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1Instrumentasi Sistem Pengaturan - 05
Sistem Instrumentasi
Pengukuran Tekanan(Measurement of Pressure)
Ir. Jos Pramudijanto, M.Eng.Jurusan Teknik Elektro FTI ITS
Telp. 5947302 Fax.5931237Email: [email protected]
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Where do we need pressure measurements?
Pressure driven equipment (IC engines, turbines, etc.)Pneumatic or Hydraulic mechanical elementsBiomedical applications (Blood Pressure, Barometric Chambers)Losses in pipes and ducts – energy efficiencyAtmospheric conditions (weather forecast, altitude)Indirect measurement of flow rate or velocityScuba divingMany, many more ...
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Pressure Terminology (1)
Tekanan Gage (Gage Preasure)Tekanan yang diukur relativ terhadap tekanan atmosfir.
Tekanan Absolut (Absolute Preasure)Pengukuran tekanan relativ terhadap titik nol tekanan.
Vacum (Reduced Vacuum)Vacum diukur relativ terhadap tekanan atmosfir.
Tekanan DifferensialPengukuran Tekanan relativ terhadap referensi tekanan.
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Pressure Terminology (2)
Tekanan StatisTekanan termodinamik yang proporsional dengantumbukan pada molekul fluida (sensor pada kecepatan lokalfluida).
Tekanan DinamisTekanan pada permukaan atau objek sesaat padatumbukan fluida (1/2ρV2)
Tekanan Stagnasi/Tekanan TotalJumlah Tekanan Satis dan Dinamis
Ptotal = Pstatis+ Pdinamis
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Various types of pressure
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Bagaimana memilih Tranduser Tekanan?Lingkungan Kerja
Range requiredAbsolute, gage, differential or vacuum
Burst strength and overpressure limitsWorking fluid chemical compatibilityOperating temperature
Temperature compensation
AkurasiSensivitasKecepatan Respons/Frequency ResponseOutput Sinyal yang dibutuhan
Analog, digital, manualBiaya?
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Types of pressure transducers
GravitationalLiquid Columns Manometers, pistons and weights
MechanicalBellow, bourdon tubes, diaphragms (equipped with displacement or strain sensors)
ElectricalPiezoelectic, Semiconductor elements (directly convert stress into voltage)
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Liquid Column ManometersU-tube and inclined manometers are simplest pressure sensing devices.
Micromanometers
AdvantagesSimpleInexpensiveCan be highly sensitive (inclined)
DisadvantagesPoor dynamic responseNo remote sensingLimited pressure range
Not practical over 100 psi
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U Tube & Well Manometers
p ghρ∆ =
“U tube” manometer Well-type manometer
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Liquid Column Manometers, continued
sinp gh gLρ ρ θ∆ = =
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Elastic Type Manometers
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Dial-type Manometer
Dial-type Manometer as a mini measurement system
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Bourdon Tubes
Bourdon Tubes (Accuracy to 0.1% FS)C-type
100,000 psi limitSpiral and Helical
Increase sensitivity1000 psi limit
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Bellows Gages• Bellows (Accurate to 0.5% FS)• Bellows gage not adequate for transient
measurements• Typically used for small pressures
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Diaphragm Gages
• Can be used for very small to very large pressure differences.• Can have rather fast response.• Can use a strain gage or displacement sensor to have a voltage
output.
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Strain gauges used with Diaphragm
Foil strain gageStrain gage is attached to elastic membrane.Narrrow span sensorsRugged sensors
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More Elastic types...
Two dummy gagesmounted elsewhere
Why do we not put 4 active gages?
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LVDT used with DiaphragmLinear Variable Differential Transformer (LVDT)
Operates under inductance ratio principleMagnetic coupling depends on position of coil.
As iron core changes position voltages change
Restricted to low vibration applications0.5 % FS accuracy availableOperating ranges from 0 to 10,000 psi
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CapacitiveThe displacement between two plates produces a change in capacitance.
Wide operating range (vacuum to 10,000 psi)Differential pressures as low as 0.01 inches H2OFast response time0.01% FS accuracy and 0.25% temperature effect
Capacitive used with Diaphragm
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Static Calibration
2cyl
mgpRπ
∆ =
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Semiconductor-type Sensors
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Silicon used with DiaphragmSilicon piezoresistive
Displacement compresses a silicon crystal. A charge is developed (lasting only a few seconds) which is proportional to the applied pressure.High frequency applications (400 kHz)
Not well suited for static pressure measurements because charge decreases rapidly.Large pressure range (2 to 20,000 psi)Accuracy is typically 1% FS
