An RTD, resistance temperature detector, is a passive temperature sensing device that operates on the principle that the resistance of a metal changes as the temperature changes. The electrical current that passes through the element...
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This article will take an in-depth look at temperature sensors.
The article will bring more detail on topics such as:
This chapter will discuss what temperature sensors are, their uses, their components, and how they function.
Temperature sensors are devices that detect and measure coolness and hotness and convert it into an electrical signal. Temperature sensors are utilized in our daily lives, be it in the form of domestic water heaters, thermometers, refrigerators, or microwaves. There is a wide range of applications of temperature sensors, including the geotechnical monitoring field and others.
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 specialized temperature sensors used to measure the temperature of the boreholes, soil, huge concrete dams, or buildings.
Temperature sensors are devices designed for measuring the degree of coolness and hotness in an object. The voltage across the diode determines the working of a temperature meter. The change of temperature varies directly proportional to the diode’s resistance.
The cooler the temperature, the lesser the resistance will be and vice-versa. A measurement of the resistance across the diode is done, and the measurement is converted into units of temperature that are readable and displayed in numeric form over readout units. In the field of geotechnical monitoring, these temperature sensors are utilized in the measuring of internal temperatures of structures such as dams, bridges, 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 in non-contact type temperature sensors measure the heat source’s radiation.
Such temperature meters are mostly used in hazardous environments such as thermal power plants or nuclear power plants. Temperature sensors are used to measure the hydration heat in mass concrete structures, in the field of geotechnical monitoring. They can also be utilized to monitor the migration of seepage or groundwater.
One of the areas that they are most commonly used is while curing the concrete since it has to be relatively warm, in order to properly set and cure. The variations of seasons cause the expansion of structures or contraction thereby, bringing an overall change to its volume.
The working principle of a temperature sensor is the voltage across the terminals of the diode. If there is an increase in the voltage, the temperature also increases. This is followed by a drop in the voltage between the terminals of the transistor of base and emitter in a diode. There are also temperature sensors that work on the principle of stress change caused by changes in temperature.
In a vibrating wire temperature meter, dissimilar metals have different linear coefficients of expansion. It mainly consists of a magnetic stretched wire of high tensile strength with two ends fixed to any dissimilar metal so that any temperature change will directly affect the tension in the wire and its natural vibration frequency.
The dissimilar metal can be made from aluminum since it has a larger linear expansion coefficient than steel. When the conversion of the temperature signal into frequency occurs, the very same read-out unit that is used for other vibrating wire sensors can also be utilized in the monitoring of temperature also.
The specially built vibrating wire sensor is the one that senses the temperature change and then the temperature change is converted into an electrical signal which is then transmitted to the read out the 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, controllers.
The detection circuit must offer an output in a usable format in order to use a temperature sensor in a control or compensation circuit. For analog circuits, usually, the output is resistance. The measurement must be converted to a digital format for processing by an MCU for digital control and compensation. Commonly, this is achieved by reading the measurement as a voltage by means of an analog to digital converter.
Typically, these have a digital interface, making it clear to communicate temperature to an MCU instead of an analog circuit. An MCU readily accesses thermocouples due to their provision of voltage. Both RTDs and thermistors have the flexibility to provide either a voltage or resistance with ease. This provides engineers with choices for the connection method of the detector to the control or compensation system. Since RTDs and thermistors output a variable resistance, this makes their integration into an analog control or compensation circuit straightforward.
For the use of an NTC thermistor in a detection circuit, a small voltage must be put across the thermistor. The temperature will be reflected by the thermistor’s resistance, with a rapid drop in its resistance as the temperature increases. If there is a requirement of a voltage output, the resistance can be converted easily to a voltage by means of three additional resistors in a Wheatstone bridge configuration as shown in the figure below.
For non-linear sensors like thermocouples and thermistors, there is a need for the output to go through a straightforward linearization. This can be implemented in a circuit that is simple for analog control and compensation. For control and compensation that is digitally based, the adjustment of the temperature that is measured can be done by the CPU using a simple lookup table that reflects the temperature/resistance chart that is incorporated in the spec sheet of the sensor.
There are many choices that engineers have in designing a temperature detection circuit to prevent overheating or implement temperature control and/or compensation functionality. For extreme temperatures, thermocouples are mostly the ideal option. When there is a requirement for the greatest accuracy, platinum RTDs provide high precision. For PCB-based applications where installing an external sensor internally can be difficult, semiconductor-based sensors make the sensing of ambient temperature near sensitive electronics possible.
For applications in the range of -40 °C to 125 °C, NTC thermistors provide an option that is less expensive than platinum RTD, thermocouple sensors, and semiconductor-based sensors. With their fast changing resistance based on the temperature, they are capable of providing superior accuracy, power efficiency, stability, reliability and responsiveness. Their flexibility also makes their integration in almost any system easy.
