Instrumentation 2 Pressure Higher Certificate in Technology (Manufacturing Technology)

1. Instrumentation 2 PressureHigher Certificate in Technology (Manufacturing Technology)

2. Pressure - Basics • Many of the processes in the modern world involve the measurement and control of pressurized liquid and gas systems • This monitoring reflects certain performance criteria that must be controlled to produce the desirable results of the process and insure its safe operation • Boilers, refineries, water systems, and compressed gas systems are but a few of the many applications for pressure gauges

3. Pressure - Basics • There are many applications for pressure sensors but we can narrow them down to two major categories: • Pressure sensing • This is the direct use of pressure sensors to measure pressure. • This is useful in weather instrumentation, aircrafts, cars, and any other machinery that has pressure functionality implemented. • Altitude sensing • This is useful in aircrafts, rockets, satellites, weather balloons, and many other applications. • All these applications make use of the relationship between changes in pressure relative to the altitude.

4. Pressure - Basics • This relationship is governed by the following equation: where h = height (m), P = pressure at altitude in Pascals or Pa, Pref = sea-level pressure in Pascals or Pa • 1 Pascal = 1 Pa = 1kg/ms² • This equation is calibrated for an altimeter, up to 36,090 feet (11,000 m). • Outside that range, an error will be introduced which can be calculated differently for each different pressure sensor. • These error calculations will factor in the error introduced by the change in temperature as we go up

5. Pressure - Basics • Example • What is the height of the aircraft in meters if the pressure reading, P, is 90kPa and Pref is 101kPa? • Solution h = [(1-(90/101)^0.19026) x 288.15]/0.00198122 h = [(1-0.978299) x 288.15]/0.00198122 h = 6.2528/0.00198122 h = 3156 meters

6. Pressure - Basics • Pressure is defined as a force per unit area e.g. kg/m² • Pressure measurements may be expressed relative to various zero references • Absolute pressure of a fluid is referenced against a perfect vacuum • Gauge pressure is referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure e.g. gauge pressure = 110 kPa – 101 kPa = 9 kPa • A standard value of atmospheric pressure has been defined to be 101.325 Pa, but is variable with altitude and weather

7. Pressure - Basics • If the absolute pressure of a fluid stays constant, the gauge pressure of the same fluid will vary as atmospheric pressure changes • Examples of absolute pressure measurements include barometric pressure, altimeters, and the Manifold Absolute Pressure (MAP) sensor used in the engine control systems • Examples of gauge pressure measurements include the tire-pressure gauge and sphygmomanometer • Differential pressure is the difference in pressure between two points. • Differential pressure gauges have two inlet ports, each connected to one of the volumes whose pressure is to be monitored

8. Pressure - Basics • Pressure sensors can be classified in five categories: • Absolute pressure sensor • This sensor measures the pressure relative to perfect Vacuum pressure (0 PSI or no pressure) • Atmospheric pressure, is about 100kPa (14.7 PSI) at sea level. Atmospheric pressure is an absolute pressure. • Gauge pressure sensor • This sensor is used in different applications because it can be calibrated to measure the pressure relative to a given atmospheric pressure at a given location • An example of gauge pressure would be a tire pressure gauge. When the tire pressure gauge reads 0 PSI, there is really 14.7 PSI (atmospheric pressure) in the tire.

9. Pressure - Basics • Vacuum pressure sensor • This sensor is used to measure pressure less than the atmospheric pressure at a given location. • Differential pressure sensor • This sensor measures the difference between two or more pressures introduced as inputs to the sensing unit • For example, if we need to know the difference of the pressure of some fluid going in a pressure boosting unit and the output pressure of that unit in a way to monitor how much we boosted the fluid pressure; we use differential sensor. • Sealed pressure sensor • This sensor is the same as the Gauge pressure sensor except that it is previously calibrated by manufacturers to measure pressure relative to sea level pressure (14.6 PSI).

