This article will take an in-depth look at temperature sensors.
The article will bring more detail on topics such as:
This section covers the basics of temperature sensors, including their purposes, key components, and operational mechanisms.
Temperature sensors measure heat and cold, converting these readings into electrical signals. These devices are commonly found in household items like water heaters, thermometers, refrigerators, and microwaves. They also have diverse applications, including in geotechnical monitoring.
A temperature sensor is a basic device that measures temperature and converts it into a readable format. Specialized sensors are designed for specific applications, such as measuring the temperature in boreholes, soil, large concrete structures, or buildings.
Temperature sensors measure how hot or cold an object is. The functioning of a temperature meter relies on the voltage across a diode. The diode's resistance changes in direct proportion to the temperature variations.
As the temperature drops, the diode's resistance decreases, and it increases with rising temperatures. The resistance measurement is converted into a readable temperature unit, which is then displayed numerically. In geotechnical monitoring, these sensors measure internal temperatures of structures like dams, bridges, and power plants.
There are many different types of temperature sensors, but the most common way that is used in their categorization is based on the mode of connection that includes contact and non-contact temperature sensors. Examples of contact sensors include thermistors and thermocouples because their contact with the objects they measure is direct, whereas non-contact type temperature sensors measure the heat source’s radiation.
These temperature meters are commonly employed in hazardous settings like thermal or nuclear power plants. They also measure hydration heat in large concrete structures and are used in geotechnical monitoring to track seepage and groundwater movement.
Temperature sensors are frequently used in concrete curing, as concrete needs to be kept warm for proper setting and curing. Seasonal changes can cause structures to expand or contract, affecting their overall volume.
A temperature sensor operates based on the voltage across a diode's terminals. As temperature rises, the voltage increases. This is accompanied by a decrease in voltage between the transistor's base and emitter terminals. Some sensors also use stress changes caused by temperature variations to function.
In a vibrating wire temperature meter, different metals have distinct coefficients of expansion. The device features a high-tensile strength magnetic wire stretched between two dissimilar metals. Temperature changes affect the wire's tension and natural vibration frequency directly.
One of the dissimilar metals used can be aluminum, which has a higher linear expansion coefficient compared to steel. The temperature signal is converted into frequency, and the same read-out unit used for other vibrating wire sensors can be employed for temperature monitoring as well.
The vibrating wire sensor detects temperature changes, converting them into an electrical signal. This signal is then transmitted to the read-out unit as a frequency.
There are three types of components in temperature sensors. There are essential components of a temperature sensor including thermocouple or extension cables and wires, as well as the sensing elements. The following are examples of components which complete the sensor: insulating beads, connectors, connecting heads, and protecting tubes. There are also associated components that are necessary in the use of sensors like converters and controllers.
To utilize a temperature sensor in a control or compensation circuit, the detection circuit must provide an output in a usable format. In analog circuits, this output is typically resistance. For digital control and compensation, this measurement needs to be converted to a digital format, which is commonly done by measuring the output voltage with an analog-to-digital converter (ADC) for processing by a microcontroller (MCU).
Semiconductor-based sensors feature a digital interface, simplifying temperature communication with an MCU, which can read thermocouples due to their voltage output. RTDs and thermistors can provide either voltage or resistance, offering engineers flexible options for connecting detectors to control or compensation systems.
RTDs and thermistors provide variable resistance, which simplifies their integration into analog control or compensation circuits. If a voltage output is needed, the resistance can be easily converted to voltage using a Wheatstone bridge configuration with three additional resistors, as illustrated below.
Non-linear sensors like thermocouples and thermistors require straightforward linearization of their output. This can be done with a simple circuit for analog control. For digital control and compensation, the CPU can use a lookup table to adjust temperatures based on the sensor’s temperature/resistance chart found in the spec sheet.
Engineers have various options for designing temperature detection circuits to prevent overheating or manage temperature control. Thermocouples are ideal for extreme temperatures, while platinum RTDs offer high precision for accuracy. For PCB-based applications where internal sensor installation is challenging, semiconductor-based sensors can detect ambient temperatures near sensitive electronics.
