Thermocouple - what is it? A lot is clear from the title. The device is a transducer used to measure the temperature of the working medium. 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 manufacture it well.
How a thermocouple works
Two dissimilar conductors are tied into 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 to be controlled, 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 varies from metal to metal. Certain types of sensors are designed for their own thermal ranges. They may also have different resistance to corrosion and aggressive environments.
Destination
Fortrouble-free operation of thermal equipment, a thermocouple is used. 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 the event of a malfunction of the gas boiler. A thermocouple for a gas stove is also needed to create an electrical ignition of the gas as soon as it starts to flow. In addition to performing the control function, the device can be used as a temperature sensor.
Dignity
Let's look at the positive characteristics of a thermocouple:
- high precision instrument;
- wide measuring range;
- possibility to fix high temperature;
- simple design;
- availability, low cost and durability of thermoelectrodes;
- easy installation and maintenance.
Flaws
Thermocouple also has its downsides:
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very low sensitivity;
- great resistance;
- nonlinearity of temperature dependence of thermo-EMF;
- need to artificially maintain the temperature of one of the ends.
Thermo-EMF of electrodes depends significantly on the presence of impurities in metals, mechanical and thermal processing. To increase it, thermopiles of several thermocouples are used.
Industrial thermocouple design features
Thermal sensors are mostly made from non-precious metals. Fromexposure to the external environment, they are closed with a pipe with a flange that serves to fasten the device. Protective fittings protect conductors from the influence of an aggressive environment and are made without a seam. The material is ordinary (up to 600ºС) or stainless (up to 1100ºС) steel. Thermoelectrodes are isolated from each other with asbestos, porcelain tubes or ceramic beads.
If the terminal is close, then the thermocouple wires are connected to it directly, without additional connectors. When the measuring device is located at a distance, when it is connected to the circuit, the free ends of the thermocouple are placed in a cast head attached to the protective tube. Inside there are brass terminal blocks on a porcelain base for connecting compensating wires made of the same materials as thermoelectrodes, but without precise and strictly controlled characteristics. They are less expensive and thicker. They are introduced into the head through a fitting with an asbestos gasket. Ceramic serves to equalize the temperature at all joints. On top is a threaded protective cap with an airtight seal.
Crimp terminations should not be installed on the wires, as they may degrade the accuracy of the readings. A ring is made from the wire and clamped under the screw.
The terminal temperature change can be corrected by an electronic device, which improves the measurement accuracy.
What can be a thermocouple. Price and features
The thermocouple can be any two welded wires of dissimilar metals. Used in industryinterchangeable materials, able to withstand high temperature, durable and high thermal EMF. There are also other static devices with high accuracy and a similar principle of operation, but thermocouples are simpler and cheaper.
Soft annealed wire made of special alloys is used as electrodes. It operates at a maximum temperature of about 1000 degrees. Alloys have stable and high thermo-emf values.
The most common sensors are those with two electrodes, where the cathode is a nickel-chromium alloy (chromel) and the anode is another metal, such as alumel (TCA thermocouple). The easiest way to connect it is with a plug.
Each type of sensor differs in operating temperature range, EMF value, mechanical and chemical resistance, 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.
Thermocouple for the boiler is available in different versions depending on the model. Prices for sensors are about 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 for the readings of the device. It is built directly into the thermocouple head. The price of the sensor becomes higher, but compensating wires are not required. Can be connected to the thermocouple headordinary copper wire.
It is preferable to take products for specific models, the specifications of which are suitable for each other. Universal devices do not differ in durability.
Types of sensors
- Type K made of nickel-chromium (TCA thermocouple) or nickel-alumel (XA), having the following properties: low price, durability, error not more than 0.4%, measurement limits from -270 to 1269 degrees. Designed to work in oxidizing and inert environments.
- L - Chromel-Kopel (THC), an inexpensive thermocouple with an upper limit of 600 degrees.
- J - iron-constantan. The sensor is the second most popular, range is from -210 to +760 deg, less durable, resistant to oxidation.
- E - nickel-chromium or nickel-constantan with higher accuracy and signal strength, the upper limit of measurement does not exceed 870 degrees.
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Precious metal sensors operate at higher temperatures but are more expensive, making them the most commonly used in industry.
thermocouple for gas boiler
Thermocouple connection options
According to the measurement methods, grounded sensors are the 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 sheath, due to which heat is quickly transferred, and the thermocouple has a small inertia. The accuracy of readings is greatly affected by electrical interference. According to this principleworking thermocouple for gas boiler. In this case, the measuring device should not be grounded, since additional circuits may appear through the ground, introducing an error in the readings.
If the junction is not in contact with the protective sheath, then this design is called ungrounded. Although this reduces the speed of the sensor, it is less susceptible to parasitic electrical interference.
The working junction can also be directly placed in the measured medium, but this method reduces the service life of the device due to corrosion. In production, such thermocouples are rarely used, but in household appliances they can be seen everywhere.
A multipoint thermocouple is used to measure temperature at multiple points.
The connection diagram is made according to the same principle, only several sensors are connected to the device located in two or more places.
How a gas boiler thermocouple works
Thermocouple - what is it? For the user, everything becomes clear when there are interruptions in the operation of gas equipment. The working junction of the thermocouple in the boiler is heated by the igniter flame. A thermo-emf equal to 20-25 mV is induced in the circuit, the value of which is sufficient to trigger the electromagnetic valve. This opens the gas supply for heating the boiler. The pilot burner always functions while the boiler is running. From it, the main burner is ignited, which heats the water. A thermocouple for a gas stove is also needed to provide electric ignition on the burners.
In addition, some cookers provide power failure protection, when the network loses gas and then re-supplies.
When the gas flare burns in the boiler, the place where the thermoelectrodes are soldered remains heated, and this ensures fuel supply. After the flame goes out, the working junction of the thermocouple cools down, and it stops generating current. This causes an emergency shutdown of the solenoid valve that shuts off the gas.
He alth check
The thermocouple for a gas boiler is checked using a solenoid valve, which is sold in specialized stores. Two flexible wires with crocodile clips must be soldered to the valve winding terminals, and then connected to the thermocouple terminals. When the working junction of a working thermocouple is heated in the flame of a gas burner or candle, the valve must operate from the generated current. The device is very convenient and unpretentious.
Checking the performance of a thermocouple is also done using a millivoltmeter. The voltage at its free ends must be at least 25 mV.
One of the reasons for turning off the igniter is a thermocouple malfunction. Poor operation can be caused by the formation of foreign formations at the soldering site. It is cleaned with sandpaper "zero". You only need to remove plaque. Otherwise, the spike will be destroyed.
If the 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 is burned out, it should be replaced. You can fuse the ends of the wires by twisting them together and connecting them to the battery terminals. In this case, you need to choose the right connection time so that the soldering 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 possible, the gas thermocouple should be replaced with a new one.
Conclusion
Thermocouple - what is it? It is a simple thermoelectric converter. The reliability and durability of the device make it an indispensable tool for shutting down a gas boiler in an emergency.
Thermocouples are also used for electric ignition in gas stoves and as temperature sensors.
