MECHANICAL TYPE TEMPERATURE MEASURING METHOD :
Mechanical temperature devices respond to temperature by producing mechanical movement. Electrical temperature devices respond to temperature by producing or changing an electrical signal.
Below are different types of mechanical temperature measuring method,
Thermometers are classified as mechanical temperature sensing devices because they produce some type of mechanical action or movement in response to temperature changes.
MERCURY IN GLASS THERMOMETERS :
Mercury in glass thermometer can be read directly and are very accurate and stable when used properly. The bulb is usually a thin- walled glass chamber that serves as a reservoir for the mercury. The stem is a glass tube that contains the capillary for the liquid. A capillary is a narrow passage within which the mercury can rise and fall. Scale is used to read the temperature.
FILLED THERMOMETERS :
Filled thermometers contain some filled quantity like a gas or a volatile liquid and rely on pressure measurement to provide temperature indications. There are four classes of filled thermometers, as they listed by the instrument society of America standards.
1. LIQUID FILLED (Except Mercury)
2. VAPOUR PRESSURE
3. GAS FILLED
4. MERCURY FILLED
1. LIQUID FILLED AND MERCURY FILLED THERMOMETERS :
Both types operate on the principle of thermal expansion, just as the mercury – in – glass thermometer does. It should be noted that the liquid or mercury is put into the system under pressure and completely fills it.
2. GAS FILLED THERMOMETERS :
This type of temperature measurement depends upon the increase in pressure of a confined gas (constant volume) with temperature increase. The system is filled under high pressure. The increase in pressure for each degree of temperature rise is therefore greater than if the filling pressure were low. Nitrogen is the most often used for such systems, because it is chemically inert and possesses a favorable coefficient of thermal expansion. Except for the size of the bulb, the gas filled system is identical to the liquid filled types.
3. VAPOUR PRESSURE THERMOMETERS :
Unlike the liquid and gas filled system which depends upon volumetric expansion for their operation, vapor - pressure thermometers depends upon the vapor pressure of liquid which only partially fills the system. As it filled partially with rise in temperature it produce vapor thus increasing vapor pressure that move pressure sensing element and we can easily read temperature. It should be noted however, that the gas filled system has the fastest response, the vapor pressure system the next fastest and the liquid filled has the slowest.
4. BIMETALLIC THERMOMETER :
With the increasing use of alloys whose coefficient of thermal expansion can be controlled. The bimetal thermometer has become a most dependable temperature measuring device are most commonly used . One of this is a nickel alloy named invar, which hardly expands at all when heated, welded together and rolled to the desired thickness, these alloys provide a bimetallic material used in the modern thermometers.