RTD elements are core components in temperature sensors, serving as the resistance element in resistance thermometers or RTDs. These elements are not typically used in their bare form but are incorporated into probes or assemblies designed to endure various conditions. Each RTD has a pre-defined resistance value at a known temperature, which changes predictably. By measuring this resistance, the temperature can be determined through calculations, tables, or instrumentation.
RTD elements are mass-produced using automated equipment, which deposits a layer of platinum onto a ceramic substrate and uses photolithography to etch an electrical current path with the desired resistance in ohms. These elements are smaller than traditional wire-wound types, offering faster response times and broader application suitability, while also reducing user costs.
Ceramic elements are used in temperature sensors and can be wound on ceramic or glass forms, or a platinum wire helix can be partially supported inside a ceramic tube. This semi-supported design offers a wide temperature range and typically the best stability. While other metals can be used, platinum is the most common, often paired with ceramic or glass insulators.
Using metals other than platinum can result in linearity issues at low temperatures and drift, affecting temperature accuracy. The need for frequent error corrections and adjustments with these metals makes platinum the preferred choice.
Thermometrics can add lead wires to temperature sensor elements according to specifications. Commonly used are 20 AWG, Teflon-coated nickel wires.
Wire-wound elements encased in glass (quartz) are available to meet specific user requirements.
This chapter will explore various classes and types of temperature sensors used in control and compensation circuits, as well as their sensor elements.
Temperature sensors come in various types, sizes, and shapes. They are generally categorized into two main classes: contact and non-contact temperature sensors.
Some temperature meters measure hotness or coldness by directly contacting the object. These contact-type sensors can detect temperature in liquids, solids, or gases across a wide range.
These temperature meters do not require direct contact with the object. Instead, they measure temperature based on the radiation emitted by the heat source.
Temperature sensors are categorized into contact and non-contact types, each further divided into specific varieties as outlined below.
This device, known as a resistance thermometer, measures temperature by utilizing the resistance of a resistance temperature detector (RTD) element, which varies with temperature. Various materials can be employed to construct the metal element of the RTD.
The materials include nickel, platinum, and copper. However, platinum is the most accurate and therefore the most expensive.
Thermocouple sensors consist of two wires composed of distinct metals, joined at two locations. The temperature variation is indicated by the voltage difference between these two wires.
While they may offer slightly less accuracy compared to RTDs, thermocouples cover a broad temperature range from -328°F to 3182°F (-200°C to 1750°C) and are typically more budget-friendly.
Thermocouples come in different types, each designed to handle particular temperature ranges. These various types are tailored to suit specific application requirements.
| Thermocouple Types | |||||
|---|---|---|---|---|---|
| Thermocouple Type | Temperature Range (°C) | ||||
| Short Term Use | Continuous Use | Class 1 Tolerance | Class 2 Tolerance | Class 3 Tolerance | |
| Type E | -40 to 900 | 0 to 800 | -40 to 800 | -40 to 900 | -40 to 904 |
| Type J | -180 to 800 | 0 to 750 | -40 to 750 | -40 to 750 | N/A |
| Type K | -180 to 1300 | 0 to 1100 | -40 to 1000 | -40 to 1200 | -180 to 1300 |
| Type N | -270 to 1300 | 0 to 1100 | -40 to 1000 | -40 to 1200 | -270 to 1304 |
| Type R | -50 to 1700 | 0 to 1600 | 0 to 1600 | 0 to 1600 | N/A |
| Type S | -50 to 1750 | 0 to 1600 | 0 to 1600 | 0 to 1600 | N/A |
| Type T | -250 to 400 | -185 to 300 | -40 to 350 | -40 to 350 | -250 to 404 |
| Type B | 0 to 1820 | 200 to 1700 | N/A | 600 to 1700 | 4 to 1820 |
Uses for Thermocouple Types:
Type E Thermocouples - Type E thermocouples use a combination of chromel (a nickel-chromium alloy) and constantan. They have a temperature range of -330°F to 1600°F (-200°C to 870°C) and provide excellent EMF (electromotive force) versus temperature characteristics. These thermocouples are suitable for use in sub-zero temperatures and inert environments but need protection when used in sulfurous conditions.
