S9 Temperature Sensor

[Zebedeo Konig]

Athermistor 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 C. 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 C for glass encapsulated thermistors or 150C for standard thermistors.

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 C but are much more expensive than copper or nickel.

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 C to 5 C but thermocouples operate across the widest temperature range, from -200 C to 1750 C.

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 C to 150 C).

Temperature sensors are vital to everyday life. These important pieces of technology measure the amount of heat an object or system is giving off. The measurements given allow us to physically sense a change in temperature. One important role of temperature sensors is prevention. Temperature sensors detect when a set high point occurs which allows time for preventative action. A good example is seen in fire detectors.

Temperature sensing is one of the most sensitive properties or parameters for industries like petrochemical, automotive, aerospace and defense, consumer electronics, and so on. These sensors are installed into devices with the purpose of measuring the temperature of a medium accurately and efficiently in a given set of requirements.

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We all use temperature sensors in our daily lives, be it in the form of thermometers, domestic water heaters, microwaves, or refrigerators. Usually, temperature sensors have a wide range of applications, the geotechnical monitoring field, being one of them.

A thermocouple (T/C) is made from two dissimilar metals that generate an electrical voltage in direct proportion to the change in temperature. An RTD (Resistance Temperature Detector) is a variable resistor that changes its electrical resistance in direct proportion to the change in the temperature in a precise, repeatable, and nearly linear manner.

The resistance across the diode is measured and converted into readable units of temperature (Fahrenheit, Celsius, Centigrade, etc.) and, displayed in numeric form over readout units. In the geotechnical monitoring field, these temperature sensors are used to measure the internal temperature of structures like bridges, dams, buildings, power plants, etc.

Contact sensors include thermocouples and thermistors because they are in direct contact with the object they are to measure. Whereas, the non-contact temperature sensors measure the thermal radiation released by the heat source. Such temperature meters are often used in hazardous environments like nuclear power plants or thermal power plants.

In geotechnical monitoring, temperature sensors measure the heat of hydration in mass concrete structures. They can also be used to monitor the migration of groundwater or seepage. One of the most common areas where they are used is while curing the concrete because it has to be relatively warm in order to set and cure properly. The seasonal variations cause structural expansion or contraction thereby, changing its overall volume.

The basic principle of working the temperature sensors is the voltage across the diode terminals. If the voltage increases, the temperature also rises, followed by a voltage drop between the transistor terminals of the base and emitter in a diode.

It primarily consists of a magnetic, high tensile strength stretched wire, the two ends of which are fixed to any dissimilar metal in a manner that any change in temperature directly affects the tension in the wire and, thus, its natural frequency of vibration.

The dissimilar metal, in the case of the Encardio Rite temperature meter, is aluminum (Aluminum has a larger coefficient of thermal expansion than steel.) As the temperature signal is converted into frequency, the same read-out unit which is used for other vibrating wire sensors can also be used for monitoring temperature also.

Contact Type Temperature Sensors: There are a few temperature meters that measure the degree of hotness or coolness in an object by being in direct contact with it. Such temperature sensors fall under the category of contact type. They can be used to detect solids, liquids, or gases over a wide range of temperatures.

Non-Contact Type Temperature Sensors: These types of temperature meters are not in direct contact with the object rather, they measure the degree of hotness or coolness through the radiation emitted by the heat source.

Thermistors or thermally sensitive resistors are the ones that change their physical appearance when subjected to a change in temperature. The thermistors are made up of ceramic material such as oxides of nickel, manganese, or cobalt coated in glass which allows them to deform easily.

Most of the thermistors have a negative temperature coefficient (NTC) which means their resistance decreases with an increase in the temperature. But, there are a few thermistors that have a positive temperature coefficient (PTC), and, their resistance increases with a rise in the temperature.

RTDs are precise temperature sensors that are made up of high-purity conducting metals such as platinum, copper, or nickel wound into a coil. The electrical resistance of an RTD changes similar to that of a thermistor.

A thermocouple usually consists of two junctions of dissimilar metals, such as copper and constantan that are welded or crimped together. One of these junctions, known as the Cold junction, is kept at a specific temperature while the other one is the measuring junction, known as the Hot junction.

A thermistor is basically a sensitive temperature sensor that reacts precisely to even minute temperature changes. It provides a huge resistance at very low temperatures. This means, that as soon as the temperature starts increasing, the resistance starts dropping quickly.

Due to the large resistance change per degree Celsius, even a small temperature change is displayed accurately by the Negative Temperature Coefficient (NTC) Thermistor. Because of this exponential working principle, it requires linearization. They usually work in the range of -50 to 250 C.

A semiconductor-based temperature sensor works with dual integrated circuits (ICs). They contain two similar diodes with temperature-sensitive voltage and current characteristics to measure the temperature changes effectively.

The Encardio Rite Model ETT-10V vibrating wire temperature meter is used for the measurement of internal temperature in concrete structures or water. It has a resolution of better than 0.1C and works similarly to that of Thermocouple Temperature Sensors. It also has a high-temperature range from -20 to 80 C.

Model ETT-10TH resistance temperature probe is specially designed for the measurement of surface temperatures of steel & measurement of the surface temperature of concrete structures. ETT-10TH can be embedded in concrete for measurement of bulk temperature inside the concrete and can even work submerged underwater.

ETT-10TH resistance temperature probes are fully interchangeable. The temperature readings will not differ by more than 1C over the specified working temperature range. This allows a single indicator to be used with any ETT-10TH probe without recalibration.

ETT-10TH temperature probe consists of a resistance-temperature curve matched thermistor epoxy encapsulated in copper tubing for faster thermal response and environmental protection. The tube is flattened at the tip so that it can be fixed to any reasonably flat metal or concrete surface for measurement of surface temperature.

The temperature probe is provided with a four-core cable used as a standard in all Encardio Rite vibrating wire strain gauges. The white and green colored wires are used for the thermistor similar to the other Encardio Rite vibrating wire sensors.

The ETT-10PT RTD (Resistance Temperature Detector) temperature probe consists of a ceramic resistance element (Pt. 100) with DIN IEC 751 (former DIN 43760) European curve calibration. The resistance element is housed in a closed-end robust stainless steel tubing which protects the element against moisture.

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