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Sistem Instrumentasi Pengukuran Tekanan (Measurement of Pressure) Ir. Jos Pramudijanto, M.Eng. Jurusan Teknik Elektro FTI ITS Telp. 5947302 Fax.5931237 Email: [email protected] Instrumentasi Sistem Pengaturan - 05 1 Where do we need pressure measurements? Pressure driven equipment (IC engines, turbines, etc.) Pneumatic or Hydraulic mechanical elements Biomedical applications (Blood Pressure, Barometric Chambers) Losses in pipes and ducts – energy efficiency Atmospheric conditions (weather forecast, altitude) Indirect measurement of flow rate or velocity Scuba diving Many, many more ... Instrumentasi Sistem Pengaturan - 05 2 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 Differensial Pengukuran Tekanan relativ terhadap referensi tekanan. Instrumentasi Sistem Pengaturan - 05 3 Pressure Terminology (2) Tekanan Statis Tekanan termodinamik yang proporsional dengan tumbukan pada molekul fluida (sensor pada kecepatan lokal fluida). Tekanan Dinamis Tekanan pada permukaan atau objek sesaat pada tumbukan fluida (1/2ρV2) Tekanan Stagnasi/Tekanan Total Jumlah Tekanan Satis dan Dinamis Ptotal = Pstatis+ Pdinamis Instrumentasi Sistem Pengaturan - 05 4 Various types of pressure Instrumentasi Sistem Pengaturan - 05 5 Bagaimana memilih Tranduser Tekanan? Lingkungan Kerja Range required Absolute, gage, differential or vacuum Burst strength and overpressure limits Working fluid chemical compatibility Operating temperature Temperature compensation Akurasi Sensivitas Kecepatan Respons/Frequency Response Output Sinyal yang dibutuhan Analog, digital, manual Biaya? Instrumentasi Sistem Pengaturan - 05 6 Types of pressure transducers Gravitational Liquid Columns Manometers, pistons and weights Mechanical Bellow, bourdon tubes, diaphragms (equipped with displacement or strain sensors) Electrical Piezoelectic, Semiconductor elements (directly convert stress into voltage) Instrumentasi Sistem Pengaturan - 05 7 Liquid Column Manometers U-tube and inclined manometers are simplest pressure sensing devices. Micromanometers Advantages Simple Inexpensive Can be highly sensitive (inclined) Disadvantages Poor dynamic response No remote sensing Limited pressure range Not practical over 100 psi Instrumentasi Sistem Pengaturan - 05 8 U Tube & Well Manometers ∆ p = ρ gh “U tube” manometer Well-type manometer Instrumentasi Sistem Pengaturan - 05 9 Liquid Column Manometers, continued ∆ p = ρ gh = ρ gL sin θ Instrumentasi Sistem Pengaturan - 05 10 Elastic Type Manometers Instrumentasi Sistem Pengaturan - 05 11 Dial-type Manometer Dial-type Manometer as a mini measurement system Instrumentasi Sistem Pengaturan - 05 12 Bourdon Tubes Bourdon Tubes (Accuracy to 0.1% FS) C-type 100,000 psi limit Spiral and Helical Increase sensitivity 1000 psi limit Instrumentasi Sistem Pengaturan - 05 13 Bellows Gages • • • Bellows (Accurate to 0.5% FS) Bellows gage not adequate for transient measurements Typically used for small pressures Instrumentasi Sistem Pengaturan - 05 14 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. Instrumentasi Sistem Pengaturan - 05 15 Strain gauges used with Diaphragm Foil strain gage Strain gage is attached to elastic membrane. Narrrow span sensors Rugged sensors Instrumentasi Sistem Pengaturan - 05 16 More Elastic types... Two dummy gages mounted elsewhere Why do we not put 4 active gages? Instrumentasi Sistem Pengaturan - 05 17 LVDT used with Diaphragm Linear Variable Differential Transformer (LVDT) Operates under inductance ratio principle Magnetic coupling depends on position of coil. As iron core changes position voltages change Restricted to low vibration applications 0.5 % FS accuracy available Operating ranges from 0 to 10,000 psi Instrumentasi Sistem Pengaturan - 05 18 Capacitive used with Diaphragm Capacitive The 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 H2O Fast response time 0.01% FS accuracy and 0.25% temperature effect Instrumentasi Sistem Pengaturan - 05 19 Static Calibration ∆p = Instrumentasi Sistem Pengaturan - 05 mg 2 π Rcyl 20 Semiconductor-type Sensors Instrumentasi Sistem Pengaturan - 05 21 Silicon used with Diaphragm Silicon 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 Instrumentasi Sistem Pengaturan - 05 22
