Pressure measurements Bachelor Degree in Chemical Engineering Course: Process Instrumentation and Control (Strumentazione e Controllo dei Processi Chimici) Measuring devices of the main process variables Pressure measurements Rev. 2.4 – April 3, 2019

PRELIMINARY DEFINITIONS ABSOLUTE PRESSURE: Pressure referenced to the perfect vacuum. BAROMETRIC PRESSURE: Pressure exerted from the atmosphere on the Earth’s surface (it changes with the altitude and the weather conditions). DIFFERENTIAL PRESSURE: Pressure difference between two different points. GAUGE (GAGE in USA) PRESSURE: Difference between the absolute and the barometric pressure. RESIDUAL PRESSURE: Absolute pressure that is referenced to the perfect vacuum and is below the barometric pressure. VACUUM: Pressure measured below the barometric pressure after taking this latter as a reference. From “Fundamentals of Instrumentation & Process Control”, 2006 Control Station absolute pressure = gauge pressure + barometric pressure 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

STANDARD ATMOSPHERIC PRESSURE The standard atmospheric pressure, referred to as P atm (n), corresponds to the value the barometric pressure exerted by the atmosphere at the sea level at 0 ºC with average weather conditions. It is defined as being equal to: P atm (n)= 101325 Pa = 1.01325 bar = 1 atm = 1.03323 kg/cm2 = 14.6959 psi = 760 mm (Hg) = 10.3323 m (H2O) 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

TYPES OF PRESSURE MEASUREMENTS Perfect vacuum pressure under measurement Atmospheric pressure Variable reference pressure Constant reference pressure Absolute pressure Gage pressure Differential pressure with a variable reference Differential pressure with a constant reference Manometer 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

DEFINITIONS Pressure sensors Quite often, all pressure sensors have been improperly called manometers even though it is actually necessary to distinguish between pressure sensors: Manometer is the instrument which measures positive gage pressures and with pressures zero-referenced to the barometric pressure. Deprimometer is the instrument that measures absolute pressures lower than the barometric pressure and has zero at barometric pressure. Measure the vacuum. Vacuum gauge is the instrument that measures the absolute pressures between the absolute vacuum and the barometric pressure, the vacuum meter has therefore the zero of the scale that corresponds to the absolute vacuum. Measure the residual pressure. Barometer is the instrument that measures only the absolute value of the atmospheric pressure (barometric pressure).  17.10.2019 Process Instrumentation and Control - Prof M. Miccio

CLASSIFICATION Pressure Sensors In the most common sensors, the pressure measurement can be performed by: 1. a variation of the liquid level; 2. a deformation of an elastic element; 3. a change of an electromagnetic quantity. 1. In the first category there are instruments based on the measurement of the level of a liquid column: Ex.: “U” manometers 2. In the second category, there are instruments based in the deformation of an elastic element because of the action of pressure forces. They are generally based on macro-deformations. e.g.: Bourdon pressure gauges; Diaphragm manometers Bellows manometers 3. In the third category there are the transducers in which a variation in pressure, which may involve a micro-deformation of an elastic element (diaphragm, wire, etc.) that produces an output signal generated by the change of a electrical property (capacity, resistance, inductance, frequency). Ex.: strain-gauge  17.10.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio PRESSURE MEASUREMENT SENSORS 1. a variation in height of liquid 2. a deformation of an elastic element 3. a variation of an electromagnetic quantity 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

LIQUID COLUMN MANOMETER or “U” MANOMETER STEVIN’S LAW Instruments based on the measurement of a level of a liquid column Measuring ranges: 10 -1500 mm Hg 0.01 - 2 bar 10 - 2500 mm H2O NOTE: they are used: in the range of low differential pressures Almost exclusively for gases and vapors as local indicator and not in automatic control  WaterColumn.swf PI 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio PRESSURE MEASUREMENT SENSORS 1. a variation in height of liquid 2. a deformation of an elastic element 3. a variation of an electromagnetic quantity 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

BOURDON PRESSURE GAUGE it consists of a closed-end elastic tube with an elliptic section, shaped as an arc, with a fixed terminal; the free terminal changes its position because of the deformation stress exerted by the pressure of internal fluid; the measurement of the terminal displacement provides the pressure measure. NOTE: They are used as local indicators in Piping & Instrumentation Diagram  PI 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

