High-accuracy temperature measurements in any environment
07 September 2018
Although temperature measurement is a common requirement for many applications, developers face significant challenges in ensuring highly accurate results. In this tutorial, Digi-Key Electronics explains how overcoming these challenges often leads to complex designs and extended design cycles – but also how new measurement devices are helping reduce the complexity.
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In building robust temperature measurement systems, designers draw on a variety of sensor types to meet their specific requirements for cost, accuracy and temperature range. Among sensor types, thermocouples are commonly used in harsh environments, with their ability to measure temperatures as low as -265°C to over 1,800°C.
Thermocouples generate a voltage that is a function of the temperature difference between the tip and its cold junction – the end of two wires used to create the thermocouple. As a result, overall measurement accuracy depends on accurate measurement of both the thermocouple voltage and the cold junction.
While thermocouples generate voltage gradients due to the Seebeck effect, other common temperature sensors, including resistance temperature detectors (RTDs), thermistors and even diodes, require an excitation current to produce a temperature-dependent voltage output. As resistive devices, RTDs and thermistors further require a precision sense resistor placed in series with the excitation current source.
The sense resistor creates a resistor network with the resistive device for ratiometric measurement of the voltage across the sensor. Finally, for each type of sensor, developers need to apply appropriate methods for converting measured results to linearised temperature data, using lookup tables or equations.
Besides dealing with the sensor, developers face multiple challenges in ensuring proper operation of the temperature measurement system. Temperature sensors are typically placed in locations exposed to harsh conditions in factories, commercial environments and homes – wherever applications require the ability to measure temperature gradients in air or fluid flow.
In industrial applications, long cable runs between sensor and measurement system inputs expose them to electrical noise, wear and tear, and external voltage sources that can damage both sensors and their measurement systems. Engineers employ a variety of methods for dealing with the various factors that can impact performance in temperature measurement systems...
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