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Measuring Temperature
Temperature is a common measurement within validation and production tests. This guide helps you select the best temperature sensor for your application needs. After you decide on your sensor, you can consider the required hardware and software to properly condition, acquire, and visualize temperature measurements.
What is Temperature
Translating temperature phenomena into a signal requires choosing from a variety of sensors. Three common sensor varieties are the thermocouple, RTD, and thermistor. Other temperature sensors such as ICs or Fiber Bragg Gratings exist, but will not be covered in this document.
Thermocouples operate under Seebeck effect principle. When two wires made of dissimilar metals are joined and heated at one end, a thermoelectric circuit is formed which causes a measurable voltage differential known as the Seebeck voltage at the “cold” end. A given pairing of metals varies in temperature range, sensitivity, and error based on the properties of those metals.
Figure 1. Illustration of the Seebeck Effect
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Each type of thermocouple consists of a selective pairing of metals. You need to understand the operating specs of the thermocouple type you select for your measurement. Some thermocouples offer a wide temperature range at the expense of a very nonlinear voltage-temperature, while others provide a smaller and more linear temperature range.
Resistance temperature detectors (RTDs) are measurement devices that operate by actively changing resistance according to the ambient temperature. RTDs typically contain a glass/ceramic core and a thin winding of metal. Platinum is often preffered due to its stability.
Figure 2. Basic Resistance Thermometer Components
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Alternative configurations use different insulation and/or winding materials, which result in different performances and temperature ranges. Another option, a thin-film RTD, consists of a thin layer of metal in between layers of insulating material. This style is best suited for surface temperature measurements because it provides more uniform contact across the surface of the RTD.
The key to the ability of an RTD to measure temperature is the thermal properties of the metal winding. If you understand these properties well, you can reliably predict the temperature at a measured resistance. The predictable resistance-temperature relationship leads to an accurate temperature measurement device.
Thermistors, like RTDs, are active measurement devices that operate by changing resistance with changes to ambient temperature. They consist of a metal oxide semiconductor pressed into a small disk or wafer coated with epoxy or glass. Since thermistors are constructed using semiconductor materials, they provide the best sensitivity of any temperature sensor device and are great for measuring small changes. They also generally have a much higher resistance than an RTD. Unlike an RTD, a thermistor is a negative coefficient device, which means its resistance decreases with an increase in temperature.
Comparing Temperature Sensors
With three different types of temperature sensors, let us compare high-level advantages and disadvantages in Table 1. For more specifics, review the sensor characteristics in Table 2.
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Advantages and Disadvantages of Temperature Sensors
Depending on what your needs are, from hardware capabilities to cost, each sensor has advantages and disadvantages. Prioritize test needs to pick the best sensor for your application.
Temperature Sensor Characteristics
As you select a sensor, understanding the impact of each characteristic on your measurement is critical.. Table 2 compares the different characteristics of the three common sensors.
After selecting the temperature sensor that best suits your application review these further sensor considerations.
Temperature Range
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The temperature range of a sensor defines the temperatures at which the sensor is rated to operate safely and provide accurate measurements.
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Thermocouple: Each type has a specified temperature range based on the properties of the metals used in creating that thermocouple.
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RTDs: Exchanges a smaller temperature range for better linearity and accuracy.
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Thermistors: Provides the lowest temperature ranges but excellent sensitivity.
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By understanding the full range of temperature, you can expose your sensor and help prevent sensor damage while ensuring better measurements.
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Linearity
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An ideal sensor would have a perfectly linear response: a unit change in temperature would result in a unit change in voltage output across the entire temperature range of the sensor.
However, no sensor is perfectly linear. Figure 3 offers an idea of the temperature-to-voltage response of the three sensors this resource examines.
Figure 5. (Left) Temperature-to-Output Response of Sensors, Figure 6. (Center) Sensitivity of Various Temperature Sensor Types, Figure 7. (Right) Response Time of Grounded versus Ungrounded Thermocouples
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Sensitivity
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The sensitivity of a given sensor indicates the percent change in measurable output for a given change in temperature. A more sensitive sensor, like a thermistor, can more easily detect small changes in temperature than a less sensitive sensor, like a thermocouple.
This sensitivity, however, comes at the expense of linearity. This can be an important factor when determining the ideal sensor choice for the temperatures you are measuring. If you intend to capture fraction-of-a-degree changes over a small temperature range, a thermistor or an RTD is more ideal. For capturing larger temperature changes over a wider range of temperatures, a thermocouple may suffice. Figure 4 gives a relative idea of the voltage.
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Response Time
Response time is the measure of time a sensor takes to respond to a change in temperature. Many factors can cause response times to increase or decrease.
A larger RTD or thermistor, for example, has a slower response time than a smaller one. In exchange for this drawback and poorer thermal shunting, a larger RTD or thermistor is less susceptible to self-heating
errors. Similarly, ungrounded thermocouple junctions provide a slower response time in exchange for electrical isolation. Figure 5 shows the relative difference in response times for ungrounded and grounded thermocouples.
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Stability
The stability of a temperature sensor is an indication of its ability to maintain a consistent output at a given temperature.
Material plays a key role in the stability of a given sensor. RTDs are often constructed of platinum for this reason as well as to ensure low reactivity. The substrate to which the platinum is bonded, however, may deform under prolonged exposure to high temperatures, which can cause additional and unexpected strain that leads to a change in measured resistance.