RTD elements are utilized in the manufacturing of temperature sensors. A resistance element is a component that senses temperature at the heart of a resistance thermometer or an RTD. They can’t be used in their bare form typically, but they may be built into a probe or assembly that enables them to withstand different conditions of their application. Each has a resistance value that is pre-specified at a known temperature which changes in a predictable fashion. In this way, by measuring the element’s resistance, that element’s temperature can be determined from calculations, tables, or instrumentation.
These elements are mass produced by automated equipment, which deposits a layer of platinum onto the ceramic substrate and utilizes photolithography in the etching of an electrical current path that corresponds to the value required in ohms. The elements have a smaller size than that of traditional wire wound elements and as a result have a response time that is fast and are suitable in more applications, while reducing the costs of the user at the same time.
Ceramic elements can be used to make temperature sensors. These elements are wound on either ceramic or glass former, or a helix of platinum wire can be semi-supported within a ceramic tube’s bores. This type that is semi-supported is capable of providing the widest temperature range of operation and typically the best stability.
Platinum is the most common type on either ceramic or glass insulators. Nickel, balco, copper, MIL-STD temperature vs. resistance curves are available too.
Thermometrics can make an addition of lead wires to the elements of a temperature sensor as per specifications. The common additions are 20 AWG, Teflon coated nickel wire.
Glass (quartz) encased elements that are wire wound are available to meet the requirements of a user.
This chapter will discuss the different classes and types of temperature sensors as applied in control and compensation circuits and temperature sensor elements.
Temperature sensors are found in different types, sizes, and shapes. There are two main temperature sensors classes: contact temperature sensors and non-contact temperature sensors.
A few temperature meters are capable of measuring the degree of hotness or coldness in an object by being in direct contact with the object. These types of temperature sensors fall under the class of contact-type sensors. They can be utilized in the detection of liquids, solids, or gasses over a broad range of temperatures.
These types of temperature meters do not measure the temperature of an object while in direct contact; rather, they measure the degree of hotness or coldness from the radiation that is emitted by the heat source.
The contact and non-contact temperature sensors are divided further into the following mentioned types of temperature sensors.
This is known as the resistance thermometer and uses the resistance of the RTD element with temperature to measure the temperature. Different types of materials can be used to make the metal.
The materials include nickel, platinum, and copper. However, the most accurate is platinum and therefore carries a high cost.
This type of sensor is made up of two wires with two metals that are different, connected at two points. The voltage between the two wires reflects the change in temperature.
Although the accuracy may be slightly reduced to a degree that is lower than an RTD, they possess a temperature range that is the most extensive from -200 °C to 1750 °C and are generally more cost-effective.
This type of temperature sensor displays a change that is precise, predictable, and large in the alteration of different temperatures. With a change this large, it means that the reflection of the temperatures occurs rapidly and accurately.
With a nature that is this large and fast, the NTC thermistor requires linearization – so there is an involvement of some mathematics.
This type of temperature sensor is the standard temperature sensor, particularly the mercury-filled glass tube. However, thermometers exist in several types. There is the glass thermometer: as above ethanol and mercury glass tube. The main liquid used in these thermometers is now ethanol.
This type of thermometer consists of a gauge and stem that are connected. There is a spring attached to a rod on the tip of the sensor, leading up to the gauge’s needle. Inside the stems, the spring sits sensing the end.
When there is an application of heat on the sensing coil, there is a creation of movement in the coil, which causes the movement of the needle in the gauge – thus the temperature is displayed.
These types of thermometers are the same in terms of how they work. There is a bulb that is filled, either with gas or liquid. This is located inside the sensing end of the probe. When heat is applied, the gas expands and the liquid heats up, which signals the rod attached to move the needle to the measured temperature.
This type of thermometer utilizes a probe such as a thermocouple or an RTD. The temperature measurement is done by the probe (sensing end) and displayed as a digital reading.
These types of temperature sensors detect temperatures from a distance by measuring the amount of thermal radiation that is being emitted by a heat source or an object.
These temperature sensors find their application in high temperatures or hazardous environments, where a safe distance must be maintained away from a particular body.
These temperature sensors are the most common type of temperature sensors that are non-contact. They are used in the following circumstances: when the target object is in motion (like within moving machinery on a conveyor belt), if the distance is great away if the surrounding environment is dangerous or extreme temperatures that are high where a contact sensor is not suitable.
Basically, a thermistor is a sensitive temperature sensor that precisely reacts to temperature changes as small as minutes. At very low temperatures, it provides huge resistance. This means that as soon as the temperature starts to increase, the resistance starts to drop quickly.
Because of the large change in resistance per degree celsius, even a small temperature change is accurately displayed by the negative temperature coefficient thermistor. The negative temperature sensor thermistor requires linearization due to its exponential working principle. Their working temperature range is usually -50 to 250 °C.
Semiconductor-based temperature sensors work with dual integrated circuits. They consist of two diodes that are similar, with voltage and current characteristics that are temperature sensitive, for measuring the changes in temperature effectively.