BOURDON PRESSURE GAUGE and ASSEMBLING ACCESSORIES Standard manometer with quadrant Ø 40 mm Quadrant with double graduation: in psi and kg/cm2 Manometer with quadrant Ø 50 mm in bar and in psi Connection Central threaded male 1/8" BSP 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

Process Instrumentation and Control - Prof M. Miccio PRESSURE MEASUREMENT SENSORS 1. a variation in height of liquid 2. a deformation of an elastic element 3. a variation of an electromagnetic quantity 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

CAPACITIVE PRESSURE TRANSDUCERS BEFORE the deformation F = (P1 - P2)* Area with P1>P2 AFTER the deformation Measurement principle: change of the CAPACITY C er relative dielectric constant e0 dielectric constant of the vacuum 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

CAPACITIVE PRESSURE TRANSDUCERS CAPACITIVE CELLS with CERAMIC MEMBRANES CELL with METALLIC DIAPHRAGM ATMOSPHERIC PRESSURE VACUUM pressione PRESSURE PRESSURE PRESSURE CELL with DOUPLE COMPENSATOR for DIFFERENTIAL PRESSURE NOTE: The dielectric fluid can move from a side to the other one. In this way, it automatically compensates the change of the dielectric fluid density. 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

TRANSDUCTION OF A CAPACITY VARIATION IN AN ELECTRIC SIGNAL Transduction deformation - capacity An a.c. Wheatstone bridge is used as rectifier in order to produce a direct signal. The sensitivity is redoubled if there are two capacities and they have opposite variations, inserting them on two different legs of the bridge. 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

ELECTRICAL RESISTANCE-BASED PRESSURE TRANDUCERS Strain-gauge It operates on the variation of an electrical resistance due to the change of the length and the cross sectional diameter (2°Ohm’s law) when a conductor is submitted to a tensile stress. thin (few microns) metallic plate; very high resistance (hundreds ohms) which changes with the deformation; extension 0.2-5% bonded with insulating material NOTE: - Cheaper sensors PROTECTION Substrate Wires Termination Strain gauge Adhesive Tratto da Dr. Chi-fu Wu, ME 4903-Special Problem in ME, GTREP, Savannah (USA), Oct. 21, 2004 Piezoelectric transducers They can be alternative to the strain-gauges. In this case the electrical resistance and the voltage across the sensible element, which is a crystal of a certain solid material (silicon or another semiconductor), changes directly because of the deformation. 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

TRANSDUCTION OF A RESISTANCE VARIATION IN AN ELECTRIC SIGNAL Transduction deformation - resistance (with Wheatstone bridge) 4 extensometers (2 for tensile and 2 for compression) on 4 legs of a bridge Valim E + e -e - +e  ADVANTAGES Accuracy 0.1% on measured value Rangeability (75 100):1 Measuring range: “The pressure range is 0 – 14000 bar” [see the University of Michigan Chemical Engineering Process Dynamics and Controls Open Textbook at http://controls.engin.umich.edu/wiki/index.php ] 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

INDUCTIVE DIFFERENTIAL PRESSURE TRANSDUCER Measure principle: Variation of the INDUCTANCE L P1 P2 Transduction deformation - inductance The deformation of the metallic diaphragm produces a translation of two units of ferrite respect to 2 fixed coils placed on the two sides of diaphragm. The deformation occurring as a consequence of the pressure difference is transduced in a inductance variation (of opposed direction in the 2 coils). This is further transduced in an electric signal by an a.c. Wheatstone bridge with rectifier or including inductances as tuning elements of oscillators (LC circuits). 17.10.2019 Process Instrumentation and Control - Prof M. Miccio

RESONATORS ON SILICON  ADVANTAGES Measurement principle: Variation of the RESONANT FREQUENCY double diapason of a “resonant” silicon mono-crystal immersed in a permanent magnetic field resonant frequency depends on the transverse deformation produced by the measuring pressure MEMS = Micro ElectroMechanical Systems  ADVANTAGES No attrition No AC/DC converter Better performances and greater stability with time (little calibration requirement) 17.10.2019 Process Instrumentation and Control - Prof M. Miccio