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Accuracy
As with any measurement application, understanding your accuracy needs is critical in ensuring reliable results. Your sensor and measurement hardware selections play a significant role in absolute measurement accuracy, but smaller details such as cabling, relative proximity to other equipment, shielding, grounding, and so on can all affect accuracy as well.
When selecting a sensor, note the specified tolerances and any factors that might impact that specification (for example, prolonged exposure to high temperatures). Also be careful to select a sensor and measurement device with similar accuracies. A tight tolerance RTD comes at a greater cost, but you may not achieve the additional accuracy if you use a low-quality measurement device
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Durability
To ensure your temperature sensors remain operational for the duration of your application, you need to understand the environment in which you are deploying them. Some sensors (thermocouples, for example) are inherently more durable because of their construction. The metals selected for a particular thermocouple, however, have different resistances to corrosion. Furthermore, a sensor encased in an isolating mineral and a protective metal sheath is more resistant to wear and corrosion over time, but it
costs more and offers less sensitivity. You should also note that different sensor configurations may have special mounting requirements to ensure a solid physical and thermal connection.
Choosing the Right Sensor
Consider four main application factors when choosing your temperature sensors:
1. Understand the measurement application and requirements.
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How quickly will the temperature change? Determine an appropriate response time.
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What is the length of deployment/how serviceable will the sensors be? Select a sensor type durable enough to reduce maintenance.
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How much accuracy is required? Consider the impact of sensor accuracy on overall measurement accuracy.
2. Determine the temperature ranges that you must measure.
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Select a sensor type that operates beyond the full range of possible temperatures.
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Consider the linearity of each type that meets your range requirements; select the type with the most linear response over your range of interest to improve voltage- or resistance-to-temperature conversion accuracy.
3. Consider the environment in which you are deploying the sensors.
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Pick a suitable sheathing material to resist any chemical exposure.
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Determine if you need isolation to prevent ground loops/noise.
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Make sure your sensors are rated to withstand vibration or abrasion if they will be exposed to those.
4. Consider how you are mounting your sensors and select an appropriate mounting style to maximize the thermal connection.
After you’ve narrowed between thermocouples, RTDs, and thermistors for your temperature measurement, there are some specific considerations for each sensor category that influence and impact the type of sensor to purchase.
Thermocouple Sensor Considerations
There are a variety of thermocouple types and constructions. The main two aspects of thermocouple sensors are the type and the sheathing configurations.
Types are generally defined by a letter designation—common types are E-, J-, T-, K-, C-, R-, and S-type. NI’s temperature hardware works with all National Institute of Standards and Technology (NIST) standard thermocouples. The thermocouple type defines the metals used to create the thermocouple; therefore, it also defines the operating range, accuracy, and linearity of the thermocouple.
The following graphs depict the voltage response of various thermocouple types over a range of temperatures.
Figure 8. (Left) Temperature Response of Different Thermocouple Types, Figure 9. (Right) Options for Thermocouple Sheathing
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Temperature Sensor Characteristics
When selecting a sensor, understand the impact of each characteristic on your measurements and be sure to select a sensor that aligns closely with your project requirements. Use the following characteristics to define your temperature sensor capabilities and performance. These characteristics apply to all temperature sensor types but with some caveats and corner cases. Table 2 compares the different characteristics of the three common sensors.
After selecting the temperature sensor you will use, thermocouples, RTDs, or thermistors, review further sensor considerations before choosing the right sensor for your applications.
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RTD Sensor Considerations
The various types of RTDs have three primary defining attributes: number of lead wires, wire materials, and physical construction.
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Number of Lead Wires
Since RTDs are active sensors, they require external excitation to produce a measurable voltage drop that can be translated into resistance. Resistance values are generally very low, meaning lead-wire resistance can cause less accurate measurements. Because of this, RTDs often come in multiwire configurations. The number of lead wire configurations can range from 2 to 4.​
Figure 10. (Left) Two-Wire RTD, Figure 11. (Center) Three-Wire RTD, Figure 12. (Right) Four-Wire RTD
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Two-wire RTDs are the simplest to use, but they can affect measurement accuracy if the resistance of the lead wires is significant relative to the measured resistance of the RTD element.
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Three-wire RTDs are the most common in industrial applications. They allow the measurement hardware to characterize and correct for lead-wire resistance, which results in a more accurate measurement. Inconsistencies in the lead-wire resistance, however, can lead to unexpected measurement errors.
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Four-wire RTDs provide separate paths for both excitation and resistance measurement. This allows for lead-wire resistance correction as well as the isolation of any noise in the excitation signal. Four-wire RTDs are also immune to mismatched lead-wire resistances, but they require measurement hardware capable of a four-wire resistance measurement.
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Wire Material
Each RTD-led wire configuration can be made up of different materials. Two key factors that influence the selection of wire materials are deployment environment and temperature range. Understanding the testing environments can limit what materials can be used under different constraints, such as can it withstand expected water submersion, chemicals, and/or abrasion. Wire material and sizing also impact lead resistance, depending on lead-wire configuration.
Figure 13. (Left) RTD Construction Styles, Figure 14. (Right) Thermistor Configurations
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