Type J Thermocouples - Type J thermocouples, similar to Type K, are general-purpose sensors made from iron and constantan, with the iron leg being positive and the constantan leg negative. They can be used either exposed or with a protective tube, which is recommended for durability. Type J thermocouples are suitable for use in vacuum, inert, and reducing environments. Like Type K thermocouples, Type J thermocouples require careful calibration and may be sensitive to electrical noise.
Type K Thermocouples - Type K thermocouples are made of Chromel®–Alumel® with small percentages of manganese and silicone. They are a general purpose thermocouple with a temperature range of -328°F up to 2462°F (-200°C up to 1350°C). Type K thermocouples need to be carefully calibrated and have small output signals. They are used in an assortment of environments including water, mild chemicals, gasses, and dry conditions. Common industries that use type K thermocouples are hospitals and food preparation. Regardless of their wide temperature range, type K thermocouples are mostly used for temperatures over 1004°F (540°C).
Type N Thermocouples: These thermocouples are composed of nicrosil (a nickel-chromium alloy) and nisil (an alloy of nickel, silicon, and magnesium). They operate within a temperature range of 32°F to 2300°F (650°C to 1260°C) and are known for their resistance to green rot and hysteresis. Common applications include use in refineries and the petrochemical sector.
Type R Thermocouples: Constructed from a combination of platinum and rhodium, Type R thermocouples can measure temperatures up to 2700°F (1480°C). They require protection through a gas-tight ceramic tube and an additional outer tube. These thermocouples offer enhanced stability and a broader temperature range compared to Type S. They are utilized in fields such as heat treatment, control sensors, semiconductor manufacturing, glass production, and metal processing.
Type S Thermocouples: Known for their precision and stability, Type S thermocouples are ideal for both high-temperature applications in the BioTech and Pharmaceutical industries as well as low-temperature scenarios. They cover a temperature range of -58°F to 2700°F (980°C to 1450°C).
Type T Thermocouples: Featuring copper and constantan, Type T thermocouples are suitable for temperatures ranging from -330°F to 700°F (-200°C to 370°C). They are used in inert atmospheres and are resistant to decomposition even in the presence of moisture. Typical applications include food production and cryogenic environments.
Type B Thermocouples: Type B thermocouples are designed for high-temperature applications and offer the highest temperature range among thermocouples, with remarkable accuracy and stability. Their construction includes platinum with 6% rhodium and platinum with 30% rhodium, operating between 2500°F and 3100°F (1370°C to 1700°C).
Type C Thermocouples: These thermocouples use tungsten and rhenium and are capable of measuring temperatures up to 4200°F (2315°C). They are suitable for use in hydrogen, inert, or vacuum environments to avoid oxidation failure. Type C thermocouples are equipped with protective sheaths made from molybdenum, tantalum, or inconel, and feature insulators such as alumina, hafnia, or magnesium oxide.
This temperature sensor type offers precise, consistent, and substantial changes in response to varying temperatures. Such a significant change allows for rapid and accurate temperature readings.
This temperature sensor, commonly seen in the form of a mercury-filled glass tube, is considered a standard type. While various thermometer types are available, traditional glass thermometers often used mercury, but today, ethanol is the predominant liquid used in these instruments.
This thermometer features a gauge connected to a stem. A spring is affixed to a rod at the sensor's tip, which moves the gauge's needle. The spring, located inside the stem, detects temperature changes at the end of the sensor.
When heat is applied to the sensing coil, it causes the coil to move. This movement translates into needle movement on the gauge, thereby indicating the temperature.
These thermometers operate similarly, featuring a bulb at the sensing end of the probe filled with either gas or liquid. When exposed to heat, the gas expands or the liquid increases in temperature, causing the rod to move and display the corresponding temperature on the gauge.
This thermometer employs a probe, such as a thermocouple or an RTD, to measure temperature. The probe's sensing end captures the temperature, which is then shown as a digital reading on the display.
These temperature sensors measure temperatures from a distance by assessing the thermal radiation emitted by a heat source or object.
These temperature sensors are used in high-temperature or hazardous environments, where maintaining a safe distance from the heat source is essential.
These temperature sensors are among the most widely used non-contact types. They are particularly useful in situations such as when the target object is moving (e.g., on a conveyor belt), when the object is located at a distance, in hazardous environments, or in extreme temperatures where contact sensors would be unsuitable.
