Pressure Measurements

• What is pressure?• The amount of force applied per unit area.

P = Force / Area

• Pascal’s Law–States whenever an external

pressure is applied to any confined fluid at rest, the pressure is increased at every point in the fluid by the amount of external pressure.

Pascal’s Law defines static fluid pressure in 3 properties as follows:

•1. Pressure works vertically to a wall face

•2. The pressure at any given point is the same in all direction.

•3. The pressure applied to a part of a fluid in a closed container is transmitted to other parts of the fluid.

Gas Laws

• 1. Boyle’s Law– States the volume of a gas varies inversely

with the absolute pressure, provided the temperature remain constant.

V1 = V2P1 P2

• 2. Charles's Law– At constant volume, the pressure of a gas varies

directly with absolute temperature.

P1 = P2T1 T2

P2 = P1 x T2 T1

• 3. General Law of Perfect Gas• Is a combination of Boyle’s and Charles’s Law

P1 x V1 = P2 x V2T1 T2

To solve for unknown P2:P2 = P1 x V1 x T2

T1 x V2

•4. Dalton’s Law–The total pressure of a confined mixture of gasses is the sum of the pressure of each gasses in the mixture.

Atmospheric Pressure

Vacuum Pressure

• The part of the loop or instrument that senses the value of a process variable and sends a corresponding predetermined output.• it exists in close proximity to the process.

PROCESS ELEMENT / SENSOR

Pressure Elements1. Manometers

• It is the simplest, most direct, and most accurate of all mechanical methods. The tubes are partly filled with the manometer liquid, which is water (for low pressure), mercury (for high pressure) and oil (for very low pressure).

PROCESS ELEMENT / SENSOR

Pressure Elements1. Manometers

PROCESS ELEMENT / SENSOR

2. Bourdon Tube

• is a pressure-sensing element consisting of a twisted tube of noncircular cross section closed at one end.

• When a process stream is routed to the open end of the tube, any increase in the pressure will cause the tube to unwind.

PROCESS ELEMENT / SENSOR

2. Bourdon Tube

BOURDON TUBE

PROCESS ELEMENT / SENSOR

2. Bourdon Tube

PARTS OF SENSING ELEMENT

PROCESS ELEMENT / SENSOR

2. Bourdon Tube (C-TYPE)

2. Bourdon Tube (SPIRAL)

Pressure Elements

2. Bourdon Tube (HELICAL)

Pressure Elements3. Bellows• It is a one-piece, collapsible, seamless metallic unit that has deep folds formed from very thin-walled tubing. The diameter of the bellows ranges from 0.5 to 12 in. and may have as many as 24 folds.

• System pressure is applied to the internal volume of the bellows.

PROCESS ELEMENT / SENSOR

3. Bellows• As the inlet pressure to the instrument varies, the bellows will expand or contract. The moving end of the bellows is connected to a mechanical linkage assembly.

Pressure ElementsPROCESS ELEMENT / SENSOR

3. Bellows

Pressure Elements4. Diaphragm

• A diaphragm pressure element is constructed as a corrugated circular plate, fixed at the rim.

• When pressure is applied to either face, its center moves but the deflection is almost un-noticeable.

• This basic construction is used for low to high pressure applications.

PROCESS ELEMENT / SENSOR

Pressure Elements 4. Diaphragm

Diaphragm

Pressure Recorder and Controller

Pressure Transmitter

Pressure Calibrator