Bench Talk

Pros & Cons of Different Temp Sensors

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Temperature is among the most measured real-world parameters, and for good reasons: It affects us personally; helps assess personal and system health; affects industrial processes, chemical activity, baking, safety, and reliability; and much more. The required accuracy for temperature measurement depends on the application, but most applications are satisfied with an accuracy of ±1°C. Beyond accuracy, some applications need to measure temperature over a wide range, while others need to measure it over a limited range but at extremes, and these variables can complicate sensor choice.

Fortunately, there are many ways to measure temperature using both direct contact and at-a-distance non-contact arrangements. Here, we will focus on contact temperature sensors.

The choice of the “right” contact temperature sensor involves balancing many competing and sometimes conflicting factors, including range, sensitivity, accuracy, interface challenges, response time, ruggedness, and size.

The appropriate choice for a wide-range, moderate-accuracy sensor will be very different than the one for a narrow-range, high-accuracy sensor; the most challenging requirement to meet is wide range with high accuracy across the entire range. Many common sensing applications, such as consumer appliances, require lower accuracy or a narrower range. In contrast, science experiments may demand high precision and stability over a range determined by the project, while automotive applications—which often embed tens of sensors—will need different combinations depending on the location and function, such as cabin interior versus under-the-hood engine management.

Among the widely used contact-type temperature sensors are thermocouples, thermistors, and resistance-temperature detectors (RTDs). These are based on clear principles of physics and materials and have specific attributes that may be beneficial, essential, or detrimental in a given situation. That's why it makes sense to better understand the principles, interfacing issues, and performance characteristics of each type.

Thermocouples

A thermocouple consists of two wires of dissimilar metals that are welded together at their tips (Figure 1). They are rugged and small due to their low thermal mass for fast response. The sensed temperature at this junction generates a thermoelectric potential (electromotive force, or voltage) roughly proportional to the temperature difference between the two tips. This thermoelectric potential is known as the Seebeck effect.^[1]

Figure 1: The commonly used schematic diagram symbol for the thermocouple shows its construction as two wires connected at a point.(Source: Author)

The Seebeck effect’s millivolt-level voltage needs cold-junction compensation to correct the temperature of the reference junction, where the thermocouple's leads connect to the signal-conditioning copper wire and connector interface circuitry (Figure 2).

Figure 2: While the thermocouple creates its own voltage and so needs no power source, it needs an external sensor and circuit for cold-junction compensation to take its connector block's ambient temperature into account. (Source: J. G. Rocha, V. Correia, M. Martins, and J. M. Cabral,^[2] Redrawn by Author)

Also, the output voltage must be corrected for its inherent non-linearities. While this was traditionally accomplished using mirrored, nonlinear analog circuitry, it is now done using a look-up table in memory or a high-order polynomial curve-fitting equation in a memory-versus-speed tradeoff.

Thermocouples are highly standardized sensors with different metal pairings to handle different temperature ranges from as low as −200°C up to nearly +2,000°C. Their sensitivity varies by thermocouple type but is fairly low, typically on the order of 40µV/°C. The voltage is translated by detailed voltage-versus-temperature tables for each type. These tables are published by the National Institute of Standards and Technology (NIST), which also provides curve-correction polynomial equations.

Thermistors

A thermistor (Figure 3) is a device composed of metal oxides formed into a tiny bead and encapsulated in epoxy or glass. Its resistance varies in rough proportion to temperature. The resistance of a negative temperature coefficient (NTC) thermistor decreases with increasing temperature. (There are also positive temperature coefficient (PTC) thermistors, but they are used for circuit protection, not temperature sensing.)

Figure 3: The IEC (left) and ANSI symbols for a thermistor make it clear that this temperature sensor is a variable resistor. (Source: Author)

Thermistors are low-cost and highly sensitive, with a resistance change on the order of tens of ohms per degree; however, they offer a limited useful temperature range. Thermistors feature good linearity over a limited temperature range, but linearity decreases over the full range.

In operation, thermistors are usually driven by a current source (typically 10mA to 100mA), and the voltage across the thermistor is measured in a three- or four-wire Kelvin arrangement to eliminate the effect of lead-voltage drop. Thermistor beads are tiny, making them a good choice for small-area sensing, but they are not as standardized as thermocouples or RTDs, so interchangeability can be an issue.

RTDs

The RTD sensing element is a wire coil or deposited film of pure metal. The resistance of this element increases with temperature in a known, constant, and repeatable manner (i.e., a positive temperature coefficient). Like the thermistor, RTDs are current-driven sensors, but with better performance and higher sensitivity. Standard models often sense from −260°C to +650°C. From a basic electrical-model perspective, the RTD is similar to the thermistor, and they even have similar schematic symbols, but their parameters are quite different (Figure 4).

Figure 4: Temperature versus resistance relationship for NTC thermistors, PTC thermistors, and RTDs. (Source: Analog Devices, Inc.)^[3]

Lower-cost, less accurate RTDs use copper, nickel, or nickel-iron wires and may be adequate for certain applications. The highest-performance RTDs use platinum wires and have a nominal resistance of 100Ω (designated by the IEC 60751 industry standard as Pt100), but other values, such as 1000Ω, are also used. One important attribute of platinum is its excellent long-term stability, often critical for scientific work. The Pt100 sensor has a resistance of 100Ω at 0°C, with a temperature resistance coefficient of 0.385Ω/°C at 0°C, while the Pt1000 version has a temperature coefficient that is a factor of ten greater than that of the Pt100.

RTDs offer a more linear response than thermocouples or thermistors, and nonlinearities can be corrected using circuitry, look-up tables, or polynomial equations. As with thermocouple linearization and curve fitting, these equations invoke higher orders and more coefficients for increasing accuracy and are thus more complex and time-consuming to execute, while the most accurate are the third-order Callendar-Van Dusen equations.

Making the Decision

At first, deciding which type of contact temperature sensor to use may seem difficult, but narrowing the choices is usually straightforward. Once the basic sensing requirements of accuracy and range are established along with other priorities, the better choice usually comes into focus. Modern ICs greatly simplify the issues by providing a high-performance analog front end for the sensors. Each cited temperature sensor—thermocouple, thermistor, and RTD—has a different set of attributes and tradeoffs, with no single type dominating across all objectives (Table 1). Note that this table provides a general view of the sensors, but many exceptions and special models exist.

Table 1. General comparison of thermocouple, thermistor, and RTD attributes

Sources

[1] https://doi.org/10.1016/j.psep.2022.03.070
[2] https://doi.org/10.1109/IECON.2009.5414829
[3]  https://www.analog.com/en/resources/analog-dialogue/articles/thermistor-temperature-sensing-system-part-1.html8422266.