Thermocouple - what is it? Much is already clear from the name. The device is a transducer used to measure the temperature of the working environment. Structurally, it consists of two conductors made of different metals, welded or soldered to each other at one end. The device is very simple, but it is difficult to make it of high quality.
How does a thermocouple work?
Two dissimilar conductors are connected in a ring. When the temperature of the joints is different, a potential difference appears between them due to the thermoelectric effect.
The principle of a thermocouple is as follows. A working junction is placed in the medium under control, and the free ends are connected to the measuring device. The greater the difference between the properties of the conductors and the thermal difference at the ends, the higher the electromotive force in the circuit (thermo-EMF).
The relationship between voltage and temperature for different metals varies. These or other types of sensors are designed for their thermal ranges. They may also have different resistance to corrosion and aggressive environment.
Appointment
A thermocouple is used for trouble-free operation of thermal equipment. What is it for a gas boiler? And how does it work? Should be considered in more detail. First of all, it provides automatic shutdown of equipment in case of malfunctions in the gas boiler. A thermocouple for a gas stove is also necessary to create electric ignition of the gas as soon as it begins to flow. In addition to performing the monitoring function, the device can be used as a temperature sensor.
Advantages
Let's consider what positive characteristics a thermocouple has:
high accuracy of the device;
wide range of measurement;
the possibility of fixing high temperature;
simplicity of design;
availability, cheapness and durability of thermoelectrodes;
ease of installation and maintenance.
disadvantages
The thermocouple has its negative sides:
Thermo-EMF of electrodes significantly depends on the presence of impurities in metals, mechanical and thermal processing. To increase it, thermopiles of several thermocouples are used.
Industrial thermocouple device features
Thermal sensors are made mostly of base metals. From the influence of the external environment they are closed with a pipe with a flange, which serves to mount the device. Protective fittings protect the conductors from the influence of an aggressive environment and are done seamlessly. The material is ordinary (up to 600º) or stainless (up to 1100º) steel. Thermoelectrodes are isolated from each other by asbestos, porcelain tubes or ceramic beads.
If the terminal is located close, then the thermocouple wires are connected directly to it, without additional connectors. When the measuring device is located at a distance, when it is included in the circuit, the free ends of the thermocouple are placed in a cast head attached to the protective tube. Inside are brass terminal blocks on a porcelain base for connecting compensation wires made of the same materials as thermoelectrodes, but not having accurate and strictly controlled characteristics. They have a lower cost and greater thickness. They are introduced into the head through a fitting with an asbestos gasket. Ceramics are used to equalize the temperature at all joints. On top is a threaded protective cover with a tight seal.
Crimp terminators must not be installed on the wires, as they may impair the accuracy of the readings. A ring is made of wire and clamped under a screw.
Correction of temperature changes at the terminals can be made by an electronic device, which increases the accuracy of measurements.
What can be a thermocouple. Price and Feature
A thermocouple can be any two welded wires of dissimilar metals. The industry uses interchangeable materials that can withstand high temperatures, durable and with large thermal emf. There are also other static devices with high accuracy and a similar principle of operation, but thermocouples are simpler in design and cheaper.
As the electrodes, soft annealed wire from special alloys is used. It works at an extreme temperature of about 1000 degrees. Alloys have stable and high values of thermo-EMF.
The most common sensors are two electrodes, where the cathode is a chromium-nickel alloy (chromel) and the anode is another metal, for example, alumel (TXA thermocouple). Its simplest connection is made using a plug.
Each type of sensor differs in the operating temperature range, the magnitude of the generated emf, resistance to mechanical and chemical influences, durability and interchangeability.
You need to purchase only those devices that meet the required parameters. This is especially true for heat resistance, otherwise the device will have to be changed soon.
The thermocouple for the boiler is available in different versions depending on the model. Sensor prices are approximately at the same level. On average, they amount to 500-600 rubles. On sale there are also sensors complete with an additional electronic converter, which compensates the readings. It is built directly into the thermocouple head. The price of the sensor becomes higher, but it does not require compensation wires. An ordinary copper wire can be connected to the thermocouple head.
It is preferable to take products for specific models whose technical characteristics are suitable for each other. Universal devices do not differ in durability.
