PROCESS INSTRUMENTAION PRESSURE MEASUREMENTLESSON-1
LINEAR VS NON-LINEAR
LINEARITY
NON-LINEAR PROCESS
LINEARITY
HYSTERESIS
HYSTERESISWhen increase the input of an instrument it gives some output readings (follow a curve), now if we decrease the same input and it gives different readings (follow another curve) then this effect of instrument is called hysteresis.
HYSTERSIS
ERRORS IN PERFORMANCE PARAMETERS
SYSTEMATIC ERRORS
ENVIRONMENT ERRORSThese errors may be avoided by,Providing air conditioning.Hermetically sealing certain components of instrumentUse of magnetic shields
RANDOM ERRORSThe cause of errors are not known or undeterminable in the ordinary process of making instrument. Such errors normally small. These may be treated as mathematically according to laws of probability
SOURCE OF ERRORSInsufficient knowledge of process parameters & design conditionsPoor designChange in process parameters, irregularities, upsets etcPoor maintenanceErrors caused by the people who operate instrumentCertain design limitations
PRESSURE
RELATION Pab-Pg-PatmThe actual pressure at a given position is called absolute pressurePressure below atmospheric pressure is called Vacuum Pressure
RELATION Pab-Pg-Patm
HYDROSTATIC EQUATION
PRESSURE MEASURMENT
PRESSURE SENSING DEVICES
COMMON DEVICESBOURDON TUBESDIAPHRAGMSBELLOWS
PRESSURE SENSING MECHANISM
BOURDON TUBE GUAGE
C-BOURDON TUBE GUAGE
BOURDON GUAGE
C-BOURDON TUBE
TYPES
SPIRAL BOURDON GUAGE
HELIX & SPIRAL TUBES
SPRING AND BELLOWS
SPIRNG & BELLOWS
SPRING & BELLOW GUAGE
DIAPHRAGMS
DIAPHRAGMS
DIAPHRAGM SENSING GUAGE
SELECTION OF DIAPHRAGM
METHODS FOR PRESSURE MEASUREMENTMost pressure instruments measure a difference between two pressures, one usually being atmospheric pressureManometer methodElastic pressure transducerElectrical pressure transducer
MANOMETERSThe manometer is simplest pressure measuring device used for gauge pressure measurement
U-Tube ManometerWell-type ManometerInclined ManometerDifferential Manometer
U-TUBE MANAOMETER
U-TUBE MANOMETERThis is used for measurement of liquid or gas pressures. The manometer is filled with a manometric fluid whose specific gravity is known. The difference between the pressures on two limbs of the manometer is a function of h (the difference between the levels of the manometric fluid in the two limbs).
DIFFERNTIAL MANOMETER
ERRORS IN MANOMETEREffect of TemperatureCapillary effectEffect of variable meniscus
MANOMETRIC FLUIDThe most common fluids of Manometer are Water, Red oil and MercuryWater is best if Air is fluid to be measuredTo minimise the effects of freezing or evaporation, Kerosene or antifreeze may be usedMercury is used when the fluids mixes with manometeric fluidWhile Red oil is used indicate reading from long distance
ADV-DIS-ADV -MANOMETERAdvantages: They are simple in construction High accuracy and sensitivityReasonable costSuitable for low pressure and differential pressuresDisadvantages:Not portableNeed levelling
INSTALLATION
PRESSURE RANGE
COMPARATORSThe general principle of comparator is to indicate the differences insize between the standard and thework being measured by means of some pointer on a scale with sufficient magnification
FEATURESAll comparators consist of three basic features1. A sensing device which faithfully senses the input signal2. A magnifying or amplifying system to increase the signal to suitable magnitude. Mechanical,Optical, Pneumatic, hydraulic andelectronic methods are used for this purpose.3. A display system (usually a scale and pointer) which utilizes the amplified signal to provide a suitable readout.
CLASSIFICATION1.Mechanical2.Optical3.Electrical&Electronic 4.Pneumatic 5.Fluiddisplacement comparators 6.Mechanical- optical comparator7.Electro-mechanicalcomparator 8.Multi-check comparator.
ELECTRICAL PRESSURE TRANSDUCERS
STRAIN GUAGE
TYPES OF SIGNALS
PNEUMATIC SIGNALSPneumatic signals are signals produced by changing air pressure in a signal pipe in proportion to measured change in a process variableThe common industry standard pneumatic signal range is 3-15 psigThe 3 psig corresponds to LRV and 15 is HRV
ANALOG SIGNALThe most common standard electrical signal is the 4-20 mA current signal
The most recent addition to process control signal tehcnologyThese are discrete levels or values that are combined in specific ways to represent process variable
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