internet communications.

Several encoding schemes are employed to enable the actual measurement information to be transmitted between machines, or in certain situations the internet infrastructure is used globally. Within Ethernet systems, one protocol that is widely used is the TCP/IP. This protocol offers assured data transmissions between two devices and Ethernet connections are supported with a wide number for encryption mechanism to ensure data security.

Conclusion

Long Range Wireless Receiver

Long Range Wireless Receiver

When it comes to nonlinear temperature sensing devices, signals have to be conditioned for linearization and error compensation. The low millivolt output of RTDs and thermocouples also needs to be compensated for by amplification. Transmission controls the accuracy of the signal. When the sensor output is converted to a 4 to 20 mA process output, a more robust signal that can be transmitted over long distances with little interference is obtained. Ethernet and Digital transmission offer signal transfer at higher rates and across greater distances.

This information has been sourced, reviewed and adapted from materials provided by OMEGA Engineering Ltd.

For more information on this source, please visit OMEGA Engineering Ltd.

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