Temperature is probably the most widely measured and frequently controlled variable in the numerous processes. This is because quite often processing and manufacture of the desired product is possible only if temperatures are accurately measured. The temperature gauges are based on the change in the physical properties, which is simple thermal expansion due to temperature makes the following two types of thermometers

 1] MERCURY IN STEEL type  thermometer
 2] B-METALLIC type thermometer MERCURY IN STEEL type  thermometer It is one of the most versatile and widely used devices in Industrial  temperature measurements .The entire assembly of bulb, capillary, and gauge is calibrated directly on the basis of pressure change changes corresponding to temperature.