
Basic Instruments
In this section we will read about

Control System

Control Valve
 I/P Converter (Current to pressure Converter)
 Valve positioner
 Parts of a pneumatic control valve positioner
 Control Valve and its types
 Solenoid Valve
 Pneumatic Cylinder
 Butterfly Valve
 Pneumatic Globe Valve
 Work of different Parts of a Control Valve
 Pneumatic globe valve types
 Characteristics of Control Valve
Pressure
This lesson is for engineering students.
Pressure
pressure is defined as the amount of force applied over a unit area. Usually involving liquids and gases, pressure is a critical component of a diverse array of applications, both those that rely on accurate pressure control as well as those that derive other values (such as depth/level or flow) based upon pressure.
Static and Dynamic Pressure
• Static pressure is the pressure of fluid or gases that are stationary or not in motion.
• Dynamic pressure is the pressure exerted by a fluid or gas when it impacts on a surface or an object due to its motion or flow.
Unit of Pressure
A kilogramforce per square centimeter (kgf/cm2), often just kilogram per square centimeter (kg/cm2), or kilopond per square centimeter is a unit of pressure using metric units. Kilogramforce per square.
Unit of Pressure
A kilogramforce per square centimeter (kgf/cm^{2}), often just kilogram per square centimeter (kg/cm^{2}), or kilopond per square centimeter is a unit of pressure using metric units. Kilogramforce per square centimeter is abbreviated ksc instead of kg/cm^{2}.
Pressure measurements can be made in a number of units. We see PSI (pounds per square inch) or bar. Other units of measure inH2O, mmHg, Pa, and many others.
Types of Pressure
1 Atmospheric Pressure
Atmospheric pressure is the force of pressure exerted by the earth’s atmosphere. Atmospheric pressure at sea level is equivalent to 14.695 psia. The value of atmospheric pressure decreases with increasing altitude.
2 Barometric Pressure
Barometric pressure is the same as atmospheric pressure.
3. Gauge Pressure
 Gauge pressure is a single pressure measurement that indicates the pressure above atmosphere. Gauge pressure represents the positive difference between measured pressure and existing atmospheric pressure. You can convert gauge pressure to absolute pressure by adding the actual atmospheric pressure value to the gauge pressure reading. For example 10 psig is equivalent to 247 psia 0 psig is equivalent to 14.7 psia.
 Vacuum
Vacuum pressure is a single pressure measurement, which also has a reference to atmospheric pressure. Vacuum pressure is the measure of the depression of process pressure below atmospheric pressure. Vacuum pressure is generally measured in cm or inches of H_{2}0. For example, 14.7 psia is equivalent to 407.5 inches of H_{2}0. Therefore, a pressure of 10 inches of H_{2}0 vacuum implies process pressure is depressed 10 inches below atmosphere. Or 10 inches of H_{2}0 vacuum is equivalent to 397.5 inches of H_{2}0 absolute. Vacuum pressure is typically measured using a gauge pressure transmitter with an elevated zero calibration.
Line Pressure
Line pressure is simply the amount of pressure, or the force per unit area, exerted on a surface by the flow parallel to a pipe wall.
Static Pressure
Static pressure is the same as line pressure.
Working Pressure
Working pressure is also referred to as line or static pressure.
Hydraulic Pressure
The Pressure that is exerted by the oil in any process line or tank is known as Hydraulic Pressure.
Pneumatic Pressure
The Pressure that is exerted by the air in any line, body or Tank is known as Pneumatic Pressure.
Differential pressure: The pressure gauge contains two process connections to measure the difference between two pressures, such as each side of a filter to measure pressure drop.