There’s no simple solution to achieving accurate temperature measurement. It’s a combination of knowing the inherent accuracy of particular sensor types, and how environmental factors can create further measurement uncertainty and the sensor calibration techniques available to reduce this uncertainty.
Thermocouples are the smallest, fastest and most durable temperature measurement solution. They can withstand very high temperatures, harsh mechanical punishment and are simple to operate. Their size allows for rapid temperature response times and the sensing junction can often be placed very close to the desired point of measurement. The durability and simplicity of this sensor type makes them ideal for embedding into devices or equipment.
However, the thermocouple is most at risk from accuracy, noise and precision error. When extreme accuracy and precision is required, many of these shortcomings can be ameliorated by good engineering practice. These include: using short runs (less than 500 ft., certainly less than 1,000 ft.), assuring that thermocouple wire is insulated and shielded, avoiding extra wire terminations, keeping wire away from high voltage equipment, and increasing the gauge of the wire. When greater accuracy is needed, some success has been found by using balanced, low-pass filtered differential amplifiers (to avoid common-mode voltage offsets), and following relatively complex calibration procedures. A common problem with thermocouples is manufacturing mistakes.
Lack of alloy homogeneity in thermocouples presents additional challenges. Deviations in metal purity and alloy homogeneity increase wire resistance resulting in deviating from the National Institute of Standards and Technology (NIST) standards. Longer runs of wire increase the chance of this type of problem. Some thought also should be given to materials selected for insulation and how the wire is shielded. When high accuracy is required, without calibration, a thermocouple type that consists of a minimum number of elements like a type T, J or G should be used.
Thermistors are ideal for applications requiring a balance of high sensitivity, accuracy and responsiveness. However, they are usually limited to a relatively narrow range of temperatures (typically less than 300˚C). Unlike thermocouples, thermistors cannot endure high temperatures or mechanical stresses, which makes them difficult to use in applications and assembly operations where these influences are not well controlled. To compensate for this limitation, the sensor can be encased in a protective metal enclosure — but this will be at the cost of thermal responsiveness. Some special thermistors are capable of working to temperatures of 1,000˚C.
Local signal conditioning is recommended for thermistors. Fortunately, it is much simpler than conditioning required for thermocouples. Thermistors tend to be larger than thermocouples, resulting in correspondingly slower responses — but faster than RTDs. Likewise, size can reduce their accuracy because they cannot be located as close to the point being measured as equivalently placed thermocouples. For thermistors, other accuracy problems exist.
Near athermistor’s maximum sensitivity point, small changes in temperature produce relatively high changes in resistance: a non-linear response. Away from the maximum sensitivity point, thermistors are less able to resolve changes in temperature. Padding resistors may be added in a voltage divider circuit to obtain a more linear response. As with thermocouples, there are some manufacturing problems.
Thermistors can be made relatively uniform in batches, but batch-to-batch variations can be problematic when high precision or accuracy is required. Additionally, there are no NIST standards for thermistors, so there may be additional manufacturer-to-manufacturer response variations.
RTDs are suitable when extremely stable and precise measurements are required, because they have less drift than other elements. They are usually the best choice when accuracy over a prolonged time is the most important quality. The accuracy and precision of RTDs often exceeds that of both thermistors and thermocouples. Accuracy is a measurement against a measurement by a superior, calibrated device. Precision is a statistic of repeatability (Figure 1). RTDs follow Deutsche Industrie Normen (DIN) and Joint Information Systems Committee (JISC) national standards.
There are fewer manufacturing problems with RTDs. With good tolerance specifications, off-the-shelf RTDs are very consistent regardless of their batch number. RTDs are very delicate. While the melting temperature of an RTD element is sufficiently high to survive many high-temperature manufacturing operations, they do not survive aggressive mechanical operations, such as compaction. It is difficult to embed RTDs into custom mechanical devices. Using metal-sheathed assemblies can protect the RTD but at the cost of response time and bulkiness. For example, Watlow manufactures cartridge heaters with embedded thermocouples. Insulation material is compressed around the thermocouple at very high pressure — called swaging. Attempts to achieve greater accuracy with an RTD have failed because they are too fragile to survive this process.
For a typical 100-ohm RTD, resistance from long lead runs and multiple terminations can become a significant source of error. This is also true for thermocouples; but it is more difficult to overcome the resulting effect on thermocouple accuracy. This is not true for RTDs. Changing from a two-wire to three- or even four-wire RTD can greatly improve accuracy. The electronics can be constructed to dynamically remove error associated with lead resistance, but there is also a trade-off in terms of cost and the number of wires required to perform the measurement. Another consideration is the environment: only the two-wire configuration is intrinsically safe.