Difference Between Thermometer and Thermostat -

Thermometer vs Thermostat

Thermostat and thermometer are two devices that are widely used in temperature measurements and controlling. This article intends to discuss the differences between these two devices.


Thermometer is a device used to measure the temperature of an object or the temperature gradient between two objects (points). There are several forms of thermometers. Mercury glass thermometers are the commercially most common type used today. The principle behind the mercury glass thermometer is the expansion of materials due to temperature. The mercury glass thermometer consists of a capillary tube with a vacuum inside and a bulb filled with mercury connected to one end. If the temperature of the mercury is increased, it will expand indicating a height in the capillary tube. This height is taken as a measurement of the temperature. The wall of the bulb is made extremely thin, to minimize the temperature gradient between mercury and the object, thereby reducing the time taken to the equilibrium. The amount of mercury used is very small; thereby the temperature drop due to the absorption of thermal energy is minimal. The capillary tube is made very thin, so that a small change in the volume will cause a large change in mercury height, thereby making the reading more accurate. Other common types of thermometers are thermocouples, Constant volume gas thermometer and silicon band gap sensors. Resolution is an essential quality of a thermometer. The resolution of a thermometer tells the minimum temperature difference that can be measured using the thermometer. Other noteworthy aspects are accuracy, thermal absorption, response time, reproducibility, recovery time, cost and mobility.


Thermostat is an instrument used to control the temperature of a system. A thermostat system consists of a temperature sensor, heat generator, and sometimes a cooling system. The operation of the thermostat is as follows.

– take an input for the temperature stabilization

– measure the temperature of the system

– turn on the heating system and turn off the cooling system, if the system temperature is lower than the defined temperature

– turn off the heating system and turn on the cooling system, if the system temperature is greater than the defined temperature.

The simplest form of thermostats is found in electric irons. It consists of a heating coil and a bimetal stripe, which the contact length is adjustable, connected in series with the power supply. The temperature control of the iron adjusts the gap between the bimetal stripe and the contact terminal. The bimetal strap is connected in such a way that if the contact terminal touches the bimetal strap the switch is in the “on” state. When the temperature of the system goes beyond the desired temperature, the bimetal stripe disconnects from the contact terminal, thus eliminating the current flow. When the system cools down, the bimetal strap returns to the normal state and touches the contact terminal.

What is the difference between thermometer and thermostat?

• Thermometer is a device used to measure the temperature; thermostat is a system used to control the temperature of a system.

• The thermometer is a passive device while the thermostat is an active device.

• Thermometer is a measurement device while the thermostat is a control device.

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