A thermistor is a temperature sensor sensitive to small temperature variations. It exhibits high resistance at low temperatures, which decreases rapidly as the temperature rises. The glass-coated NTC thermistor is known for its high accuracy, with even the smallest resistance changes per degree Celsius being promptly detected.
Negative Temperature Coefficient (NTC) thermistors need linearization because their response follows an exponential relationship. They operate within a temperature range of -58°F to 482°F (-50°C to 250°C). Linearization is also necessary for NTC thermistors due to their sensitivity and response speed.
In a detection circuit, an NTC thermistor operates by passing a small voltage through it. The thermistor’s resistance, which decreases quickly with rising temperature, reflects the temperature changes.
For temperature ranges between -40°F and 257°F (-40°C and 125°C), NTC thermistors offer a more cost-effective alternative to platinum RTDs, thermocouples, and semiconductor sensors. Their rapid resistance change allows for high accuracy, energy efficiency, stability, and responsiveness. NTC thermistors are also easily integrable into various systems.
Semiconductor-based temperature sensors utilize dual integrated circuits, comprising two similar diodes. These diodes have voltage and current characteristics that are sensitive to temperature changes, enabling precise temperature measurements.
Semiconductor-based sensors provide a linear output but are less accurate within a narrow range of 1 to 5°C. They have the slowest response time, typically 5 to 6 seconds, and operate within a limited temperature range of -94°F to 302°F (-70°C to 150°C).
This temperature sensor is designed for measuring internal temperatures in water or concrete structures. It offers a resolution better than 0.1°C and operates similarly to a thermocouple sensor.
It also has a wide temperature range, from -4°F to 176°F (-20°C to 80°C).
This chapter will discuss the applications and benefits of temperature sensors. It will also discuss temperature control using temperature sensors.
Temperature sensors play a crucial role in measuring temperature across diverse applications and industries. They are ubiquitous, found in both industrial environments and daily life. Here are some examples of how temperature sensors are used:
Temperature sensors are employed to monitor a range of environments and equipment, including power plants and manufacturing processes. They measure water temperatures in reservoirs and boreholes, assess temperature-induced stress and volume changes in dams, and study the impact of temperature on other installed instruments.
Temperature sensors are used in scientific research and biotechnology monitoring.
Temperature sensors are employed in various medical applications, including patient monitoring, medical devices, thermodilution, humidifiers, gas analysis, cardiac catheters, ventilator flow tubes, and dialysis fluid temperature management.
Temperature sensors are used to measure inlet air temperature, exhaust gas temperature, engine temperature, and oil temperature.
Temperature sensors are employed in kitchen appliances such as ovens and kettles, as well as in various white goods.
Temperature sensors are used in air conditioning and heating ventilation systems, whether for residential or commercial applications.
Temperature sensors are employed in refrigerated vans and trucks to monitor and control temperature during transport.
Temperature sensors offer several advantages, including:
When choosing a temperature sensor for a specific application, several factors need to be taken into account. Consider the following aspects when selecting a temperature sensor:
Various types of temperature sensors are designed to measure different temperature ranges and may offer higher accuracy within specific ranges. Before purchasing, ensure that the sensor's temperature range matches the requirements of your application. This information is typically detailed in the sensor's datasheet.
Applications may have specific accuracy requirements. Note that thermocouples generally exhibit greater long-term stability variance compared to thermistors and RTDs.
The choice of temperature sensor depends on the available space within the application. A smaller device may be necessary if space is limited. Additionally, the package style of the sensor affects how it connects to the application and measures temperature, making it a crucial factor to consider.
The placement and selection of a temperature sensor are significantly influenced by the environmental conditions in which it will operate. Factors such as humidity, vibrations, and other environmental elements can impact the sensor's stability and accuracy. Additionally, electrical noise can affect readings, particularly when detecting small temperature changes. Ensuring that the sensor is used in a low-noise environment is essential for obtaining accurate measurements.
To ensure optimal performance, it is crucial to thoroughly assess the conditions and environment where a temperature sensor will be installed. In addition to evaluating the sensor's structure, it is important to protect any connections, such as lead wires, from harsh and unstable conditions.
