Thermocouple - what is it? Thermocouple for gas boiler

Thermocouple - what is it? Much is clear already from the title. The device is a transducer used to measure the temperature of the working medium. Structurally it represents two conductors from different metals welded or soldered to each other at one end. The device is very simple, but it is difficult to produce it qualitatively.

How the 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 the thermocouple is as follows. On Wednesday, which is monitored, the working junction is placed, 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 own thermal ranges. They can also have different resistance to corrosion and aggressive environments.

Appointment

For accident-free operation of thermal equipment, a thermocouple is used. What is this 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 failures in the operation of the gas boiler. A thermocouple for a gas stove is also needed to create an electric gas pod as soon as its supply begins. In addition to performing the monitoring function, the device can be used as a temperature sensor.

Advantages

Let us consider the positive characteristics of a thermocouple:

• High accuracy of the device;

• Wide measuring range;

• The possibility of fixing a high temperature;

• Simplicity of design;

• Availability, cheapness and durability of thermoelectrodes;

• Simplicity of installation and maintenance.

disadvantages

The thermocouple has also negative sides:

• Very low sensitivity;

• Great resistance;

• Nonlinearity of temperature dependence of thermo-emf;

• The need to artificially maintain the temperature of one of the ends.

The thermo-emf of the electrodes essentially depends on the presence of impurities in metals, mechanical and thermal processing. To increase it, thermal batteries of several thermocouples are used.

Industrial thermocouple device features

Thermal sensors are made mostly of base metals. From the impact of the external environment they close the pipe with a flange, which serves to fix the device. The protective armature protects conductors from the influence of an aggressive environment and is made without a seam. The material is ordinary (up to 600 ° C) or stainless (up to 1100 ° C) 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 to it directly, without additional connectors. When the measuring instrument is positioned at a distance, when it is connected to the circuit, the free ends of the thermocouple are placed in the 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 not having precise and strictly controlled characteristics. They are of lower cost and greater thickness. They are injected into the head through a nipple with an asbestos gasket. Ceramics serves to equalize the temperature at all junctions. On top is a threaded protective cover with a sealed seal.

Do not install crimp terminals on the wires, as they may degrade the accuracy of the readings. From the wire make a ring and clamp it under the screw.

Correction of temperature changes on the terminals can be made by an electronic device, which increases the accuracy of measurements.

What a thermocouple can be. Price and characteristics

Thermocouple can be any two welded wires of dissimilar metals. In the industry used interchangeable materials that can withstand high temperatures, long-lived and with a large thermo-emf. There are also other static devices with high accuracy and a similar operating principle, but thermocouples are simpler in design and cheaper.

As an electrode, soft annealed wire is used from special alloys. It operates at a maximum temperature of about 1000 degrees. Alloys have stable and high values of thermo-emf.

The most common are sensors with two electrodes, where the cathode is a chromium-nickel alloy (chromel), and the anode is another metal, for example, alumel (thermocouple TXA). The simplest connection is made using a plug.

Each type of sensor differs in the operating temperature range, the value of the generated EMF, resistance to mechanical and chemical influences, durability and interchangeability.

Acquire 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 executed in different versions depending on the model. The prices for the sensors are approximately at the same level. On average, they are 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 no compensation wires are required. A conventional copper wire can be connected to the thermocouple head.

It is preferable to take products for specific models, the technical characteristics of which are suitable for each other. Universal devices do not differ in durability.

Types of sensors

1. Type K of nickel-chromium (thermocouple TXA) or nickel-alumel (CA), having the following properties: low price, durability, error not more than 0.4%, measuring range from -270 to 1269 degrees. Designed for operation in oxidative and inert media.

2. L - chromel-copel (TCA), an inexpensive thermocouple with an upper limit of 600 degrees.

3. J is iron-constantan. The sensor ranks second in popularity, the range is from -210 to +760 deg, less durable, resistant to oxidation.

4. E - nickel-chromium or nickel-constantan with higher accuracy and signal magnitude, the upper limit of measurements does not exceed 870 degrees.

5. Sensors made of noble metals work at a higher temperature, but have a high cost, which is why they are most often used in industry.

Thermocouple connection options

By the methods of measurement, the most common are grounded sensors. At them the ends of a wire are welded in one knot, finished with a special probe. The sleeve is in contact with the protective outer sheath, so that heat is transferred quickly, and the thermocouple has a low inertia. The accuracy of the indications is greatly influenced by electrical interference. The thermocouple for a gas boiler works on this principle. The measuring device must not be grounded at this, because additional contours can enter the ground, introducing an error in the readings.

If the junction does not come into contact with the containment, then such a structure is called ungrounded. Despite the fact that at the same time the speed of the sensor decreases, it is less susceptible to parasitic electrical interference.

The working junction can also be directly placed in the medium to be measured, 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.

To measure the temperature at several points, a multi-zone thermocouple is used. The connection scheme for it is done on the same principle, only a sensor is connected to several sensors located in two or more places.

How does a thermocouple with a gas boiler work?

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 from the flame of the ignitor. In the circuit, a thermal emf is applied equal to 20-25 mV, the values of which are sufficient for the operation of the solenoid valve. This opens the gas supply to the boiler heating. The pilot burner always functions while the boiler is running. From it, the main burner, which is heating the water, is lit. A thermocouple for a gas stove is also necessary to provide an electric ignition on the burners.

In addition, some slabs are protected against failures in the feed, when gas is lost in the network, and then fed again.

When the gas flare in the boiler burns, the place of adhesion of thermoelectrodes remains heated, and thus fuel supply is ensured. After the flame goes out, the working junction of the thermocouple cools, and it ceases to produce a current. In this case, an emergency shutdown of the solenoid valve closing the gas occurs.

Check of serviceability

The thermocouple for the gas boiler is checked using a solenoid valve, which is sold in specialized stores. To the terminals of the valve winding, two flexible wires with crocodile clips should be soldered, 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 spark plug, the valve must function from the current being generated. The device is very convenient and unpretentious.

The thermocouple is also tested with a millivoltmeter. The voltage at its free ends must be at least 25 mV.

One of the reasons for switching off the ignitor is a malfunction of the thermocouple. Bad work can be caused by the formation of foreign objects in the place of adhesion. It is cleaned with a nest of "zero". You need to delete only the raid. Otherwise, the spike will be destroyed.

If the thermocouple wire is broken, it can be connected with a short piece of an ordinary copper wire. It is important that the contact points are at the same temperature.

If the thermocouple for the boiler has burnt out, it must be replaced. You can fuse the ends of the wires, twisting them together and connecting them to the battery terminals. In this case, you need to choose the correct connection time, so that the spike is of high quality. In addition, it must be done so that the thermocouple wires do not get metal from the terminals of the battery. The place of twisting in the welding process is usually immersed in graphite powder. If you assemble a device with a LATR, you can adjust the current, and the joint will be of high quality. All these measures are temporary, if the case is convenient, the gas thermocouple must be replaced with 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 down the gas boiler in the event of an emergency.

Thermocouples are also used for electric ignition in gas stoves and as temperature sensors.