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The choice of thermal resistance or thermocouple, how to correctly select a temperature sensor, characteristics

Temperature sensor, features of choice

A temperature sensor is a device, a box in which an element is located, when power is applied, that changes its resistance depending on the measured temperature.

Properly selected high-quality choice of temperature sensor is the key to the production of quality products

Temperature is the main parameter in most technological processes, as well as in everyday life. Temperature measurement in technological processes is especially important, since the quality of products depends on it. To avoid product defects, it is necessary to choose the right temperature sensor and ensure measurement accuracy.

Consider the reasons for product defects:

1. Slow sensor response to sudden temperature changes
Let's say you have a fast-flowing process. But the temperature of the medium or body changes abruptly, so when choosing a sensor, it is necessary to take into account the reaction speed so that it doesn’t work out, as in the famous cartoon about Carlson: “Citizen, your milk has run away ...”
2. Incorrect selection of the sensor for the temperature range

As a result, frequent shutdowns of the technical process. The secondary device writes: "the sensor is not found", because a sensor up to 500 ° C is selected, and in the technological process there are temperature runouts above the limit.

Result: production is stopped because an alarm has been triggered.
3. Incorrect sensor installation

For example, to measure outdoor air temperature, the sensor should be installed on the north side of the building under a canopy, and not next to a window or vent.

The same is true in the technological process: if the temperature is not measured at a given point, we will get unreliable data.

Result: defective products.

4. Measurement accuracy
There are various tolerance classes and measurement errors. In some processes, accuracy is unimportant, but in some it is critical. Example: to control the temperature in the incubator, a thermocouple with an error of 2.5 ° C was chosen, but such an error is too large for this process! Result: the egg set is ruined.

So what do you need to know when choosing a sensor to avoid temperature errors in your process and ensure quality products?

Типы материалов
Resistance thermometers
When measuring temperature, the resistance at the output of the sensor changes.
The most popular NSH:
• copper 50M, 100M
• platinum 50P, 100P, Pt100, Pt500, Pt1000
• nickel Ni1000

NSH is a function or table of values in accordance with GOST R 6651-2009, which defines the "resistance-temperature" relationship.

Thermocouples
When the temperature changes, the EMF changes (GOST R 8.585-2001). The main NSH sensors manufactured by OWEN:
DTPC (XA, chromel-alumel)
DTPL (XK, chromel-copel)
DTPJ (LCD, iron-constantan)
DTPN (HH, nihrosil-nisil)
DTPS (PP, platinum-rhodium-platinum 30%)

Температурные кривые


Нелинейные зависимости

Thermistors
When the temperature changes, the resistance at the output changes abruptly. The most commonly used thermistors are 10 kΩ at 25 °C.

Resistance thermometer sets are paired sensors and transducers. Used as part of a heat meter for heat metering.


NTS_RTS
1. Temperature Range

This is the first thing to find out and understand for which process the sensor will be used.

Рабочий диапазон
2. Speed

The delay in temperature measurement is related to the speed of the measuring unit and the thermowell. The thinner the sensor armature, the less the sensor response to temperature changes. The sensor sleeve is filled with sand or thermal paste. If the sensor is installed in a protective sleeve, then the distance between the inner wall and the sensor sleeve must be minimal.

BUT! Don't forget about pressure! The smaller the diameter of the mounting part, the lower the pressure the converter is designed for. In addition, the speed is affected by the installation of the sensor..

3. Sensor installation

The sensitive element of the sensor (located at the end of the mounting tube) is installed directly in the measurement area.

To measure air temperature:
• When measuring the outdoor air temperature, the sensor is installed on the north side of the building under the canopy. Avoid installation near windows.
• When measuring indoor air temperature, avoid sunny places and stagnant air.
• When measuring the air temperature in the furnace, the working junction or mounting part is brought to the zone close to the heating zone of materials.

When installing the sensor in a pipeline:

Монтаж
• The mounting part of the sensor must be 2/3 in the pipeline. The sensing element is mounted on the axis or slightly below.
• If the sensor is consignment note, then we clean the surface of the pipe, apply heat-conducting paste, press the sensor to the pipe with a clamp, cover the thermometer with thermal insulation from above.

4. Error

Resistance thermometers and thermocouples have tolerance classes.
• For resistance thermometers: AA, A, B, C.
• For thermocouples tolerance classes 1 and 2.

It can be seen from the graphs that at temperatures from -196 to 150 ° C, resistance thermometers will be more accurate. At temperatures above 300 ° C - more precisely thermocouples.

Переходные процессы
Характеристики термопар

The author of this article is the OWEN company: https://owen.ru/forum/showthread.php?t=31527

#Sensortemperature, #thermistor, #thermal resistance, #thermocouple

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