Temperature is the main parameter

A correctly selected temperature sensor is the key to producing 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 the products depends on it. To avoid product defects, it is necessary to select the correct temperature sensor and ensure the measurement accuracy. Let's look at the reasons for product defects.

1. Slow sensor response to sudden temperature changes.

For example, you have a fast flowing process. But the temperature of the medium or body changes abruptly, therefore, when choosing a sensor, it is necessary to take into account the reaction rate.

2. Incorrect selection of the sensor for the temperature range.
As a result, there are frequent shutdowns of the technical process. The secondary device writes: “sensor not found”, because the selected sensor is up to 500 ° C, and in the technological process there are temperature overshoots above the limit.
Result: stopping the technological process, because the alarm has been triggered.

3. Incorrect sensor installation.
For example, to measure the outdoor temperature, the sensor should be installed on the north side of the building under a canopy, not near a window.
So it is in the technological process: if the temperature is not measured at a given point, we will get inaccurate data.
Result: defective products.

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

So, what you need to know when choosing a sensor to avoid errors in temperature measurement in the process and to ensure the production of quality products?

Типы материалов
The Russian Federation has а National standard 6651-2009 "State system for ensuring the uniformity of measurements. Thermal converters of resistance from platinum, copper and nickel. General technical requirements and test methods" (The NS 6651-2009).

The NS 6651-2009 has a concept "The nominal static characteristic" (NSC). The NSC (Nominal Static Characteristic) is a function or table of values that defines the resistance-temperature relationship.

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

There is NS R 8.585-2001 "State system for ensuring the uniformity of measurements. Thermocouples. The nominal static characteristics of the conversion" (NS R 8.585-2001).

Thermocouple electromotive force
When the temperature changes, the electromotive force also changes (NS R 8.585-2001).
The main NSC sensors produced by the OWEN company:
DTPK (HA, chromel-alumel)
RTPL (HC, chromel-copel)
RTPJ (ZhK, iron-constantan)
DTPN (NN, nichrosil-nisil)
RTAs (PP, platinum-rhodium-platinum 30%)

Температурные кривые

Нелинейные зависимости
When the temperature changes, the output resistance changes abruptly. Most commonly used thermistors are 10 kΩ at 25 ° C.
Resistance thermometer kits are matched sensors and transducers. They are used as part of a heat meter for heat metering.

1. Temperature range
This is the first thing to find out. It is necessary to understand for which process technology the sensor will be used.
Рабочий диапазон
2. Rapidity of action

The delay in temperature measurement is related to the speed of the measurement unit and thermowell. The thinner the sensor armature, the lower the sensor's response to temperature changes. The sensor well is filled with sand or thermal grease. If the sensor is installed in a thermowell, the distance between the inner wall and the sensor well must be as small as possible.

Attention! Don't forget about pressure! The smaller the diameter of the mounting part, the lower the pressure the transmitter is designed for. In addition, the installation of the sensor affects the performance.

3. Installing the sensor
The sensing 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 temperature, the sensor is installed on the north side of the building under a canopy. Avoid installing next to windows.
• When measuring the room temperature, avoid sunny places and stagnant air.
• When measuring the air temperature in the furnace, the working junction or the mounting part is brought to the zone close to the heating zone of materials.

When you installing the sensor in a pipeline:
• The sensor mounting part must be 2/3 in the pipeline. The sensing element is installed on the axis or slightly below.
• If the sensor is attached, then we clean the pipe surface, apply heat-conducting paste, press the sensor to the pipe with a clamp, cover the thermometer with thermal insulation on top.


4. Accuracy
There are tolerance classes for resistance thermometers and thermocouples.
• For resistance thermometers: АА, А, В, С.
• For thermocouples, tolerance classes 1 and 2.

The graphs show that at temperatures from -196 to 150 ° C, resistance thermometers will perform more accurate measurements. At temperatures above 300 ° C - thermocouples.

Переходные процессы
Характеристики термопар
The author of this article is OWEN : https://owen.ru/forum/showthread.php?t=31527
#Sensor, #temperature, #thermistor, #thermalresistance, #thethermocouple
Russian version
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