201309 CLD 20202 Chapter 2-Pressure Measurement.ppt
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8/14/2019 201309 CLD 20202 Chapter 2-Pressure Measurement.ppt
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CLD 20202
Process Instrumentation
CHAPTER 2
PressureMeasurement
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Objectives
After finish this lesson, you will be able
to:
Define the pressure
Differentiate the types of pressure
Understand the characteristic of liquidpressure & factor that affected
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Understand the characteristic of gaspressure
Apply the different unit of pressure
measurement Understand the principle, operation,
advantages & disadvantages of pressure
measurement device
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Pressure
Defined as normal force exerted by a fluid per
unit area (gas & liquid)
Units of pressure N/m2 (Pascal, Pa)
Other units = bar, atm, kgf/cm2, lbf/in2 = psi
Ama
AFP
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Unit of Measurement
English unit -pound per square inch(psi)
(SI) unit - Newton per square meter
(Pascal) Common units
Kilograms per square centimeter (kg/cm2)
Kilopascals (kPa)Bar ( 1 bar = 100,000 Pa)
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referenced to column of fluid
inches of water (in H2O)
millimeter of mercury (mm Hg)torr
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indicated pressure includes or excludesatmospheric pressure
psig psia
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Example
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Absolute, Gauge & Vacuum
Pressure Several common ways to measure pressure
1. Zero absolute pressure
2. Standard atm pressure
Gauge pressure (psig)refer to atm pressure
(14.7psia)
Absolute Pressure (psia)refer to zero abs pressure(vacuum)
Vacuum Pressure (-50mmHg)refer atm pressure
& measured inve direction
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Most pressure measuring devices calibrated to
read zero in the atmosphereGauge Pressure
Pressure below atmospheric
pressure
Vacuum Pressure
atmabsgauge PPP
absatmvac PPP
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Example 1
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Solution
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Example 2
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Solution
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Hydrostatic Pressure
Pressure a fluid exerts on an object or
container walls
Applied to determine LEVEL of liquid in tanks
P = hydrostatic pressurerho = density of liquid
H = height of the liquid
G = gravitational force
gHP
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Variation of pressure with depth
P in a fluid at a rest
does not change in the
horizontal direction
In the presence of a
gravitational field, P
increases with
depthmore fluid rests
on deeper layers
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Liquid Pressure Characteristic
Influencedby :
Position within static
fluid but at a given
position (height)
Unaffectedby :
Shapeof the confining
boundaries
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Factors Affecting LiquidPressure
Density
Depth
SurfacePressure
P proportionalto density
P proportionalTo depth
Any P acting on theSurface (P atm)
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Gas Pressure Characteristic
Gas expand to fill entire area thatcontains them.
2 factors that affect a gas
Volume Temperature
BOYLES LAWdescribes the inversely
proportional relationship
between the absolute
PRESSURE and VOLUME of
gas, if the TEMPERATURE
kept constant
CHARLES LAWfor a fixed mass of gas at
constant volume, PRESSURE
directly proportionalto the
TEMPERATURE
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BOYLES LAW
Volume of gas inversely proportional to pressure
V 1 / PV = constant / P
P*V = constant 1/P
V
P
V@ single T
P1*V1= constant = P2*V2
P1V1= P2V2
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CHARLES LAW: (1746-1823)
Pressure is proportional to absolutetemperature (assuming V and n constant)
P T (Temp in kelvin)
P = constant * T
T
P
Also,Volume of gas is proportional to the
amount of gas present
V n v = constant * n
@ConstantV and n
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Now with the equations
Liquids do not obey the ideal gas lawsVirtually no change with P (hydraulics), small change with T
P = constant x T
v = constant x n
P x V = constant
P1V1 = P2V2
P x V = constant x T x constant x n
P x V = constant x n x T
P x V = R x n x T
PV = nRT
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Typical Pressure Transmitter consists 2 parts:
1. Primary Element2. Secondary Element
Pressure Measurement Devices
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Pressure Measurement Devices
Primary element- pressure sensor / pressure element- converts to: physical motion that is proportion toapplied pressure (mechanical /electrical value)
- read by the secondary element.
Secondary element- pressure measuring instrument
- electronics (transducer)- convert output (primary element) tosignal such as 4-20mA.
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electronic-based sensor (strain gages)have better response and higheraccuracy than mechanical-based types
(bourdon) pressure measuring instruments -
pressure transducer
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THANK YOU
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