They give a linear output, however, but are less accurate at 1 to 5 °C. These types of temperature sensors have the slowest responsiveness (5 seconds to 6 seconds) across a temperature range that is the narrowest (-70 °C to 150 °C)
This type of temperature sensor is used to measure the internal temperature in water or concrete structures. It exhibits a resolution of better than 0.1 °C and its working is similar to that of a thermocouple temperature sensor.
Its temperature range is also high ranging from -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.
The function of temperature sensors is to measure temperature in many various applications and industries. Temperature sensors are all around us, present in both industrial settings and everyday life. The following are examples of the applications of temperature sensors:
Temperature sensors are utilized to monitor various environments and machinery, power plants, and manufacturing. Temperature sensors are used to measure water temperatures in reservoirs and boreholes. They can also be used to interpret temperature-related stress and changes in volume in dams. Temperature sensors are also utilized in the study of the temperature effect on other installed instruments.
Temperature sensors are utilized in science and biotech monitoring.
Temperature sensors are utilized in the monitoring of patients, in medical devices, in thermodilution, in humidifiers, gas analysis, cardiac catheters, ventilator flow tubes, and dialysis fluid temperature.
Temperature sensors are used for measuring inlet air temperature, exhaust gas, engine temperature, and oil temperature.
Temperature sensors are used in kitchen appliances (ovens, kettles, etc.) and also in white goods.
Temperature sensors are utilized in air conditioning devices and heating ventilation devices, either domesticated or commercial.
Temperature sensors are used in refrigerated vans and trucks.
The benefits of temperature sensors include:
There are some factors that must be considered when selecting a temperature sensor for a certain application. The following are factors that must be considered when opting for a temperature sensor:
Different types of temperature sensors are capable of measuring different ranges and might be more accurate within a certain range. It is important to make sure that the range of the temperature sensor is checked, and also the range of your application that is expected before purchasing. The temperature range of the temperature sensor is provided on the datasheet.
An application may require a certain degree of accuracy; the variance of thermocouples in long term stability is higher than that of thermistors and RTDs, so you must be aware of this. The temperature sensor offering the highest accuracy tends to be the NTC thermistors that are glass coated.
The type of temperature sensor selected is determined by the space that is available within the application. If there is limited space, then a device that is smaller is required. How the temperature sensor will be connected to the application and how the temperature is going to be measured is determined by package style, therefore package style is an important consideration.
Temperature sensors are utilized in the effective control of temperature. Five keys to effective temperature control are going to be discussed below:
The last decision that is 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 that are used in commercial temperature control which are: 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. Sensor bodies that are heavy offer a slower and 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 fast-responding.
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 goal of any temperature control system is to balance the heating and cooling capabilities of the system. If the energy balance is mismanaged, this may result in temperature instability. There are two basic types of controls which are proportional output control and on/off output control.
The easiest type of control to understand is the on/off control. Its decision making on temperature deviation is based on the temperature setpoint of the system. At a temperature that is predetermined, either below or above the setpoint, there is an activation of the full capacity of the heating or cooling system by the system. The only way to stop further cooling or heating is by turning off the conditioning equipment. If the mass process is larger than the heating or cooling capacity and the temperature fluctuation that is allowed is wide, on/off control can be an ideal control method that is cost-effective.
However, there are some practical limitations with it. Because the capabilities of a system are often significantly higher than the load in different situations, only turning the cooling or heating equipment on and off is generally impractical.
One approach that can be used to enhance the capacity control of on/off control systems is the addition of different levels or stages of capacity to the cooling or heating system.
The process control can be more manageable by keeping all of the equipment subsystem functions predictable. With the advent of energy; management improvements, some energy saving schemes can have an effect that is destabilizing on the temperature control of the process. As loads change, many new systems will automatically compensate several variables in response to this change.
The most commonly adjusted variables are supply fluid temperature, process fluid volume, and heating or cooling capacities. The adjustment of several subsystems such as supply temperatures, process pressures, and fluid flow will change the system’s characteristics, and these changes are capable of creating temperature instability and cause problems in control. For instance, consider a plant chiller system in which the rate of flow of the fluid of the process is changed.
The reduced rate of flow can change heat-transfer characteristics exponentially in some components of the system and heat exchangers. The loss of heat transfer may result in temperature instability at the critical point and can cause the malfunctioning of the compressor or other components.
Many of the proportional controllers fall into three design actions or types: (PID) proportional plus integral and derivative, (PI) proportional plus integral, and (P) proportional only. Engineers have been provided with more flexibility in their designs with technical advances in small process controllers.
The improvement of controllers has increased to such a degree that they are capable of often compensating for system design flaws and unruly characteristics. The simplest controllers are proportional controllers and they are the basis for most small electronic devices. A proportional controller is able to work off a deviation of temperature and applies an output signal on the basis of a linear calculation in which a change in a given deviation results in a linear 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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