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how to select and use the right temperature sensor

by:JVTIA     2020-10-28
Instruments (I)Pvt. Ltd.
It is one of the largest manufacturers of temperature sensors in India and has 35 years of experience in the field of temperature measurement.
Tempsens is an ISO 9001 certified company with NABL accredited laboratories.
We produce thermocouple, RTDs, sleeves, compensation cables, calibration equipment, thermometers, industrial heaters and related accessories.
We also offer calibration services for various temperature sensors, including thermometers in the temperature range40°C to 1500°C.
Our products are exported to more than 40 countries.
How to select and use the appropriate temperature sensor each sensor has its own specifications within a specific temperature range.
Now the user will decide which type of sensor is best for his/her application.
The choice of the sensor depends on a variety of specifications, namely: Application, tolerance, accuracy, and beyond most temperature ranges.
The problem now is to distinguish between different types of sensors, including the temperature range, tolerance, accuracy, interchange and relative advantages and disadvantages of each type of sensor.
Reviewing the basics of RTD and thermocouple RTD contains a sensing element that is a resistor that changes the resistance as the temperature changes.
This change in resistance is well understood and repeatable.
The sensing element in RTD usually contains a coil or a conductive film mesh cut with a conductor pattern.
The extension cord is connected to the sensing element, so the resistance can be measured from a distance.
The sensing element is then encapsulated so that it can be placed in a position in the process where it will reach the same temperature that exists in the process.
On the other hand, the thermocouple contains two electrical conductors made of different materials, which are connected at one end.
The end of the conductor exposed to the process temperature is called the measuring junction.
Point at the end of the thermocouple conductor (
Usually the place where the conductor is connected to the measuring device)
When the measurement and reference junction of the thermocouple are at different temperatures, a millivolt potential is formed inside the conductor, called the reference junction.
Knowing the type of thermocouple used, the size of the millivolt potential inside the thermocouple and the temperature of the reference junction, the user can determine the temperature of the measured junction.
The Millivolt potential generated in the thermocouple conductor varies depending on the material used.
Some materials make better thermocouple than others, because the millivolt potential produced by these materials is easier to repeat and has been established.
These thermocouple are given specific type names such as Type E, Type J, Type K, Type N, type T, type B, Type R, and Type S.
The temperature limit of RTD and thermocouple the material used in RTD and thermocouple has a temperature limit, which may be an important consideration in their use. 1.
As mentioned earlier, RTD consists of sensing elements, wires connecting sensing elements to measuring instruments, and some kind of support for locating sensing elements in the process.
Each of these materials sets a limit on the temperature that RTD can expose.
Table 1: available temperature range platinum-sensitive element material and temperature limit material-
Nickel-260 °C to 650 °C
Copper-100 °C to 300 °C
Sensing elements in 70 °C to 150 °C nickel/iron 0 °C to 200 °F RTD typically contain platinum wire or film, ceramic housing and ceramic cement or glass for sealing sensing elements and supporting component wires.
Typically, platinum sensing elements can be exposed to temperatures up to about 650 °c.
Other materials such as nickel, copper and nickel/ferroalloy can also be used, but their useful temperature range is much lower than that of platinum.
The wires connecting the sensing elements to the reading or control meter are usually made of materials such as nickel, nickel alloy, tin-plated copper, silver-plated copper or nickel-plated copper.
The wire insulation used also directly affects the temperature exposed by RTD.
The table contains commonly used wires and insulation materials and their maximum operating temperature. 2.
There are E, J, K, N, T, R, S and B types of thermocouple materials.
These types of thermocouple can be divided into two categories: base metal and precious metal thermocouple. Type E, J.
K, N and T thermocouple are called base metal thermocouple because they are made of common materials such as copper, nickel, aluminum, iron, chromium and silicon.
There are preferred conditions of use for each type of thermocouple, such as the use of bare J-type thermocouple (Iron/Constantine)
It is usually limited to the maximum temperature of 540 °c and is not recommended for use in oxidation or vulcanization atmosphere due to deterioration of the iron conductor.
Bare T-type thermocouple (
Due to the deterioration of the copper conductor, it is not used at more than 370 °c.
The temperature range of these thermocouple types is included in Table 3.
R-Type, S-type and B-type thermocouple are known as precious metal thermocouple because they are made of platinum and platinum.