10. Pressure - Basics • Static pressure is uniform in all directions, so pressure measurements are independent of direction in an immobile (static) fluid • Flow, however, applies additional pressure on surfaces perpendicular to the flow direction • This directional component of pressure in a moving (dynamic) fluid is called dynamic pressure • An instrument facing the flow direction measures the sum of the static and dynamic pressures; this measurement is called the total pressure or stagnation pressure • Since dynamic pressure is referenced to static pressure, it is neither gauge nor absolute; it is a differential pressure

11. Pressure - Basics • Dynamic pressure is used to measure flow rates and airspeed • Dynamic pressure can be measured by taking the differential pressure between instruments parallel and perpendicular to the flow • Pitot-static tubes, for example perform this measurement on airplanes to determine airspeed • The presence of the measuring instrument inevitably acts to divert flow and create turbulence, so its shape is critical to accuracy and the calibration curves are often non-linear

12. Pressure - Manometer • The most accurate way to measure low air pressure is to use a Manometer which is a Hydrostatic gauge • It consist of a vertical column of liquid in a tube whose ends are exposed to different pressures • The column will rise or fall until its weight is in equilibrium with the pressure differential between the two ends of the tube • Hydrostatic gauge measurements are independent of the type of gas being measured, and can be designed to have a very linear calibration • They have poor dynamic response

13. Pressure - Manometer • The simplest design is a closed-end U-shaped tube, one side of which is connected to the region of interest • Any fluid can be used, but mercury is preferred for its high density and low vapor pressure • The units of measurement commonly used are inches of mercury (in. Hg), using mercury as the fluid and inches of water (in. w.c.), using water or oil as the fluid • Simple hydrostatic gauges can measure pressures ranging from 100 Pa to above atmospheric

14. Pressure - Manometer • Fig. 2.1. In its simplest form the manometer is a U-tube about half filled with liquid. With both ends of the tube open, the liquid is at the same height in each leg. • Fig. 2-2. When positive pressure is applied to one leg, the liquid is forced down in that leg and up in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the pressure. • Fig. 2-3. When a vacuum is applied to one leg, the liquid rises in that leg and falls in the other. The difference in height, "h," which is the sum of the readings above and below zero, indicates the amount of vacuum

15. Pressure - Manometer • The difference in fluid height in a liquid column barometer is proportional to the pressure difference P - Po = pgh where P = unknown pressure , p = density liquid, g = gravity, h = height, Po = atmospheric pressure • Density of water for example is 1 kg per 1000cm³ • Density of mercury for example is 13.6 kg per 1000cm³

16. Pressure - Manometer • Example: • What is the pressure of the gas being measured by a simple u-tube manometer using the following information: h = 10cm, Po = 101kPa, p = 1kg/1000cm³, g = 9.81m/s² • Solution P – Po = pgh P – Po = (1000kg/m³)(9.81m/s²)(0.1m) P – Po = 981Pa = pressure difference between atmospheric and test pressures Therefore the pressure of the gas being measured is: P = 101000Pa – 981Pa = 100019Pa = 100.019kPa if the column of liquid rose up at the test side

17. Pressure – Variations in manometer designs

18. Pressure – Variations in manometer designs

19. Pressure – Variations in manometer designs

20. Pressure - Bourdon Tube gauge • A Bourdon gauge uses a coiled tube which as it expands due to pressure increase causes a rotation of an arm connected to the tube • The pressure sensing element is a closed coiled tube connected to the chamber or pipe in which pressure is to be sensed • As the gauge pressure increases the tube will tend to uncoil, while a reduced gauge pressure will cause the tube to coil more tightly • This motion is transferred through a linkage to a gear train connected to an indicating needle. The needle is presented in front of a card face inscribed with the pressure indications associated with particular needle deflections • In a barometer, the Bourdon tube is sealed at both ends and the absolute pressure of the ambient atmosphere is sensed • Differential Bourdon gauges use two Bourdon tubes and a mechanical linkage that compares the readings • Note that a Bourdon gauge can measure liquid pressure as well as gas pressure

21. Pressure - Bourdon Tube gauge • This particular gauge is a combination vacuum and pressure gauge used for automotive diagnosis • The left side of the face, used for measuring manifold vacuum, is calibrated in centimeters of mercury on its inner scale and inches of mercury on its outer scale. • The right portion of the face is used to measure fuel pump pressure and is calibrated in fractions of 1 kgf/cm² on its inner scale and pounds per square inch on its outer scale

