Measurement of Pressure

The fluid pressure is usually measured with reference to standard atmosphere (i.e. 760mm of mercury/101.325 kPa). Any differential pressures are often expressed as gauge/vacuum pressure. The pressure measuring devices mostly employed in fluid systems are generally grouped under two categories; liquid manometers and mechanical gauges .

The liquid manometers work on the principle of balancing the column of liquid whose pressure is to be determined by same/another liquid column. Depending on the application, magnitude of pressure and sensitivity requirement, the manometers can be selected. The most commonly used liquid as manometric fluid are mercury, water, alcohol and kerosenes etc. Most of the case, for gauge pressure measurements, mercury is widely used as manometric fluid because it has non-evaporating quality under normal conditions, sharp meniscus and stable density. For some pressure differences and low level vacuum, water can be considered as working fluid in the manometer. Manometers can be employed to measure pressures in the range of 0.4 Pa to 200 kPa.

The liquid manometers become bulky for handling higher pressure measurements. In such cases, mechanical gauges are normally employed. These gauges employ elastic elements which can deflect due to pressure acting on it. The deflection obtained by action of pressure is mechanically magnified and made to operate a pointer moving in a graduated dial. Some of these mechanical devices are dead-weight pressure gauges, bourdon tube pressure gauge, elastic diaphragm pressure gauges, pirani and McLeod gauges (vacuum measurement) etc.

Measurement of Temperature

Temperature is a thermodynamic property of a fluid which is measured as a change with respect to another temperature-dependent property. In practical aspects, the temperature is gauged by its effect on quantities such as volume, pressure, electrical resistance and radiant energy. The temperature sensing devices working on these techniques are classified in the following categories;

• •  Thermometers (changes in physical dimension and gas/vapour pressures)

•  Resistance temperature detectors (RTD), Thermistors, Thermocouples, Semiconductor sensors (changes in electrical properties)

•  Pyrometers (Changes in thermal radiation)

Among all the devices, the electrical temperature sensors are mostly used particularly when automatic/remote recording is desired. Radiant sensors are used for noncontact temperature sensing, either in high temperature applications (combustors) or for infrared sensing at low temperatures. These are optical devices and can be adapted to whole-field temperature measurements known as thermal imaging. The most familiar type of temperature sensing device is the thermometer that appears in laboratories and households because of its ease in use and low cost. Some of the important and commonly used temperature devices are discussed here.

Resistance Thermometers and Thermistors: Traditionally, the resistance elements sensitive to temperatures are made out of metals which are good conductors of electricity (e.g. nickel, platinum, copper and silver). The operating ranges for this class of devices fall between – 250ºC to 1000ºC. They are commonly referred as resistance temperature detectors (RTD) and provide a linear temperature-resistance relation.

(7.1.7)

Here, R is the resistance at temperature T, R₀ is the resistance at reference temperature and is the temperature coefficient of resistance depending on the material (Fig. 7.1.5)

There are certain classes of semiconducting materials (such as metal oxides of cobalt, manganese and nickels) having negative coefficient of resistances. These devices are called as thermistors for which the resistance temperature relation is non-linear as given below;

(7.1.7)

where, is a constant for a given thermistor. The practical operating ranges for the thermistor lie approximately between – 100ºC to 275ºC.

 

Fig. 7.1.5: Representation of a resistance temperature gauge.