To safeguard a temperature sensor and its wiring from environmental conditions, protective measures such as durable metal sheaths that resist wear and corrosion can be employed. While these sheaths enhance protection, they can also increase the sensor's cost and impact its sensitivity. Moreover, some sensor configurations require specific mounting considerations to ensure a reliable thermal connection.
Temperature sensors play a crucial role in effective temperature control. Below are five key factors for achieving efficient temperature management:
In system design, selecting a temperature sensor is often one of the final decisions. This choice is typically influenced by the compatibility with the system's process controller, as well as factors like availability and cost. However, essential selection criteria such as sensor mass, operating temperature, signal strength, reading sensitivity, and ambient conditions are frequently overlooked.
In commercial temperature control, three primary types of sensors are commonly used: thermistors, thermocouples, and resistive thermal devices (RTDs). Each sensor type has distinct characteristics, and within each category, different styles are designed to meet specific temperature ranges, output signals, and process compatibility based on their construction materials.
The sensor should be chosen to align with the specific parameters of the design application. Sensor mass is a critical characteristic to consider: heavier sensors generally have a slower and more dampened response to temperature changes, while lighter sensors respond more quickly. For applications requiring precise temperature control, a lighter sensor is often preferred due to its faster response time.
The placement of the temperature sensor is critical, even if it seems straightforward. In some cases, physical obstacles during field installation may necessitate modifications to the initial design location. If placing the sensor in the ideal position proves challenging, it's important to consider potential compromises and alternative solutions.
Nevertheless, the sensor's placement is crucial to the overall success of the system. It is essential not to sacrifice system efficiency for the sake of easier sensor installation. Below are some tips to avoid common pitfalls:
The primary objective of any temperature control system is to achieve a balance between heating and cooling capabilities. Mismanagement of this energy balance can lead to temperature instability. The two fundamental types of controls used are proportional output control and on/off output control.
The on/off control is the simplest type of temperature control to understand. It operates based on the system's temperature setpoint, activating the full capacity of the heating or cooling system when the temperature deviates from this setpoint. The system continues to operate at full capacity until the temperature reaches the setpoint, at which point the equipment is turned off. On/off control can be a cost-effective method, especially when the mass of the process is larger than the heating or cooling capacity and when a wide temperature fluctuation is acceptable.
However, there are practical limitations to this approach. Since a system's capabilities often exceed the load requirements in various situations, simply turning the heating or cooling equipment on and off is usually impractical.
To improve the control capacity of on/off systems, one approach is to incorporate multiple levels or stages of capacity into the heating or cooling system.
Process control becomes more manageable when all equipment subsystem functions are predictable. However, energy-saving measures and management improvements can sometimes destabilize temperature control. Many modern systems are designed to automatically adjust several variables in response to changes in load, helping to maintain stability despite these fluctuations.
Commonly adjusted variables in temperature control systems include supply fluid temperature, process fluid volume, and heating or cooling capacities. Modifying subsystems such as supply temperatures, process pressures, and fluid flow can alter the system’s characteristics, potentially leading to temperature instability and control issues. For example, in a plant chiller system, changing the rate of flow of the process fluid can impact overall performance.
A reduced flow rate can significantly alter heat-transfer characteristics in some system components and heat exchangers. This loss in heat transfer efficiency may lead to temperature instability at critical points and could potentially cause the compressor or other components to malfunction.
Proportional controllers typically come in three design types: (PID) proportional plus integral plus derivative, (PI) proportional plus integral, and (P) proportional only. Advances in small process controllers have given engineers greater flexibility in their designs.
Advancements in controller technology have enhanced their ability to compensate for system design flaws and unpredictable characteristics. The simplest type of controller is the proportional controller, which forms the basis for many small electronic devices. A proportional controller operates by responding to temperature deviations, generating an output signal based on a linear calculation where changes in deviation result in a proportional change in output.
A temperature sensor can also be defined as a simple instrument that measures the degree of coldness or hotness and then converts it into a readable unit. There are different types of temperature sensors, including thermocouples, thermistors, RTDs, etc. Each temperature sensor has its own unique characteristics, which makes it suitable in specific types of applications. However it is important to consider the temperature range, accuracy, size, and stability of a temperature sensor for optimal performance within a specific application.
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