In daily operations, we often encounter the use of temperature sensors, thermocouples and RTDs are used as temperature sensing components, but their principles and functions are different. What are the differences?
1. Differences in operating principles
The thermocouple is made of two different conductors or semiconductor materials soldered or twisted, divided into hot end and free end, the hot end is pierced into the equipment that needs to measure the temperature, and the cold end is placed outside the equipment, if the temperature of the two ends is different, the thermoelectric potential will occur in the thermocouple circuit, because the thermoelectric potential is a function of the measured temperature, and the value of the electromotive force can be converted into the temperature value. Thermal resistance is based on the property that the resistance value of the conductor changes with the change of temperature, converts the change of resistance into an electrical signal, and then measures the temperature.
2. Structural differences
The structure of a thermocouple
There are 3 types of thermocouple tip joints, as shown in the figure below. According to the type of thermocouple, wire diameter, and operating temperature, it can be joined by gas welding, butt welding, resistance welding, arc welding, silver welding, etc.
In industrial applications, in order to facilitate installation and extend the service life of thermocouples, the method of adding an external sleeve is usually used. Casings are generally divided into protective tube type and armored type.
The structure of a thermal resistance
There are 3 types of component shapes for RTDs, and ceramic package types are now dominant. The ceramic encapsulated version is used for RTDs with protective tubes as well as for armoured RTDs. The diameter of the bare platinum wire is about a few tens of microns for the ceramic and glass encapsulated type, and about 0.05 mm for the mica plate type. The lead wires are made of platinum alloy wires, which are much thicker than the component wires.
In industrial applications, thermocouples and RTD protective sleeves are simply the same shape, how to identify them without a nameplate and without knowing the signal?
The first thing is to look at the lead wire of the temperature measuring element, usually the thermocouple only has two lead wires, if there are three lead wires, it is a thermal resistance. However, if there are four lead-out wires, the resistance value needs to be measured to determine whether it is a two-branch RTD or a four-wire RTD. If it is infinite, then it is a double RTD, and a pair of pinouts with a resistance value of almost zero is a RTD. If the resistance of two pairs of lead wires is between 10 and 110, it is a single four-wire thermal resistance, and its resistance value is close to what graduation number of thermal resistance, then it is the thermal resistance of the graduation number.
If there are only two lead-out wires, you can use a digital multimeter to measure the resistance value to determine it, because the resistance value of the thermocouple is very small, and the thermal resistance is simply zero; If the resistance value is very small when measuring, it may be a thermocouple.
The RTD will also have a minimum resistance value greater than 10 at room temperature. Commonly used thermal resistance has Pt10, Pt100, Pt1000 platinum thermal resistance, Cu50, Cu100 copper thermal resistance four graduation numbers, at room temperature 20 °C, its resistance value Pt10 is 10.779, Pt100 is 107.794, Pt100 is 1077.94, Cu50 is 54.285, Cu100 is 108.571. When the room temperature is greater than 20°C, the resistance value is greater, and the resistance value of the two can be mostly distinguished. If it is a thermal resistance, you can also know what kind of thermal resistance it is.
How can I tell the difference on the job site?
Thermocouples: Thermocouples have positive and negative electrodes, and compensation wires are also positive and negative. The first thing is to make sure that the connection is made and the equipment is correct. In operation, the common ones are short circuit, open circuit, poor contact (can be distinguished by a multimeter) and transformation (according to the appearance of the color to identify), when viewing, the thermocouple and the secondary meter should be separated. The method I judge in practice is for your reference: use something to short the compensation line on the secondary meter, the meter indicates the room temperature (if not, the meter is bad), and then short the thermocouple terminal, the meter indicates the ambient temperature of the thermocouple location (not, the compensation line is faulty), and then use the multimeter MV file to roughly estimate the thermoelectric potential of the thermocouple (if normal, please check the process).
Thermal resistance: nothing more than short circuit, and open circuit, can be judged with a multimeter. In operation, if there is a doubt about a short circuit, as long as the resistor end is removed from a wire to see the appearance, such as to the maximum, the thermal resistance is short-circuited, zero-back, and the wire is short-circuited, so as to ensure that the normal connection and equipment, the meter value is low or unstable, and the protective pipe may enter the water. Appears maximum, RTD breaks, appears minimum, short circuit. Generally speaking, RTDs are used for temperatures below 300 degrees Celsius, and thermocouples are used for temperatures above 300 degrees. As the temperature changes, the resistance of the resistance changes and the thermoelectric potential of the thermocouple changes.
Finally, how to choose a thermocouple and a RTD?
Selection according to the scale of temperature measurement: thermocouples are generally selected above 500°C, and thermal resistance is generally selected below 500°C;
Selection according to measurement accuracy: select thermal resistance with high accuracy requirements, and select thermocouples with low accuracy requirements;
Selection according to the measurement scale: the thermocouple generally refers to the "point" temperature, and the thermal resistance generally refers to the spatial average temperature.
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