Overview of Temperature Sensors

Publish Date: Aug 19, 2016 | 106 Ratings | 2.95 out of 5 | Print

Table of Contents

  1. Overview
  2. Steps for selecting a temperature sensor
  3. Temperature sensor characteristics
  4. Next Steps
  5. References

1. Overview

You can choose from a variety of sensors to translate temperature phenomena into a measurable signal. Three common sensor varieties are the thermocouple, RTD, and thermistor. Each has its own operating principles, benefits, considerations, and drawbacks. This document provides context and information to help you select the best temperature sensor for your application needs.

In addition to the characteristics of the sensors themselves—operating range, sensitivity, linearity, response time, and so on—you must consider the requirements each sensor type imposes on the measurement hardware. For example, thermocouples require no current excitation (unlike thermistors) but do need cold-junction compensation, which is available only in certain measurement hardware systems. To better familiarize yourself with the measurement hardware necessary for temperature measurements, download the Engineer's Guide to Accurate Sensor Measurements.

Table 1 provides a high-level comparison of these temperature sensor types:

 

Table 1: Advantages and Disadvantages of Temperature Sensor Types

 

Note: You can use other temperature measurement sensors such as ICs or Fiber Bragg Gratings that are not discussed in this article.

Table 2 shows the different characteristics of the three common sensors this white paper examines:

 

Characteristic

Thermocouple

RTD

Thermistor

Temperature Range

Excellent

-210 °C to 1760 °C

Great

-240 °C to 650 °C

Good

-40 °C to 250 °C

Linearity

Fair

Good

Poor

Sensitivity

Low

Medium

Very High

Response Time

Medium to Fast

Medium

Medium to Fast

Stability

Fair

Good

Poor

Accuracy

Medium

High

Medium

Susceptible to Self-Heating?

No

Yes, Minimal

Yes, Highly

Durability

Excellent

Good

Poor

Cost 

Lowest

High

Low

Signal Conditioning Requirements

Cold-Junction Compensation

Amplification

Open Thermocouple Detection

Scaling

Excitation

Lead Resistance Correction

Scaling

Excitation

Scaling

Table 2: Comparison of Temperature Sensor Types

 

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2. Steps for selecting a temperature sensor

  1. Thoroughly understand the measurement application and requirements.
    • How quickly will the temperature change? Determine an appropriate response time.
    • What is the length of deployment/how serviceable will the sensors be? Select a sensor type durable enough to reduce maintenance.
    • How much accuracy is required? Consider the impact of sensor accuracy on overall measurement accuracy.
  2. Determine the temperature ranges that you must measure.
    • Select a sensor type that operates beyond the full range of possible temperatures.
    • 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.
    • Pick a suitable sheathing material to resist any chemical exposure.
    • Determine if you need isolation to prevent ground loops/noise.
    • Make sure your sensors are rated to withstand vibration or abrasion if you are exposing them to those.
  4. Consider how you are mounting your sensors and select an appropriate mounting style to maximize the thermal connection.
  5. Select the necessary measurement hardware to condition, acquire, analyze, and display/save the temperature signals. Learn more in the Engineer's Guide to Accurate Sensor Measurements.

 

 

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3. Temperature sensor characteristics

Use the following characteristics to define your temperature sensor capabilities and performance. These apply to all types of temperature sensors but with some caveats and corner cases. 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.

Temperature range

The temperature range of a sensor defines the temperatures at which the sensor is rated to operate safely and provide accurate measurements. Each type of thermocouple has a specified temperature range based on the properties of the metals used in creating that thermocouple. RTDs offer a smaller temperature range in exchange for better linearity and accuracy, and thermistors provide the lowest temperature ranges but excellent sensitivity. Understanding the full range of temperatures you can expose your sensor to can help prevent sensor damage while  ensuring better measurements.

Linearity

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. In reality, however, no sensor is perfectly linear. Figure 1 offers an idea of the temperature-to-voltage response of the three sensors this white paper examines.

Figure 1: Temperature-to-Output Response of Sensors

Sensitivity

The sensitivity of a given sensor indicates the percent change in measureable 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 2 gives a relative idea of the voltage.

Figure 2: Sensitivity of Various Temperature Sensor Types.

 

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 3 shows the relative difference in response times for ungrounded and grounded thermocouples .

Figure 3: Response Time of Grounded Versus Ungrounded Thermocouples

 

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.

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.

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.

Cost

As with any aspect of a project, cost can be a key limiting factor. In high-channel-count applications, for example, the linearity benefits of RTDs may be outweighed by the relative increase in cost versus thermocouples. You must also consider the added cost of wiring, mounting, and signal conditioning  when considering total system cost.

Signal conditioning requirements

Each type of temperature sensor requires some level of signal conditioning to adequately acquire and digitize the measured signal for processing. The measurement hardware you select can be just as important in ensuring accurate measurements as the sensor and can mitigate or exacerbate the shortcomings of each sensor type. These signal conditioning features include the following:

  • Amplification
  • Cold-junction compensation (thermocouples only)
  • Filtering
  • Excitation (RTDs and thermistors only)
  • Offset error adjustment
  • Scaling to temperature units
  • Lead resistance correction
  • Channel-to-channel isolation
  • Open thermocouple detection (thermocouples only)

To learn more about these and other hardware considerations for temperature measurements, download the Engineer's Guide to Accurate Sensor Measurements .

 

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4. Next Steps

For specifics on the theory of operation and variations of each sensor type, refer to the sensor-specific whitepapers below:

 

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5. References

http://www.omega.co.uk/temperature/Z/pdf/z019-020.pdf

http://www.engineeringtoolbox.com/temperature-sensors-d_448.html

http://www.itsirl.com/admin/pdfmanual/1420797923pt100acc.pdf

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