Working principle and classification of temperature transmitter - part 2

Principle and material of thermal resistance temperature measurement

Thermal resistance temperature measurement is based on the characteristic that the resistance value of metal conductor increases with the increase of temperature. Thermal resistors are mostly made of pure metal materials. At present, platinum and copper are most used. In addition, nickel, manganese and rhodium have been used to make thermal resistors.

Type of thermal resistance

1) Ordinary thermal resistance

According to the temperature measurement principle of thermal resistance, the change of measured temperature is directly measured by the change of thermal resistance value. Therefore, the change of resistance of various wires such as the outgoing line of thermal resistance body will affect the temperature measurement.

2) Armored thermal resistance

Armored thermal resistance is a solid body composed of temperature sensing element (resistor), lead wire, insulating material and stainless steel sleeve. Its outer diameter is generally φ 2-- φ 8mm, min φ mm。 Compared with ordinary thermal resistance, it has the following advantages: ① small volume, no air gap inside, small measurement lag in thermal inertia; ② Good mechanical performance, vibration resistance and impact resistance; ③ Can bend, easy to install ④ long service life.

3) End thermal resistance

The end face thermal resistance temperature sensing element is wound by specially treated resistance wire and is close to the end face of the thermometer. Compared with the general axial thermal resistance, it can more accurately and quickly reflect the actual temperature of the measured end face, and is suitable for measuring the end face temperature of bearing bush and other parts.

4) Flameproof thermal resistance

Through the junction box with special structure, the explosion of explosive mixed gas inside its shell caused by spark or arc is limited in the junction box, and there will be no overload explosion on the production site. Flameproof thermal resistance can be used for temperature measurement in places with explosion hazard in bla-b3c area.

For the temperature transmitter, you can see the following introduction

1. Purpose of temperature transmitter

The temperature transmitter is used for temperature measurement. In this paper, the temperature transmitter is defined as a temperature measuring device that uses the temperature sensor for temperature detection and converts the signal of the temperature sensor into standard current signal and standard voltage signal through the conversion circuit.

2. Classification of temperature transmitters

The signal output interface is divided into 4 ~ 20mA, 0 ~ 10mA, 0 ~ 20mA, 0 ~ 5V, 0 ~ 10V, or digital / frequency interface and other interfaces;

In terms of structure and installation form, it can be divided into wall mounted type, air duct type and probe rod type;

From the distance between the temperature sensor and the conversion circuit, it can be divided into integrated type or split type;

There are also explosion-proof and other types of products.

3. Basic principle of temperature transmitter

The temperature sensor is usually thermal resistance, thermistor, integrated temperature sensor, semiconductor temperature sensor, etc., and then the signal of the temperature sensor is converted into standard current signal or standard voltage signal through the conversion circuit.

4. Main technical parameters of temperature transmitter

Temperature measurement range

Temperature measurement accuracy

Output form of temperature measurement signal

Structure and installation form of temperature transmitter

5. Selection of temperature transmitter

Select according to the measurement range

You can refer to the type of temperature sensor used. See temperature sensor selection;

Select according to accuracy requirements

You can refer to the type of temperature sensor used. See temperature sensor selection;

Select according to signal interface

4 ~ 20mA, 0 ~ 10mA, 0 ~ 20mA, 0 ~ 5V, 0 ~ 10V, or digital / frequency interface and other interfaces can be selected;

It shall be selected according to the structural form and installation requirements. For example, wall mounted type shall be selected for indoor installation.

Thermal resistance PT100: the change of resistance value is achieved by sensing temperature change

Temperature transmitter:

1. Calculate the current temperature by confirming the difference of resistance value

2. The corresponding standard signal (4-20mA) value is output according to the range of thermal resistance

Namely: temperature change - thermal resistance - resistance change - temperature transmitter - 4 ~ 20mA signal

For example, the range of Pt100 is: - 199.9 degrees - 600.0 degrees

After the temperature transmitter converts this into a standard signal, the corresponding 4mA is - 199.9 degrees and 20mA is 600.0 degrees

The current temperature value can be known by confirming the output current of the transmitter.