What is Temperature Sensor
A temperature sensor is an electrical instrument that measures the temperature of the air, liquid, and solid matter in a wide range of industries and applications. A temperature sensor works by providing a readable temperature measurement on a meter from electrical signals produced inside a temperature probe. The working principle of a temperature sensor depends on the voltage across the diode inside the temperature probe. The change in temperature is directly proportional to the diode's resistance. For example, the warmer the temperature, the more resistance, and vice-versa.
Advantages of Temperature Sensor
Corrosion-proof and rugged construction
The temperature sensors are described as having corrosion-proof and rugged construction, indicating that they are durable and can withstand harsh environments.
Adaptability to data loggers and data acquisition systems
The temperature sensors are mentioned to be adaptable to data loggers and data acquisition systems, suggesting that they can be easily integrated into existing monitoring systems.
Waterproof
The temperature sensors are said to have 'O' ring protection against water ingression, making them completely waterproof. This feature ensures their reliability and longevity in wet or submerged environments.
Unaffected by changes in atmospheric pressure
The temperature sensors are stated to be unaffected by changes in atmospheric pressure, indicating that they can provide accurate temperature readings regardless of variations in air pressure.
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Temperature Sensor Types
Negative Temperature Coefficient (NTC) thermistor
A thermistor is a thermally sensitive resistor that exhibits a continuous, small, incremental change in resistance correlated to variations in temperature. An NTC thermistor provides higher resistance at low temperatures. As temperature increases, the resistance drops incrementally, according to its R-T table. Small changes reflect accurately due to large changes in resistance per ℃. The output of an NTC thermistor is non-linear due to its exponential nature; however, it can be linearized based on its application. The effective operating range is -50 to 250 ℃ for glass encapsulated thermistors or 150℃ for standard thermistors.
Resistance Temperature Detector (RTD)
A resistance temperature detector, or RTD, changes the resistance of the RTD element with temperature. An RTD consists of a film or, for greater accuracy, a wire wrapped around a ceramic or glass core. Platinum makes up the most accurate RTDs while nickel and copper make RTDs that are lower cost; however, nickel and copper are not as stable or repeatable as platinum. Platinum RTDs offer a highly accurate linear output across -200 to 600 ℃ but are much more expensive than copper or nickel.
Thermocouples
A thermocouple consists of two wires of different metals electrically bonded at two points. The varying voltage created between these two dissimilar metals reflects proportional changes in temperature. Thermocouples are nonlinear and require a conversion with a table when used for temperature control and compensation, typically accomplished using a lookup table. Accuracy is low, from 0.5 ℃ to 5 ℃ but thermocouples operate across the widest temperature range, from -200 ℃ to 1750 ℃.
Semiconductor-based temperature sensors
A semiconductor-based temperature sensor is usually incorporated into integrated circuits (ICs). These sensors utilize two identical diodes with temperature-sensitive voltage vs current characteristics that are used to monitor changes in temperature. They offer a linear response but have the lowest accuracy of the basic sensor types. These temperature sensors also have the slowest responsiveness across the narrowest temperature range (-70 ℃ to 150 ℃).
What Applications Use Temperature Sensors
Medical applications
Temperature sensors are used to quickly and accurately measure a patient's body temperature. They are also used in MRI imaging machines and portable ultrasound scanners.
Appliances in homes
Temperature sensors are used in many appliances that you may not know about. They are found in refrigerators to keep food and drinks cold, in ovens used to cook food to a specific temperature, and in air conditioners/wall heaters. They are also found in battery chargers to prevent undercharging and overcharging of appliances.
Vehicles
Temperature sensors are located in the radiators of different vehicles. These warn you if the engine is getting too hot, thus preventing the engine from exceeding its temperature limits. They are also used in climate control settings, allowing you to cool or heat the interior of your vehicle.
Oil extraction
Temperature sensors are the foundation of safe and effective practices in the oil extraction industry. Oil drilling rigs are equipped with built-in temperature sensors that notify workers when they need to stop drilling.
HVAC systems
HVAC systems require temperature sensors to provide the optimal temperature for a specific room or building. They can also be used to detect leaks, such as in air conditioning units.
Chemical industry
The chemical industry uses high quality and effective temperature sensors to measure the extremely high temperatures in chemical reactions.
Renewable energy
Renewable energy sources need to produce energy efficiently to function; therefore, they rely on temperature sensors to regulate and measure temperatures. Wind turbines, biomass combustion applications, solar heat pumps and geothermal monitoring all require temperature sensors.
Integrated circuits
Integrated circuits are found in the desktop computers, laptops, cell phones and other electronic devices we use every day. They rely on integrated silicon temperature sensors to avoid overheating.
How to Choose the Right Temperature Sensor
Temperature Range: Determine the temperature range over which you need to measure. Some sensors are suitable for a wide range, while others are more limited.
Accuracy: Consider the level of accuracy required for your application. Some sensors, like RTDs, provide high accuracy, while others, like thermocouples, offer a broader range but with slightly lower accuracy.
Response Time: Different sensors have different response times. In applications where quick temperature changes need to be captured, such as in control systems, a fast response time is crucial.
Stability: Some sensors, like RTDs, are known for their stability over time. If long-term accuracy is important for your application, stability becomes a critical factor.
Linearity: Ensure that the sensor provides a linear response within the temperature range of interest. This simplifies the calibration and conversion of electrical signals to temperature readings.
Environmental Conditions: Consider the environmental conditions in which the sensor will be used. Some sensors are more suitable for harsh environments or conditions with electromagnetic interference.
