Process Variables

Hot options for temperature measurement

Thermistors, thermocouples and RTDs offer a range of devices

By Tom Stevic, contributing editor

Several distinct methods exist for industrial temperature measurement. The most common is direct temperature sensing of an object, fluid or gas. A sensor of some type is placed in direct contact with the object to be measured. The sensor changes its electrical or mechanical properties in proportion to the object’s current temperature in a repeatable, measurable fashion. Connecting the sensor to some device that converts the sensor’s change to some meaningful value, such as degrees Celsius (°C), degrees Fahrenheit (°F) or Kelvin (K) creates a control device. Once the object’s temperature is determined, the result is displayed, recorded or used as an input to some control action.

The sensor’s construction and the materials used determine the accuracy and operational temperature range. Perhaps the simplest temperature sensor is a simple bimetal switch or thermostat. The sensor operates by connecting two dissimilar metals. Because metals expand and contract at different rates, the sensor mechanically changes shape as the temperature changes. This change in shape can move a mechanical pointer across a fixed reference for display purposes and if connected to a set of contacts or mercury switch can supply an on-off control signal.

Thermistors are available in two types—negative temperature coefficient (NTC), which decrease resistance as the temperature increases, and positive temperature coefficient (PTC) devices, which increase resistance as the temperature increases. Thermistors are inexpensive devices but require calibration for accurate measurements. Thermistors are available as nonprobe-style elements that can be directly bonded to the device being measured or encased in a tube-like probe that protects the thermistor element from the environment. The electrical resistance of a thermistor is nonlinear and needs a signal-conditioning circuit.

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Thermocouples are temperature measurement devices that use the Seebeck effect. In short, Thomas Seebeck (1770-1831) discovered that, by placing a junction of two different metals in different temperatures, a voltage is produced between the “hot junction” and the “cold junction.” This voltage differential is used to calculate the temperature differential. The voltage created is measured in milliVolts (mV).

Different metal pairs are using the construction of a thermocouple. The thermocouple type designation is expressed as a single capital letter. Type J uses a metal pair of iron (Fe) and constantan copper-nickel (Cu-Ni). Type T uses copper (Cu) and constantan copper-nickel (Cu-Ni). Each thermocouple type has a specific range of temperatures to maintain accuracy. Various manufacturers have published tables that cover all available types, useful temperature range and precision.

Thermocouples are available in a wide variety of styles, from bare junctions to flexible tape to enclosed probes. Probe-style sensors often have a grounded version and an ungrounded version. A grounded probe has the hot junction bonded to the probe material; ungrounded probes keep the junction insulated from the probe material. A grounded probe takes a longer time to react to a temperature because the insulating material must be heated along with the junction. However, an ungrounded probe can provide some measure of protection against electrical interference, such as electromagnetic force (EMF) or radio frequency interference (RFI). A grounded thermocouple reacts faster to a change in temperature but may create a path for noise to enter the electrical circuit.

A disadvantage to using thermocouples in industrial applications is the field wiring from the sensor to the control device. Thermocouple sensor wires cannot be attached to a copper terminal block and then converted to standard copper hookup wire to complete the connection to the measurement apparatus. This limitation can be overcome by using specialty terminal blocks and thermocouple extension wire using the same metals as the thermocouple. Another solution is to place a signal conditioner between the sensor’s wires and converting the signal to a 4-20 mA or 0-10 V signal or even an IIoT module. Standard electrical shielding practices are required in any case.

When operated within its operational temperature range, even a two-wire RTD is more accurate than a thermocouple.

Resistance temperature detectors (RTDs) do not produce a voltage difference as thermocouples do. RTDs take advantage of the physical properties of some metals that vary resistance as a function of temperature. Typically copper, nickel or platinum are used as thin wires wrapped around a ceramic core. A known voltage is applied to the RTD element, and then the voltage drop across the element is measured. In measuring this voltage, the RTD’s temperature can be calculated. Most industrial RTDs are classified as PT100 (100 Ω at 0°C) and PT1000 (1000 Ω at 0°C). However, there are many specialty types available. The sensors are available in two-wire, three-wire and four-wire models. The electrical hookups are complex, but, in this case, more wires mean higher accuracy. When operated within its operational temperature range, even a two-wire RTD is more accurate than a thermocouple. However, an RTD is more expensive than a thermocouple. RTD elements are rather fragile, so they are almost always enclosed in some protective probe.

Most of the configurations are available for thermocouples. Many temperature sensor manufacturers have off-the-shelf stock and the ability to custom-build about any configuration that can be dreamed up. Consider the lead time to build a custom sensor and the time it would take to replace it. Temperature sensors do break, regardless of the type. Periodic testing and calibration tests can reduce the chance of a malfunctioning sensor causing machine downtime.