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Elements of measurement system:

Elements of measurement system:

There are different types of measurement system

1- Temperature measurement
2- Pressure Measurement
3- Flow Measurement
4- Level Measurement
5- Process control

fig. Different measurement process
1. Temperature Measurement: 

Devices used for temperature measurement are different thermometers, thermocouples, or digital thermometers. Thermometers contain a mercury or any other fluid. Temperature is directly proportional to an expansion of mercury. As soon as the temperature increases it causes fluid to expand. Hence, the temperature can be measured by measuring the volume of fluid. There are many temperature measurement devices available such as thermocouples, Resistive temperature devices (RTD), Thermistors, etc. Heat can directly be related to temperature. If the temperature of object is high, it has more energy and if the temperature is low then it has less energy.

Temperature is measured in Celsius and Fahrenheit. Conversion from Celsius to Fahrenheit: 
°F = (9/5)°C + 32

2. Pressure Measurement:

Pressure is defined as force per unit area that a fluid exerts on its surroundings. Pressure, P, is a function of force, F, and area, A:
                                              P = F/A
The SI unit for pressure is the pascal (N/m2), but other common units of pressure include pounds per square inch (psi), atmospheres (atm), bars, inches of mercury (in. Hg), millimetres of mercury (mm Hg).
There are different pressure measurement devices:

Manometer: A manometer is a scientific instrument used to measure gas pressures. Open manometers measure gas pressure relative to atmospheric pressure. A mercury or oil manometer measures gas pressure as the height of a fluid column of mercury or oil.

Diaphragm pressure gauge: A diaphragm pressure gauge is a device, that uses a diaphragm with a known pressure to measure pressure in a fluid. It has many different uses, such as monitoring the pressure of a canister of gas, measuring atmospheric pressure, or recording the strength of the vacuum in a vacuum pump.

Bellows: Bellows are used to measuring pressure and they can be made of cascade capsules. It is basically manufactured by fastening together many individual diaphragms.

Strain gauge pressure transducer: Strain Gauges can be used as a secondary element in pressure measurement. They can be joined together with bellows and diaphragms to effectively measure pressure.

Piezoelectric pressure transducer: Piezo-electricity is the ability of some materials mostly crystals to generate an electric potential in response to applied mechanical stress. In this transducer, the piezoelectric effect is used in certain materials such as quartz to generate rapid electrical signals and measuring the strain upon the sensing mechanism due to pressure. Common types of piezoelectric pressure transducers are charge mode type and low impedance voltage-mode type.

3. Flow Measurement: Flow measurement rely on forces produced by the flowing stream as it overcomes a known constriction, to indirectly calculate flow. Flow may be measured by measuring the velocity of fluid over a known area.

The common types of flow-meters with industrial applications are listed below:

a) Obstruction type (differential pressure or variable area)
b) Inferential (turbine type)
c) Electromagnetic
d) Positive-displacement flow-meters
e) Fluid dynamic (vortex shedding)
f) Anemometer
g) Ultrasonic
h) Mass flow-meter.

4. Level Measurement: Level devices operate under three main different principles:
  • The position (height) of the liquid surface
  • The pressure head
  • The weight of the material
There are two methods used to measure the level of a liquid:

      i.   Direct Methods
      ii.  Indirect or inferential Methods

i. Direct Methods: 
  • The direct method measures the height above a zero point. 
  • Direct methods for level measurement are mainly used where level changes are small and slow such as sump tanks and Bulk storage tanks.
  • Direct methods are simple and reliable. 
There are four types of direct level measurement devices:

      1. Dip-sticks & Dip-Rods
      2. Weighted gauge tape
      3. Sight Gauges, and
      4. Floats.

5. Process Control: Process control is an engineering mechanism that uses continuous monitoring of an industrial process’ operational variables (e.g. temperature, pressure, etc) and algorithms and then uses that information to adjust variables to reach product output specifications and objectives.

Process control Instrument has mainly two aspects:

i.  Measurement: It gives an idea of the operating conditions of a plant and measures various process like temp, press, flow and level.
ii. Control: The information obtained from measurement enables us to control a process.

Need for process instruments:

- Quality of product
- Improved productivity
- Safety of process and people
- Environment considerations.

Written by : Anuj Yadav
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