FIELD INSTRUMENTS

                                               FIELD INSTRUMENTS

What is field instrument ?   

 Primary or measuring elements and final elements are called field instruments,Field instruments are used to measure and control the  process  variables, such as temperature, pressure, level , flow and quality .

Types of process measuring and final controlling  instruments:

1. Pressure measuring instruments.
2. flow measuring instruments.
3. Temperature measuring instruments.
4. Level measuring instruments.
5. Quality measuring instruments.
6. Control valves 
7. ON/OFF valves
8. MOV
9. Flame scanner and Ignition Transformer  

            Pressure measuring instruments

What is pressure?

                              Pressure: It is defined as Force per unit Area. P = F/A

                   Units      : bar, Pascal, kg / cm², lb / in².

Types of Pressure Measurement:

There are four types of pressure measurement required when monitoring industrial processes,

they are: ·

1. Atmospheric Pressure
2. Gauge Pressure
3. Vacuum Pressure
4. Absolute pressure

Atmospheric Pressure:

This is the pressure experienced on the earth’s surface due to the surrounding air. It may be considered that each square meter of the earth’s surface supports a column of air, exerting a pressure of 1.013 bars. The pressure reduces progressively towards the top of the column. This is illustrated by the difference between pressure at sea level and that at the top of a high mountain. It also varies due to changes in temperature and composition of air.

Gauge Pressure:

Consider a gauge connected to an air supply and reading 6 bar. The reading is called gauge

pressure. This gives the pressure over and above that due to the atmosphere in any sealed

system.

Vacuum Pressure:

This is pressure below atmospheric, that is where a degree of vacuum exists and it may be

expressed in one of two ways. Atmospheric pressure may be used as a datum, and the degree of vacuum expressed as a number of pressure units below this datum, or in terms of absolute pressure using absolute zero pressure as a datum.

Absolute Pressure:

If a vessel with a gauge attached and open to the atmosphere is then sealed, the pressure

gauge reading remains zero. Existence of atmospheric pressure can be provided by completely exhausting the vessel when the gauge will read a vacuum (or suction) corresponding to “Absolute Zero” of pressure. Pressure measured from this datum is referred to as absolute pressure. 

Thus it follows that:Absolute pressure = gauge pressure + atmospheric pressure

Different methods of pressure measurements 

1. Mechanical

2. Electrical 

ELECTROMAGNETIC FLOW METER

   Proximity sensors: