How to choose a a temperature measurement device

Preventing product damage, ensuring sterilization, determining biological health, ensuring mixture blending, controlling chemical reactions (including food cooking), ensuring drying, curing and out-gassing all start with temperature measurement. It is the physical variable most often measured in industrial processes, making its optimization vital. 

To provide a real world example, temperature measurement sensors play an extremely important role in heat-treatment of metals, like the structural steel used in buildings and metals used in aircrafts. In these cases, a manufacturer must be able to guarantee that the metal was heat-treated in a particular way to ensure the metal has the desired properties. Since it is not possible to simply look at something and assess whether it was properly heat-treated, optimizing temperature measurements and finding the right device is key. 

Starting off, Table 1 shows the different sensor types and their physical property that is sensitive to temperature:

The temperature sensor chosen for a particular process depends on its cost, range of operation, sensitivity, response time, repeatability and its ability to survive its environment. There is usually some measurement range overlap, and more than one sensor type may be suitable for an application. For example, biological systems can often be monitored with non-contact infrared (IR) detectors, thermistors or silicon diodes.

Occasionally, the only practical way to monitor an internal temperature is to embed a temperature sensor into a product during the manufacturing process. An example is an electric motor with an embedded resistive temperature detector (RTD), or a thermistor on one of the motor's copper windings.

A thermistor is a type of resistor, generally made of ceramic or polymer, whose resistance varies significantly with temperature.  Unfortunately, thermistors require more complicated software to account for their very non-linear temperature response. 

RTDs use such metals as platinum or nickel. RTDs are useful over larger temperature range, while thermistors typically achieve a higher precision within a limited temperature range, typically −90 °C to 130 °C.

A few industrial processes operate at such high temperatures that only a few sensor types can survive and give repeatable results after temperature cycling. Steel making requires measuring up to 1700 ºC, requiring use of either thermocouples, which can be immersed in molten metals for internal measurements, or IR thermometry. IR optical sensors are sometimes used for high temperature measurements but they can only measure hot surface temperatures. Non-contact IR thermometry may be the only choice if the product is moving or if penetrations are not allowed in the product, for example, ceramic firing in a kiln. Non-contact methods can only measure surface temperatures; internal temperatures cannot be measured.

Table 2 provides on overview of temperature sensor applications:

Measuring with thermocouples

A thermocouple is a sensor for measuring temperature that consists of two electrical conductors made from different metal alloys. Typically, the two conductors are built into a cable that has a heat-resistant outer sheath. At one end of the cable, the two conductors are mechanically and electrically connected together by crimping, welding or other means. This end of the thermocouple (known as the hot junction) is placed at the location that is to be monitored. The other end of the thermocouple (the cold junction, also known as the reference junction) is connected to a thermocouple measurement system. The term "hot" junction is somewhat of a misnomer, since this junction will be subjected to a temperature below that of the reference junction if relatively cold temperatures are being measured. 

Thermocouples generate an open-circuit voltage proportional to the temperature difference between the hot and reference junctions. 

It is important to have an accurate cold junction measurement, even when several thermocouplesare connected to the same board. If the cold junction sensor is too far away from the thermocouple, there may be a significant measurement error – more than 1º C. To avoid such errors, some measurement systems, including Sensoray’s model 2608 and 2418, uses eight cold junction sensors to ensure each thermocouple is less than half an inch from a cold junction sensor. 

It is not unusual to attach the hot end of a thermocouple sensor next to an electrical heater or to the machine frame used by high voltage motors. These two conditions often lead to high voltages finding their way onto thermocouple sensors. Thermal cycling of electrical heaters often weakens their electrical insulation, resulting in a high common mode voltage being applied to the millivolt-level thermocouple signal. This condition obscures the temperature signal and in the worst case destroys the measurement electronics. One solution is to isolate the electronics and let them float up to the high common mode voltage of the thermocouple. Sensoray’s model 2418 provides such channel-to-channel isolation and isolation to its Ethernet data link.

Analog/digital (A/D) products that measure resistance must have a wide measurement range to work with a wide range of resistance values that sensors produce. All supported resistive sensors, including RTDs and thermistors, are acquired by applying a constant voltage across the sensor or by forcing a constant current through the target sensor. Sensoray products use both types: constant voltage for high resistance sensors and constant current for low resistance sensors. Sensoray uses a constant current in a four-wire measurement, two for applying current and two for measuring the voltage at the sensor, to eliminate the effects of signal losses in the connecting wires. 

Precision temperature measurements require special considerations

Sensors must be chosen to survive their environment and produce repeatable results after repeated exposure to temperature extremes. Thermocouples and RTDs can survive high temperatures but sacrifice resolution; they need more complex circuitry, for example, cold junction measurement (for thermocouples) and a four-wire measurement (for RTDs). The millivolt-level signal from thermocouples limits high resolution measurements in noisy industrial environments. 

Malfunctioning machinery can add high common mode voltages that obscure the tiny sensor signals and they can destroy measurement circuitry. Isolated measurement circuitry such as Sensoray’s 2418 minimizes the effects of high common mode voltages while resisting high voltage damage.  

Some applications require resolutions of 0.001ºC. Thermistors provide this resolution at the expense of a limited temperature range; however, unlike thermocouples and RTDs they cannot survive high temperature environments. Thermistors have the fastest time response to temperature changes. They have the added advantage of being inexpensive and not requiring an additional cold junction measurement.

The reversed biased silicon diode is the least expensive sensor. It has a well-defined voltage versus temperature curve, but does not have the thermistors’ high sensitivity – nor can it survive the high temperature environments where thermocouples excel. The reversed biased diode is sometimes built into integrated circuits designed for temperature measurement. The added circuitry makes the diode’s non-linearity appear linear.