Temperature is a physical quantity which is defined as the hotness or coldness of any object or substance. We can calibrate temperature in different scales and in different units according to our requirements. Kelvin (k) is the international system of units (SI) of temperature, other than Kelvin temperature is measured in Celsius scale (C) and Fahrenheit scale (F). We can easily convert one unit of temperature into another by using some calculations. For measuring temperature we use different methods and devices. Till now many methods have been developed for measuring temperature. Almost all the methods depend upon the measuring some physical property of a working material that varies with temperature. Following are the most commonly used temperature measuring devices.
- Liquid in Glass Thermometer
- Electric Resistance Thermometer
- Radiation Thermometry
- Silicon Diode
- Bimetallic Devices
- Bulb and Capillary Sensor
- Constant volume Gas Thermometer
- Sealed Bellows
- Constant Pressure Gas Thermometer
LIQUID IN GLASS THERMOMETER:
It is the most common type of temperature measuring device. We all are aware of it and use it many times for measuring our body temperature in case of fever. As name suggests this device consists of a small diameter glass tube containing some liquid. The most commonly used liquid is mercury. But due to some hazardous effect of mercury, other fluids for example organic liquid or some alcohol which are less toxic replaced mercury. The liquid is chosen in such a way that it expands quickly as per slight change in temperature. Thermal expansion of liquid takes place into the thermometer. The volume of the liquid changes with change in the temperature. The small change in the volume drives the mercury column relatively long way. The space in the glass tube is filled with nitrogen or partial vacuum. A scale is marked on the periphery of the thermometer to measure the temperature as liquid expands. The scale is calibrated properly according to the temperature variation. The accuracy of this type of thermometer is based on the manufacturing process. It can measure only limited range of temperature. It is cheap and easy to use and no power source is required.
- It is mostly used in hospitals and homes for measuring body temperature.
- In air craft application to measure the atmosphere condition which is suitable for flight.
- It is used in meteorological and oceanographic applications. It helps to measure weather forecast models.
ELECTRIC RESISTANCE THERMOMETER:
Electric resistance thermometer was developed after the liquid in glass type thermometer. In this, the change in the resistance of a metal wire due to change in the temperature is measured. When electric current flows through the wire then the wires scatter of each other due to the electric resistance. The wires used are of platinum because platinum do not react with air and due to its non corrosive property. These wires are normally wound into a coil and placed in a ceramic tube or they may be attached like wheat stone bridge circuit.
- It can measure temperature to a higher degree of accuracy
- It is very sensitive so that a small change in the temperature can be measured.
- It is mostly used for calibrations of other thermometers due to its higher degree of accuracy and sensitivity.
It is based upon the radiation emitted by the body. All the objects emit infrared radiations with intensity proportional to its temperature. These radiations detect by radiation detector known as radiation thermometers. Radiation thermometer consists of a number of series of optics that focus infrared lights onto a special electronic detector. This electronic detector is made up of semiconductor material. Semiconductor materials produce electric current which is directly proportional to the intensity of infrared radiation. This device has electronics circuit so we can say that this device measure temperature electronically.
- It is used to measure the temperature without any physical contact.
- It is helpful to measure temperature of objects which are at some distance.
- Radiation thermometers measure temperature at very faster rate. So it is used where quick results are required.
Thermocouple measures the temperature with the variation in the voltage. As the temperature increases the voltage is also increases. Temperature is directly proportional to the voltage. But it is not necessary that the variation between voltage and temperature is linear or not. The thermocouple should be placed inside the metal, ceramic or glass shield to protect it from the exposure of environment. Different types of thermocouples are used to measure the temperature these are Single Hole (SH series), Double Hole (DH series), Fish Hole (FH series) and Double Hole Oval (OV series).
APPLICATIONS: It is mostly used in laboratories.
As name suggests it is a type of diode which is made up of silicon. This diode is used to measure low temperatures. Its construction is same as a diode. This device is developed specially to measure cryogenic temperature. The temperature of cryogenic engine and fuel used also measured by this small device. The conductivity of the silicon diode is linearly dependent on the cryogenic temperature and its conductivity increases linearly in the low cryogenic regions.
APPLICATIONS: To measure very low temperature at cryogenic region.
As per the name of this device the temperature is measured by the means of bimetal i.e. two metals inside the temperature sensor. It is made up of two metals strips having different thermal expansion coefficient. This device works due to the expansion of metals when they are heated. These metals bonded together and mechanically linked to a pointer. When they are heated one side of the bimetallic strip will expand more than the second one. This expansion shows by the pointer of this device which is calibrated according to the proper temperature range. In some practical situations the bimetallic system consists of a bimetallic strip which can be helical or spirally wounded depends on the size of the sensor and the temperature range to be measured. When the temperature variation takes place the bimetallic rotate an attached spindle which is indicated by the pointer attached to it. This device can measure temperature from -80oC to +600oC. So that we can easily use it to measure high temperatures. The main advantage of this device is, it does not use any power source for their operation. But these are not so accurate like electrical devices such as thermocouple.
- It is most commonly used in household thermostats.
- The other main application of bimetallic device is in circuit breakers.
BULB AND CAPILLARY SENSOR:
Bulb and capillary sensor consist of a bulb, capillary tube, diaphragm and spring. These are arranged in a proper sequence. It is used at remote location. By using this we will measure temperature inducts, pipe and tanks. The bulb of this device is filled with liquid, some gas or refrigerant as per our requirement which temperature range has to be measured. Its working is very simple. Expansion of the fluid in the heated bulb exerts pressure, this pressure is transmitted to the diaphragm by the means of capillary tube due to this a translated movement is generated. This linear movement indicates the temperature. The linear movement is directly proportional to the temperature. This device is very cheap and easy to work. The bulb should be placed horizontal in position.
It is another main device which is used to measure the temperature of fuel and air. It is filled with gas or vapour or some liquid as per our requirement and availability. It indicates the temperature change with the variation of volume and pressure causing due to expansion or contraction. This device is very sensitive in case of gas filled in it. The liquid filled bellow can be work as transducer applications.
- It is used in case of transducers
- Used to measure the temperature in vacuum.
- It is also used to measure temperature of fuels.
CONSTANT VOLUME GAS THERMOMETER:
Here gas used is works as thermometric property. As name suggests constant volume gas thermometer consists of a fixed amount of gas in some container. This work on the principle that changes in gas pressure is directly proportional to the change of the temperature of gas. For a constant volume gas thermometer if we use the ideal gas equation then we will see that temperature only depends only on the pressure as other properties are constant. Mostly we use air as gas at room temperature but if we have to measure low temperature then in such a case we prefer helium rather than air because helium has boiling point close to absolute zero. Pressure sensors are used to measure the pressure inside the container and this value converts into temperature by using some calibration. We can easily measure the temperature variation by measure the variation of pressure inside the container.
CONSTANT PRESSURE GAS THERMOMETER:
For a constant pressure gas thermometer if we use the ideal gas equation then we will see that temperature depends only on the volume as other terms are constant. Thus for measuring temperature, the change in volume of the thermometric fluid is the change in temperature. As thermometer has the cross sectional area, in this the volume change depends upon the height or length of the thermometric fluid contained in the thermometer i.e. while measuring our body temperature we look at the markings of the thermometer which are along the length of thermometer. So by measuring volume change we can easily measure the temperature change.
This is all about differnce types of thermometers. If you have any query regarding this article, ask by commenting. If you like this article, don’t forget to share it on social networks. Subscribe our website for more informative articles. Thanks for reading it.