A thermostat is a simple device that can be found in a car's cooling system, in various household or climatic appliances, as well as in automation systems in industrial production.
Content
What is a thermostat
A thermostat is a discrete mechanical device that, upon reaching a predetermined temperature setpoint, changes its state or the state of its electrical contacts.
These contacts, for example, can be used in relay circuits, starting or stopping various units, or transmitting information about reaching the temperature to the APCS automation system. The word itself comes from two Greek words: "θερµο-", which means heat and "στατός" - standing, motionless.
Unlike analog temperature sensors such as thermocouple or resistance thermometer, the thermostat will not show the true temperature value at a certain point in time. Its task is only to “work” at the pre-set temperature value, that is, change its state. After that, depending on the type of thermostat, the actions required for regulation are carried out.
Thermostats are used in devices or systems that heat or cool something to a given temperature. For example, in refrigerators, heating appliances, automotive engine cooling systems, industrial furnaces, etc.
What does a thermostat consist of and what is its principle of operation
The design of the thermostat and its principle of operation depend on the type of sensing element used. These can be bimetallic plates or metal capsules with capillary tubes filled with liquid or gas.
A bimetallic plate is two dissimilar metal strips with different coefficients of thermal expansion, which are welded together. During heating, one of the metal plates expands more, which leads, when the set temperature is reached, to its bending or straightening.
Mechanically moving in this way, the bimetallic plate can close or open electrical contacts or, for example, open a coolant valve.
Another common type of thermostat is capillary.Its work is based on the first law of thermodynamics, according to which, when the temperature in a thermodynamic system changes, it must perform mechanical work until it reaches an equilibrium state.
The capillary thermostat includes the following elements:
- a metal capsule containing a working fluid (eg, glycol);
- a capillary tube connecting the sensor to the thermostat control unit;
- a control unit or an electromechanical relay, with which the temperature setpoint is set.
When the metal capsule is heated, the volume of its contents changes, which through the capillary tube presses on the relay membrane and when the specified temperature is reached, its contacts close or open.
For all types of thermostats, the temperature is set mechanically by turning the adjusting screw, or it is rigidly set by the manufacturer to a certain temperature.
The purpose of the thermostat
As mentioned above, the main task of the thermostat is to control the temperature regime. The scope of use of thermostats is very wide: from an iron in a small apartment to huge ovens in industrial facilities. They are used in various equipment, heating systems, air conditioners and household appliances.
The thermostat makes their use safe and at the same time comfortable, since temperature control is carried out automatically.
Thermostats can be already built-in or used additionally, for example, in water mixers, to adjust the operation of gas boilers or heating gender.
The car thermostat is a mandatory component for the engine cooling system.It helps to maintain the required temperature without overheating.
Species and types
Thermostats can be divided according to the operating temperature range:
- Devices operating at high temperatures from +300 to 1200 °C.
- Medium level thermostats: -60 to 500 °C.
- With the lowest temperature range (cryostats): less than -60 °С. They work together with additional sources of cold.
Also, thermostats are classified depending on the stability and accuracy of operation. They are characterized by the deviation from the set temperature:
- 5 - 10 ° C - the worst indicator of the thermostat.
- 1 - 2 ° С - for an air thermostat this is a good indicator, but for a liquid one it is mediocre.
- 0.1 ° C - excellent for air, medium - for liquid.
- 0.01 °C - not achievable for an air thermostat, good for a liquid thermostat of a special design.
- 0.001 °C - this indicator can only be achieved in metrological liquid thermostats.
How to test the thermostat
To test the thermostat, the following method can be used: when the temperature setting is changed, characteristic clicks should be heard when the temperature value is equal to the ambient temperature - the contacts close and open.
If the thermostat is removable, then you can try to heat its sensitive element and check the operation.
If, for example, we consider the thermostat in the oven, then by setting a certain temperature, after warming up, you can observe the burner flame: if it has decreased and remains at the same level, then everything is in order.The accuracy of the result obtained can be achieved using a thermometer.
Correct operation of the thermostat setpoint can be done using a thermometer or multimeter with thermocouple. This method is suitable, for example, for a washing machine. A tester will also help, which, by connecting to the thermostat contacts, will show their closing and opening.
What could be a thermostat problem?
The main problems with this device of all varieties are constantly closed or open contacts, regardless of temperature. Another of the malfunctions is a large error, that is, a discrepancy between the temperature indicators given.
Thermostat vs thermostat - what's the difference?
Temperature controllers are a more capacious concept. Thermostats are part of them.
Modern thermostats have analog inputs from sensors, with the ability to display the measured temperature on the display, and analog and discrete outputs for process control. They have the ability to record measured parameters in memory and display a graph of the regulation process.
The task of the thermostat is much simpler - to switch contacts at the set temperature value.
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