A Guide to Temperature Sensor Selection

In putting together any circuit for temperature detection, part of your job as a designer is keeping costs under control. This responsibility can and should influence your choice of a temperature sensor. Let the application requirements guide your selection and make the most affordable decision that meets all your needs for reliability, accuracy, and performance. Many practical considerations influence your sensor selection.

Temperature Range

Temperature range is one of the first matters to consider when picking out a temperature sensor. When your operating environment is above 1000 °C, for instance, your choice is limited to robust thermocouples. Fortunately, very few applications involve environments that extreme.

As a rule of thumb, operating temperature ranges are narrow for most industries, including automotive, medical, industrial, and consumer use. Components that rely on semiconductors restrict the temperature range even further. Consumer and commercial application MCUs are generally rated for a range between 0 °C and 85 °C. In industrial applications, a slightly wider range (-40 °C to 100 °C) is typical. Automotive MCUs feature an even wider range at -40 °C to 125 °C. Conditions within these boundaries allow engineers to pick out conventional temperature sensors.

Containment

Components designed for temperature detection often need packaging based on how they are to be used. When hot oil temperatures need to be measured, for instance, a sensor relying on semiconductors cannot be immersed directly in the liquid.

Inexpensive sensors are often packaged in epoxy. Hermetic glass enclosure is a suitable choice for operating in high-temperature environments. Glass packaging also protects sensors from a range of other potential problems, such as debris and liquid. When strength and durability are high priorities, sensors can be housed in stainless steel. The universal rule is that sensor cost rises along with the complexity of the housing.

Sensors are produced in many different shapes and sizes. Making the right form choice can improve performance, accuracy, and reliability. All temperature sensors generate some heat themselves because of the power they use. Self-heating increases environmental temperatures around the sensor, and this can impede accuracy and lead to errors.

NTC thermistors give the designer the ability to reduce the impact of self-heating by increasing sensor mass. Relatively minor size increases can reduce self-heating significantly. Calling for a thermistor that measures 3 x 3 x 3 mm rather than one that measures 2 x 2 x 2 mm, for example, triple the sensor's volume and mass. Only thermistors deliver this flexibility. Semiconductor sensors are fixed in size by their nature. Mass adjustment is likewise much more limited with wire-based sensors such as RTDs and thermocouples.

Stability

Many design features, including packaging, material selection, and construction, can influence a temperature sensor's long-term drift. An NTC thermistor coated in epoxy will drift by 0.2 °C every year. The same sensor sealed in a glass will drift by only 0.02 °C per year. Platinum RTDs are excellent choices for maximising stability. full-wire RTDs drift by just 0.002 °C/year and film RTDs drift by 0.05 °C/year. Semiconductor-based and thermocouple sensors are much less stable, with annual drift rates of (respectively) 2 °C and 1 °C.

Stability is critical whenever an application demands many years of continuous operation from a sensor. Stability problems can be reduced if the system is designed for regular calibration. Calibration is something of a trade-off, as it may increase the complexity and cost of maintenance significantly. In an ideal design, sensors will be stable enough to last for the system's entire expected operational lifetime.

Accuracy

A system designed for temperature control and compensation will lose reliability and functionality over time if it does not include a reliable detection circuit. A temperature detection circuit's accuracy is influenced by several factors, particularly resolution and responsiveness. The importance of accuracy is paramount when precision in temperature control is required. Accuracy still matters even when temperature merely needs to be maintained reliably.

As an extended example, consider a system where a fan prevents reliability-reducing overheating in the MCU. The fan is activated any time the temperature exceeds a certain upper threshold. A cheap thermocouple may introduce inaccuracy of as much as 5 °C to this measurement. Such thermocouples also commonly feature slow response times, as much as 20 s. With a temperature sensor like this, the system may have to endure another 20 s of rising temperatures before the fan is activated. (Consider that even more time will be required for the fan to reduce the temperature, too.) If the thermocouple also features a stability of 1 °C per year, a 10-year operational lifespan introduces another 10 °C measure of variability into the system.

This system's engineer must bear in mind that the thermocouple detector circuit described above will, in a worst-case scenario, be reporting temperatures 5 °C low and 20 s late.

Accuracy could be increased by calibrating the circuit as part of the manufacturing process. This is an expensive step, though.

The more common and rational choice will be to lower the fan activation threshold to account for the potential inaccuracy of the sensor. This reduces the chance of pushing the system past its reliability limit and causing overheating. Yet this option, too, requires tradeoffs. A generous threshold will activate preventative measures (the fan, in this case) sooner and more often than necessary. This increases power consumption and reduces the reliability of the fan. These are costs and issues that could be avoided by designing a detection circuit with better responsiveness and accuracy.

Of all the basic sensor types available in the -50 °C to 250 °C range, an NTC thermistor delivers the highest accuracy. With the right model of sensor and packaging, stability can be maintained for a long time with accuracy ranging from 0.05 to 1.5. Responsiveness is also excellent with NTC thermistors; common reaction times range from 0.12 to 10 s. In contrast, platinum RTD and semiconductor-based sensors are very slow responders. Typical times are between 1 and 50 s and 5 and 60+ s, respectively. With a detection circuit based on an NTC thermistor, the system can already be taking corrective action while slower sensors haven't yet registered a temperature change.

NTC thermistors allow engineers to build systems with tighter temperature thresholds. This reduces fan wear and power consumption by limiting unneeded cooling. NTC thermistors also feature high accuracy even when constrained to small temperature ranges thanks to their responsiveness and wide-ranging output resistance. These factors contribute to thermistors' popularity for vast numbers of embedded applications.

Noise Immunity

Electrical noise and lead resistance can both have an adverse effect on accuracy if a sensor is susceptible to them. Lead resistance does not matter to thermocouples, but they are highly susceptible to electrical noise. (The problem is more severe with cold junction thermocouples.) Semiconductor-based sensors are immune to lead resistance, but they need the right board layout to protect them from electrical noise. Platinum RTDs reverse the situation: They are nearly immune to electrical noise but struggle with lead resistance. Platinum RTDs lose the most accuracy in 3 and 4-wire layouts. Because they have high initial resistance, NTC thermistors are well-protected against both lead resistance and electrical noise.

Cost

Speaking generally, within any type of sensor, detection circuit cost rises directly along with the circuit's accuracy. Robust design features also cost more. Platinum RTDs are the most expensive option in the -50 °C to 250 °C temperature range, averaging $6. Semiconductor-based sensors are the next most expensive at roughly $0.9. Despite their reputation for low cost, thermocouples sit in the middle of the pack with an average cost of $0.5. NTC thermistors are extremely affordable; a single sensor hermetically sealed in glass costs less than $0.2. Less robust NTC thermistors can be even cheaper, costing less than $0.05 per unit when purchased in bulk.