22. Pressure - Bourdon Tube gauge

23. Pressure - Bourdon Tube gauge Calibration • Calibration occurs just before the final assembly of the gauge to the protective case and lens • The assembly consisting of the socket, tube, and movement is connected to a pressure source with a known "master" gauge • A "master" gauge is simply a high accuracy gauge of known calibration • Adjustments are made in the assembly until the new gauge reflects the same pressure readings as the master • Accuracy requirements of 2 percent difference are common, but some may be 1 percent, .5 percent, or even .25 percent • Selection of the accuracy range is solely dependant upon how important the information desired is in relationship to the control and safety of the process

24. Pressure - Bourdon Tube gauge Calibration • Most manufacturers use a graduated dial featuring a 270 degree sweep from zero to full range • These dials can be from less than 1 inch (2.5 centimeters) to 3 feet (.9 meter) in diameter, with the largest typically used for extreme accuracy • By increasing the dial diameter, the circumference around the graduation line is made longer, allowing for many finely divided markings • These large gauges are usually very fragile and used for master purposes only • Masters themselves are inspected for accuracy periodically using dead weight testers, a very accurate hydraulic apparatus that is traceable to the National Bureau of Standards in the United States

25. Pressure - Bourdon Tube gauge Applications • The varied applications account for the wide range in design of the case and lens enclosure • Some dials are illuminated by the luminescent inks used to print the graduations or by tiny lamps connected to an outside electrical source • Gauges intended for high pressure service usually are of "dead front" safety design, a case design feature that places a substantial thickness of case material between the Bourdon tube and the dial • This barrier protects the instrument viewer from gauge fragments should the Bourdon tube rupture due to excess pressure • The internal case design directs these high velocity pieces out the back of the gauge, away from the viewer

26. Pressure – Dead-Weight Tester • Pressure transducers can be recalibrated on-line or in a calibration laboratory • Laboratory recalibration typically is preferred, but often is not possible or necessary • In the laboratory, there usually are two types of calibration devices: deadweight testers that provide primary, base-line standards, and "laboratory" or "field" standard calibration devices that are periodically recalibrated against the primary • Of course, these secondary standards are less accurate than the primary, but they provide a more convenient means of testing other instruments.

27. Pressure – Dead-Weight Tester • A deadweight tester consists of a pumping piston with a screw that presses it into the reservoir, a primary piston that carries the dead weight, and the gauge or transducer to be tested • It works by loading the primary piston (of cross sectional area A), with the amount of weight (W) that corresponds to the desired calibration pressure (P = W/A) • The pumping piston then pressurizes the whole system by pressing more fluid into the reservoir cylinder, until the dead weight lifts off its support

28. Pressure – Dead-Weight Tester • Example • If the area of the piston used in the Dead weight is 5mm² and the mass of the moving part (piston and holder) is 0.3kg, what mass is required to be added to balance a pressure of 5MPa applied to the piston area, taking g = 9.81m/s ² ? • Solution W = weight = mg = (0.3kg + mass weight)(9.81m/s ² ) P = W/A 5000000 Pa = [(0.3kg + mass weight)(9.81m/s ² )]/(0.005m²) 5000000 Pa (0.005m²) = [(0.3kg + mass weight)(9.81m/s ² )] 25000 kgm/s² = 2.943 kgm/s² + [(mass weight)(9.81m/s ² )] 25000 kgm/s² - 2.943 kgm/s² = [(mass weight)(9.81m/s ² )] [25000 kgm/s² - 2.943 kgm/s²]/9.81m/s² = mass weight mass weight = 2548.11 kg

29. Pressure – Dead-Weight Tester • Today's deadweight testers are more accurate and more complex than the previous instrument but the essential operating principles are the same • In the United States, the National Institute of Standards & Technology (NIST) provides certified weights and calibrates laboratory piston gauges by measuring the diameter of the piston • Deadweight testers can be used to calibrate at pressure levels as low as 5 psig (35 kPa) and as high as 100,000 psig (690 MPa) • Tilting type, air-lubricated designs can detect pressures in the mm Hg range • NIST calibrated deadweight testers can be accurate to 5 parts in 100,000 at pressures below 40,000 psig (280 MPa)

30. Pressure – Dead-Weight Tester • For an industrial quality deadweight tester, error is typically 0.1% of span • A typical secondary standard used for calibrating industrial pressure transducers contains a precision power supply, an accurate digital readout, and a high-accuracy resonant (quartz) pressure sensor • It is precise enough to be used to calibrate most industrial pressure transducers, but must be NIST-traceable to be used as an official calibration standard • The best accuracy claimed by the manufacturers is typically 0.05% full scale