These thermocouple are used for applications that exceed the capabilities of base metal thermocouple.
The rated operating temperature of Type R and Type S thermocouple is between 540 °C and 1480 °C, and the rated operating temperature of type B thermocouple is between 540 °C and 1700 °C.
When it is expected to be exposed for a long time at temperatures above 13700 °c, it is prudent to specify a B-type thermocouple to increase the life of the thermocouple.
If maintained near the upper limit of use for a long time, the R & S type thermocouple experiences significant grain growth.
Since there are no sensing elements for the thermocouple, they do not have many temperature limiting materials for RTD.
The thermocouple is usually constructed using bare conductors and then insulated in compacted ceramic powder or molded ceramic insulator, a structure that allows the thermocouple to be used at a much higher temperature than RTD.
In temperature measurement, tolerances, precision, and interchange tolerances and precision are the most misunderstood terms.
The term tolerance refers to a specific requirement, usually by adding or subtracting some quantity.
On the other hand, precision refers to an infinite number of tolerances within a specified range.
For example, RTD contains a sensing element that is manufactured to have a specific resistance at a specific temperature.
The most common example of this requirement is the DIN standard.
In order to meet the requirements of the DIN standard, the resistance of the RTD must be 100 Ohm ± 0. 12 % (or 0. 12 Ohms)
Considered a B-Class sensor at 0 °C (
The A-level sensor is 100 Ohm ± 0. 06%.
Tolerance of 0.
12 ohm is only suitable for resistance at 32 °c and cannot be applied to any other temperature.
Many vendors will provide the RTD with a table of interchanges that provide the user with a tolerance table at a specific temperature.
On the other hand, the way the thermocouple is specified is different from RTD because they are made differently.
Unlike the sensing elements in RTD, the mV potential generated in the thermocouple is a function of the material composition and the metallurgical structure of the conductor.
Therefore, the thermocouple is not specified at a specific temperature, but is specified with an error limit covering the entire temperature range.
These limits assigned to the thermocouple are called standard error limits or special error limits.
Table 3 contains standard and special error limit specifications for each standard thermocouple type.
It must be noted that the error value limit listed in Table 3 applies to the new thermocouple before use.
Once the thermocouple is exposed to process conditions, a change in the thermocouple conductor may result in an increase in error.
Users are encouraged to conduct tests on a regular basis to determine the condition of the thermocouple used in high reliability or near precision applications.
Comparison of response time of different sensors each temperature sensor has its own advantages and disadvantages.
Advantages of RTD: RTD is often used in applications where repeatability and accuracy are important considerations.
The properly constructed platinum RTD has a very repeatable resistance
Temperature properties over time.
If a process will operate at a specific temperature, a specific resistance of RTD at that temperature can be determined in the laboratory and will not change significantly over time.
RTD also allows for easier interchanges, as their original changes are much lower than those of the thermocouple.
For example, the standard error limit of the K-type thermocouple used at 400 °c is ± 4 °c. A 100-
The interchange degree of Ohm DIN, grade B platinum RTD is ± 2.
2 °c at the same temperature.
RTD can also be used in conjunction with standard meter cables for connection to display or control equipment where the thermocouple must have a matching thermocouple wire to obtain accurate measurements.
RTD weakness: in the same configuration, you can expect to pay RTD 2 to 4 times more than base metal thermocouple.
RTD is more expensive than a thermocouple because more construction is required to make RTD, including making sensing elements, connecting extension lines, and assembling sensors.
Due to the structure of the sensing element, the RTD is not performing as well as the thermocouple in high vibration and mechanical impact environments.
The temperature of the RTD is also limited to about 650 °c, where the thermocouple can be used up to 1700 °c.
Strength of the thermocouple: the thermocouple can be used at temperatures up to 1700 °C, usually at a lower cost than RTD, and in a smaller size (
About. 020\'\' dia)
To achieve faster response to temperature.
The thermocouple is also more durable than the RTD, so it can be used in high vibration and impact applications.
Weakness of the thermocouple: when exposed to medium or high temperature conditions, the thermocouple is more unstable than the RTD.
In key applications, to verify performance, the thermocouple should be removed and tested under controlled conditions.
When connecting a thermocouple sensor to a thermocouple meter or control device, a thermocouple extension cord must be used.
Use of instrument wires (plated copper)
Errors are caused when the ambient temperature changes.
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