Cost: Different sensors come at different price points. Ensure that the sensor you choose fits within your budget while still meeting your requirements.
Sensor Size and Form Factor: The physical size and form factor of the sensor may be important, especially in applications with limited space.
Calibration and Interfacing: Consider the ease of calibration and the interfacing requirements for the sensor. Some sensors may require specialized interfaces or signal conditioning.
Long-Term Reliability: For applications where reliability over time is essential, choose a sensor with a proven track record of long-term performance.
Industry Standards: Some industries have specific standards or requirements for temperature sensors. Ensure that the sensor you choose complies with these standards if applicable.
Measurement Method: Decide whether you need contact or non-contact temperature measurement. For non-contact measurement, consider infrared sensors or thermocouples, while contact methods include RTDs and thermistors.
Power Consumption: If your application has power constraints, consider the power consumption of the sensor.
Mounting and Installation: Consider the ease of mounting and installing the sensor. Some sensors may require special mounting considerations.
The operation of temperature sensors relies on various physical properties that change predictably with temperature alterations. One of the most common principles used in these sensors is the change in electrical resistance, known as the resistance temperature detector (RTD) or thermistor. Others operate based on the thermoelectric effect (thermocouples) or infrared radiation detection.
RTDs use materials whose electrical resistance changes with temperature. As the temperature increases, the resistance of the material also increases. This change in resistance is measured and converted into a temperature reading.
Thermistors function similarly to RTDs but exhibit a more significant change in resistance with temperature, allowing for higher sensitivity in certain applications.
Thermocouples utilize the principle of generating a voltage when two different metals are joined together at two different temperatures. The voltage produced is directly proportional to the temperature difference, enabling temperature measurement.
Infrared Sensors detect temperature by measuring the radiation emitted by an object. They work by capturing the infrared energy emitted and converting it into an electrical signal, which is then processed to determine the object's temperature.
Select the Right Sensor
The last decision made in the system design is often the selection of a temperature sensor. Typically, the decision is driven by the compatibility of the process controller of the system and often comes down to availability and cost. Unfortunately, the critical selection criteria that include sensor mass, operating temperature, signal strength, reading sensitivity, and operating ambient are overlooked frequently.
There are three types of sensors used in commercial temperature control: thermistors, thermocouples, and resistive thermal devices (RTDs). Each type of temperature sensor exhibits its own characteristics and various styles within each type are designed for specific temperature ranges, output signals and process compatibility based on the construction materials.
The sensor must be selected in such a way that it matches specific design application parameters. An important characteristic is the sensor mass, and it is important to consider it when making a final selection. Heavy sensor bodies offer a slower, dampened response to process changes, whereas a light sensor mass offers a quicker response to process temperature changes. Typically, if the temperature control is more exacting, the sensor will be lighter and will respond more quickly.
Install the Sensor in the Correct Location
The location of the temperature sensor of the system is crucial, even though it might be obvious. In some situations, field installation of a temperature sensor is affected by physical obstacles that are responsible for the initial design location to be modified. When the placement of the temperature sensor in the correct location is difficult, there must be a consideration of compromises and alternatives.
However, the sensor's position is the key to the success of every other part of the system. There must never be a compromise of the system efficiency for ease of sensor installation. Listed below are a few tips to avoid common pitfalls:
The sensing location must be made sure that it is at the entry point of the position of the critical temperature.
The mixing of the process fluid must be ensured, and there should be no cold or hot currents that could fool the sensor.
It should be ensured that the location of the sensor is not negatively affected by fluid velocity changes.
It should be ensured that no heating or cooling sources will affect the process fluid. For instance, an uninsulated piping section after the sensor but prior to the critical point of the temperature of the process can affect the temperature delivered to the critical point.
How to Correct Use Temperature Sensor
Select a location for installing the Temperature Sensor in which the temperature distribution of the sensing object will not change.
Make sure that the length of the Temperature Sensor's protective tubing is sufficient to touch or insert into the sensing object. The length of metallic protective tubing must be at least 20 times its diameter, and the length of non-metallic protective tubing must be at least 15 times its diameter.
Do not repeatedly bend the Temperature Sensor at the same point. The minimum allowable bending radius of sheathed Temperature Sensors is approximately five times the protective tubing diameter. Bending part of the sensor at an acute angle and then extending again may result in broken internal wires or cracks in the element. Do not bend the soldered sections.
Do not bend the protective tubing while measuring low temperatures, which will cause the protective tubing to become fragile.
Do not bend sheathed Temperature Sensors to within 100 mm of the end to protect the sensing section.
Do not allow the temperature of the section connecting the protective tubing and lead wire to exceed 70℃ for exposed-lead models or 100℃ for heat-resistive models.
Do not allow the temperature of the terminal box to exceed 100℃ for exposed-terminal models or 90℃ for enclosed-terminal models.
Do not subject the ceramic protective tubing of high-temperature thermocouples to sudden heating or cooling. Ceramic protective tubing has a low resistance to thermal shock. Either preheat the protective tubing or gradually heat to the required temperature.
Do not use standard lead wires in locations subject to strong bending stress or on moving parts.
If the thermocouple has crimp terminals, use them only as a secondary means of securing the thermocouple. The thermocouple junction of the thermocouple is at the crimped section of the crimp terminals, so a temperature difference occurs between the screw fixating section and the thermocouple junction. Confirm the difference in temperature between the location that is to be measured and the temperature measured by the thermocouple in advance at the actual application temperature.
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