31. Pressure – Semiconductor Pressure Transducers - Basics • These sensors are electronic components that provide an electrical signal and have essentially no moving parts • Many gauges today already have these sensors mounted within the case to send information to process control computers and controllers • These sensors are intrinsically safe, allowing their use in flammable or explosive environments • However, the mechanical gauge does not require the electrical power source or the computer equipment needed by the electronic sensor

32. Pressure – Semiconductor Pressure Transducers - Basics • A pressure transmitter is a standardized pressure measurement package consisting of three basic components: a pressure transducer, its power supply, and a signal conditioner/retransmitter that converts the transducer signal into a standardized output • Pressure transmitters can send the process pressure of interest using an analog pneumatic (3-15 psig), analog electronic (4-20 mA dc), or digital electronic signal • When transducers are directly interfaced with digital data acquisition systems and are located at some distance from the data acquisition hardware, high output voltage signals are preferred • These signals must be protected against both electromagnetic and radio frequency interference (EMI/RFI) when traveling longer distances

33. Pressure – Semiconductor Pressure Transducers - Basics • Transducer accuracy refers to the degree of conformity of the measured value to an accepted standard • It is usually expressed as a percentage of either the full scale or of the actual reading of the instrument • In case of percent-full-scale devices, error increases as the absolute value of the measurement drops • Repeatability refers to the closeness of agreement among a number of consecutive measurements of the same variable • Linearity is a measure of how well the transducer output increases linearly with increasing pressure

34. Pressure – Semiconductor Pressure Transducers – Strain Gauge • When a strain gage is used to measure the deflection of an elastic diaphragm or Bourdon tube, it becomes a component in a pressure transducer • Strain gage-type pressure transducers are widely used • Strain-gage transducers are used for narrow-span pressure and for differential pressure measurements • Essentially, the strain gage is used to measure the displacement of an elastic diaphragm due to a difference in pressure across the diaphragm • These devices can detect gauge pressure if the low pressure port is left open to the atmosphere or differential pressure if connected to two process pressures • If the low pressure side is a sealed vacuum reference, the transmitter will act as an absolute pressure transmitter

35. Pressure – Semiconductor Pressure Transducers – Strain Gauge • Strain gage transducers are available for pressure ranges as low as 3 inches of water to as high as 200,000 psig (1400 MPa) • Inaccuracy ranges from 0.1% of span to 0.25% of full scale • Additional error sources can be a 0.25% of full scale drift over six months and a 0.25% full scale temperature effect per 1000¡ F.

36. Pressure – Semiconductor Pressure Transducers – Strain Gauge

37. Pressure – Semiconductor Pressure Transducers – Strain Gauge

38. Pressure – Semiconductor Pressure Transducers – Potentiometric • The potentiometric pressure sensor provides a simple method for obtaining an electronic output from a mechanical pressure gauge • The device consists of a precision potentiometer, whose wiper arm is mechanically linked to a Bourdon or bellows element • The movement of the wiper arm across the potentiometer converts the mechanically detected sensor deflection into a resistance measurement, using a Wheatstone bridge circuit • The mechanical nature of the linkages connecting the wiper arm to the Bourdon tube, bellows, or diaphragm element introduces unavoidable errors into this type of measurement • Temperature effects cause additional errors because of the differences in thermal expansion coefficients of the metallic components of the system • Errors also will develop due to mechanical wear of the components and of the contacts

39. Pressure – Semiconductor Pressure Transducers – Potentiometric • Potentiometric transducers can be made extremely small and installed in very tight quarters, such as inside the housing of a 4.5-in. dial pressure gauge • They also provide a strong output that can be read without additional amplification • This permits them to be used in low power applications • They are also inexpensive • Potentiometric transducers can detect pressures between 5 and 10,000 psig (35 KPa to 70 MPa) • Their accuracy is between 0.5% and 1% of full scale, not including drift and the effects of temperature

40. Pressure – Semiconductor Pressure Transducers – Potentiometric

41. Pressure – Semiconductor Pressure Transducers – Piezoelectric • When pressure, force or acceleration is applied to a quartz crystal, a charge is developed across the crystal that is proportional to the force applied • The fundamental difference between these crystal sensors and static-force devices such as strain gages is that the electric signal generated by the crystal decays rapidly • This characteristic makes these sensors unsuitable for the measurement of static forces or pressures but useful for dynamic measurements