Types of sensors
Type K is made of nickel-chromium (TXA thermocouple) or nickel-alumel (XA), which has the following properties: low price, durability, error of not more than 0.4%, measurement limits from -270 to 1269 degrees. Designed for use in oxidizing and inert environments.
L - chromel-copel (THC), an inexpensive thermocouple with an upper limit of 600 degrees.
J is iron constantan. The sensor ranks second in popularity, the range is from -210 to +760 degrees, less durable, resistant to oxidation.
E - nickel-chromium or nickel-constantan with higher accuracy and signal size, the upper limit of measurements does not exceed 870 degrees.
Sensors made of precious metals operate at higher temperatures, but have a high cost, and therefore are most often used in industry.
Thermocouple Connection Options
By measurement methods, grounded sensors are most common. Their ends of the wire are welded into one knot, ending with a special probe. The sleeve is in contact with the protective outer shell, due to which heat is quickly transferred, and the thermocouple has a low inertia. The accuracy of the readings is greatly influenced by electrical noise. According to this principle, a thermocouple for a gas boiler works. In this case, the measuring device must not be grounded, since additional circuits may appear through the ground, introducing an error in the readings.
If the junction does not come into contact with the protective sheath, then this design is called ungrounded. Despite the fact that the speed of the sensor is reduced, it is less susceptible to stray electrical interference.
The working junction can also be directly placed in the measured medium, but this method reduces the life of the device due to corrosion. Such thermocouples are rarely used in production, and in household appliances they can be seen everywhere.
A multizone thermocouple is used to measure temperature at several points.
The connection diagram for her is done according to the same principle, only several sensors located in two or more places are connected to the device.
How a thermocouple works with a gas boiler
Thermocouple - what is it? For the user, everything becomes clear when interruptions in the operation of gas equipment occur. The working junction of the thermocouple in the boiler is heated by the igniter flame. A thermo-emf of 20-25 mV is induced in the circuit, the value of which is sufficient for the operation of the electromagnetic valve. This opens the gas supply for heating the boiler. The pilot burner always functions while the boiler is operating. The main burner that heats the water is ignited from it. A thermocouple for a gas stove is also necessary to provide electric ignition on the burners.
In addition, some cookers provide protection in the event of a power failure when gas is lost in the network and then supplied again.
When a gas flame is burning in a boiler, the place of soldering of the thermoelectrodes remains heated, and due to this, fuel is supplied. After the flame goes out, the working junction of the thermocouple cools down, and it ceases to generate current. In this case, an emergency shutdown of the electromagnetic valve that shuts off the gas occurs.
Health check
The thermocouple for a gas boiler is checked using an electromagnetic valve, which is sold in specialized stores. Two flexible wires with crocodile clamps must be soldered to the terminals of the valve winding, and then connected to the terminals of the thermocouple. When the working junction of a working thermocouple is heated in the flame of a gas burner or a candle, the valve must function from the generated current. The device is very convenient and unpretentious.
The thermocouple is also tested using a millivoltmeter. The voltage at its free ends must be at least 25 mV.
One of the reasons for the ignitor to trip is a thermocouple malfunction. Poor performance may be caused by the formation of foreign matter at the adhesion site. They clean it with sandpaper “null”. Only plaque needs to be removed. Otherwise, the spike will be destroyed.
When a thermocouple wire breaks, it can be connected with a short piece of ordinary copper wire. It is important that the contact points are at the same temperature.
If the thermocouple for the boiler burns out, it should be replaced. You can fuse the ends of the wires by twisting them together and connecting to the battery terminals. In this case, you need to choose the connection time correctly so that the spike turns out to be of high quality. In addition, it is necessary to make sure that metal from the battery terminals does not get on the thermocouple wires . The place of twisting during the welding process is usually immersed in graphite powder. If you assemble a device with LATR, then you can adjust the current, and the joint will turn out to be of high quality. All these measures are temporary, if necessary, the gas thermocouple should be replaced by a new one.
Conclusion
Thermocouple - what is it? It is a simple thermoelectric converter. Reliability and durability of the device make it an indispensable means of shutting off the gas boiler in the event of an emergency.
Thermocouples are also used for electric ignition in gas stoves and as temperature sensors.