Pressure Measurement

Muhajir Ab. RahimSchool of Mechatronic Engineering

Universiti Malaysia Perlis

Introduction

• Pressure is define as force divided by area. (exerts on solid, gas, liquid)

• The unit of pressure is in Pa, psi, atm, bar, torr

P = F/AF = force in newton/ lb

A = area in m2/ in2

1 psi = 6895 pa

1 bar = 105 Pa

1 atm = 101325 Pa

1 torr = 133.3 Pa

QUIZ

Convert 2 atm to psi. Given 1 psi = 6897 Pa

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.

Pressure Measurement

• Absolute pressure is the pressure measured wrt a vacuum (unit = psia)

• Gauge pressure is the pressure measured wrt atmospheric pressure (unit = psig)

• Atmospheric pressure is the pressure on the earth’s surface due to the weight of gases in the earth’s atmosphere (14.7psi)

Zero Pressure

Pressure due to Atmosphere

Gauge Pressure at point of interest

Absolute Pressure at point of interest

Exercise

Find the absolute pressure, if a pressure gauge reads 8.3psi, while the atm pressure is 14.7psi.

Pabs = Pat + Pg

= 14.7 + 8.3

= 23 psi

Hydrostatic Pressure

• Hydrostatic pressure is the pressure in a liquid.• The pressure increases as the depth in a liquid increases, due to its

weight.• In term of equation, P = ρgh

ρ = density in kg/m3

g = acceleration due to gravity (9.8m/s2)

h = depth in liquid in m

P = pressure in Pa

P = ρwhρw = weight density in lb/ft3

h = depth in liquid in ft

P = pressure in lb/ft2

Exercise

If a pool has a water with a depth of 6 ft. Find the pressure at the bottom of the pool in Pa and psi. (assume density = 103kg/m3)

Pressure MeasuringInstrument

• The techniques for pressure measurement is depending on pressure level. (moderate, very high, very low)

• Very high pressure level is higher than 1000 atm• Very low pressure level is below than 133 Pa (1 torr)

1. Moderate Pressure Measurement

- Manometer

- Elastic elements (diaphragm, bellows, capsules, bourdon tubes, spiral, helix)

Pressure MeasuringInstrument

2. High Pressure Measurement

- Electrical Resistance pressure gauge

3. Low Pressure Measurement

- McLeod gauge

- Pirani gauge

- Ionisation gauge

Manometer

• Manometer is the simplest device for measuring static pressure.

• It contains water/ mercury or any other suitable fluid in the manometer tube.

• When a pressure line is connected to one column of manometer, the fluid in the column will be forced down, and the fluid in the other will rise.

• By measuring the difference in height of the fluid in the two columns, the pressure of the inlet can be expressed in inches of fluid.

Types of Manometer

U-tube Manometer Well-type Manometer

Incline-tube Manometer

Elastic Elements

• Elastic elements, when subjected to pressure, get deformed.

• Measurement of the deformation, gives an indication of pressure value.

• The deformation may be measured by mechanical or electrical means.

• Example of elastic elements are; diaphragms, capsules, bellows, Bourdon or helical tubes.

Elastic Elements

Flat diaphragm Corrugated diaphragm Capsule

C-shape Bourdon tubeStraight tubesBellows

Spiral Bourdon tubeTwisted Bourdon tube Helical Bourdon tube

Electrical ResistancePressure Gauge

• The concept of operation is based on electrical resistance change in a conductor when applied directly to a pressure.

• The sensing element consist of a loosely wound coil of relatively fine wire, and it will be compressed when high pressure applied on it.

• The length and cross section of the wire affect its electrical resistance, when pressure applied at a rate determined by the bulk of modulus material.

McLeod Gauge

• It compresses the low pressure gas so that the increased pressure can be measured.

• The change in volume and pressure can then be used to calculate the original gas pressure, providing that the gas not condensed.

Pirani Gauge

• It consist of platinum filament and thermocouple enclosed in a chamber.

• The pressure measurement is based on the relation of heat conduction and radiation from a heating element to the number of gas molecules per unit volume in the low pressure region.

Ionization Gauge

• It can be used to measure pressure down to about 2 torr.• The gas is ionized with a beam of electrons and the

current is measured between two electrodes in the gas.• The current is proportional to the number of ions per unit

volume, which also proportional to the gas pressure.

How to select?

1. You must understand the pressure range, accuracy and the response time of the instrument.

2. Parameter such as hysterisis and stability can be obtained from manufacturer’s specification.

3. Other aspect such as cost and maintenance also should be considered.

4. The liquid in the tube must not interact with the pressure gas. (contamination, evaporates)