42. Pressure – Semiconductor Pressure Transducers – Piezoelectric • When pressure is applied to a crystal, it is elastically deformed • This deformation results in a flow of electric charge (which lasts for a period of a few seconds) • The resulting electric signal can be measured as an indication of the pressure which was applied to the crystal • These sensors cannot detect static pressures, but are used to measure rapidly changing pressures resulting from blasts, explosions, pressure pulsations (in rocket motors, engines, compressors) or other sources of shock or vibration • Some of these rugged sensors can detect pressure events having "rise times" on the order of a millionth of a second

43. Pressure – Semiconductor Pressure Transducers – Piezoelectric • The output of such dynamic pressure sensors is often expressed in "relative" pressure units (such as psir instead of psig), thereby referencing the measurement to the initial condition of the crystal • The maximum range of such sensors is 5,000 or 10,000 psir • The desirable features of piezoelectric sensors include their rugged construction, small size, high speed, and self-generated signal • On the other hand, they are sensitive to temperature variations and require special cabling and amplification

44. Pressure – Semiconductor Pressure Transducers – Piezoelectric

45. Pressure – Semiconductor Pressure Transducers – Piezoresistive • Piezoresistive pressure sensors operate based on the resistivity dependence of silicon under stress • Similar to a strain gage, a piezoresistive sensor consists of a diaphragm onto which four pairs of silicon resistors are bonded • Unlike the construction of a strain gage sensor, here the diaphragm itself is made of silicon and the resistors are diffused into the silicon during the manufacturing process • The diaphragm is completed by bonding the diaphragm to an unprocessed wafer of silicon

46. Pressure – Semiconductor Pressure Transducers – Piezoresistive • If the sensor is to be used to measure absolute pressure, the bonding process is performed under vacuum • If the sensor is to be referenced, the cavity behind the diaphragm is ported either to the atmosphere or to the reference pressure source • The silicon diaphragm is shielded from direct contact with the process materials by a fluid-filled protective diaphragm made of stainless steel or some other alloy that meets the corrosion requirements of the service • Piezoresistive pressure sensors are sensitive to changes in temperature and must be temperature compensated • Piezoresistive pressure sensors can be used from about 3 psi to a maximum of about 14,000 psi (21 KPa to 100 MPa).

47. Pressure – Semiconductor Pressure Transducers – Optical • Optical pressure transducers detect the effects of minute motions due to changes in process pressure and generate a corresponding electronic output signal • A light emitting diode (LED) is used as the light source, and a vane blocks some of the light as it is moved by the diaphragm • As the process pressure moves the vane between the source diode and the measuring diode, the amount of infrared light received changes • The optical transducer must compensate for aging of the LED light source by means of a reference diode, which is never blocked by the vane • This reference diode also compensates the signal for build-up of dirt or other coating materials on the optical surfaces

48. Pressure – Semiconductor Pressure Transducers – Optical • The optical pressure transducer is immune to temperature effects, because the source, measurement and reference diodes are affected equally by changes in temperature • Moreover, because the amount of movement required to make the measurement is very small (under 0.5 mm), hysteresis and repeatability errors are nearly zero • Optical pressure transducers do not require much maintenance • They have excellent stability and are designed for long-duration measurements • They are available with ranges from 5 psig to 60,000 psig (35 kPa to 413 MPa) and with 0.1% full scale accuracy

49. Pressure – Semiconductor Pressure Transducers – Optical

50. References • http://www.omega.com/literature/transactions/volume3/pressure.html • http://www.omega.com/literature/transactions/volume3/pressure2.html • http://www.omega.com/literature/transactions/volume3/pressure3.html • http://en.wikipedia.org/wiki/Pressure_measurement • http://www.efunda.com/formulae/fluids/manometer.cfm • http://www.upscale.utoronto.ca/PVB/Harrison/Manometer/Manometer.html • http://www.dwyer-inst.com/htdocs/pressure/ManometerIntroduction.cfm • http://tpub.com/machines/9c.htm • http://www.answers.com/topic/pressure-gauge • http://www.answers.com/topic/pressure-measurement-1 • http://en.wikipedia.org/wiki/Pressure_sensor • http://en.wikipedia.org/wiki/Piezoresistive_effect