sign in sign up
<<
Home , Pressure measurement

MAC0010.1177/0020294014551627Tech Talk (4) Measurement BasicsTech Talk (4) Basics 551627research-article2014

Themed Paper

Measurement and Control 2014, Vol. 47(8) 241­–245 Tech Talk: (4) Pressure © The Institute of Measurement and Control 2014 Reprints and permissions: Measurement Basics sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0020294014551627 mac.sagepub.com

John E Edwards and David W Otterson

I. Introduction Absolute pressure pa is the pressure perpendicular to the flow direction, while above a total , and having little impact on surfaces parallel to Accurate and reliable pressure pressure p is the pressure above or the flow direction. This directional measurement is a requirement for the g below p giving component of pressure in a moving safe operation of most industrial atm (dynamic) is called dynamic processes. It is probably the p = p + p for all p where p is pressure. An instrument facing the flow measurement parameter most applied by a g atm g g negative if less than p direction measures the sum of the static the Instrument Engineer. The object of atm and dynamic ; this pressure measurement is to produce a To avoid sign confusion, pressures measurement is called the total pressure. dial indication, control operation or a below atmospheric pressure are referred Since is referenced to signal, typically the standard 4–20 mA, to as p giving , it is neither gauge nor that represents the pressure in a vac absolute; it is a differential pressure. process. p = p – p for p < p While static gauge pressure is of Pressure measurement is obtained a atm vac g atm primary importance in determining net from the effects of pressure which cause We can see from Table 1 that loads on pipe or vessel walls, dynamic position movement, change in resistance 1 atm = 14.696 psi = 1.01325 which is pressure is used to measure flow rates or other physical effects which are then equivalent to 0 psig and 0 barg. We can and airspeed. Dynamic pressure can be measured. The most common pressure deduce that 30 psig = 44.696 psia = 3.082 measured by taking the differential sensors or primary pressure elements bara and 10 psia = 4.696 psi pressure between instruments parallel employ a Bourdon tube, diaphragm, vac = −4.696 psig. It is recommended and perpendicular to the flow. Pitot– bellows, balance or variable that absolute pressures are stated as static tubes, for example, perform this capacitance arrangement. Some other ‘psia’ or ‘bara’ and gauge pressures are measurement on aircraft to determine methods are also outlined later in this stated as ‘psig’ or ‘barg’ to prevent airspeed. The presence of the measuring article. confusion. instrument inevitably acts to divert flow Gauge pressure is the unit most and create turbulence, so its shape is II. Pressure Units and encountered, with a good example being critical to accuracy and the Terminology vehicle tyre pressures which are in gauge curves are often non-linear. pressure. A gauge pressure device will Dynamic pressure can be expressed as Process pressure is defined as the force indicate zero pressure when vented to applied to a surface area, for example, atmosphere. pd = 0.5 ρ V2 kg/m2. The SI unit for pressure is Absolute pressure includes the effect (Pa), but bar is more commonly used for of atmospheric pressure with the gauge where pd is the dynamic pressure (Pa), ρ process measurement. Table 1 shows pressure. An absolute pressure indicator is the density of fluid (kg/m3) and v is the the relationships for the more common would indicate atmospheric pressure (not velocity (m/s). pressure units. scale zero) when vented to atmosphere. Pressure is a relative measurement B. Differential Pressure defined as either gauge or absolute. A. Static and Dynamic Pressure Measurement Gauge pressure varies with atmospheric pressure, which in turn varies with the Static pressure is uniform in all directions, Differential pressure (dp), as the term altitude above sea level and the weather so pressure measurements are implies, is the pressure difference conditions. The relationship between independent of direction in a stationary between two points of measurement. these definitions is shown in (static) fluid. Flow, however, applies Typical applications include pressure Figure 1. additional pressure on surfaces drops in ventilation systems, across

October 2014 Vol 47 No 8 l Measurement and Control 241

Downloaded from mac.sagepub.com by D W Otterson on February 19, 2015 Themed Paper

Tech Talk: (4) Pressure Measurement Basics

Readability and location will determine Table 1. Pressure units conversion the size of gauge specified. Generally, From To the larger the gauge diameter, the more accurate will be the reading as more 2 psi kg/cm bar mm Hg atm graduations can be incorporated. psi 1 0.07031 0.06895 51.715 0.06805 Manufacturers’ recommendations vary, but in general, the normal operating kg/cm2 14.223 1 0.9807 735.6 0.98692 pressure of a gauge should be at around bar 14.504 1.0197 1 750.06 1.01972 75% of the scale presuming an adequate design/overpressure safety margin. mm Hg 0.01934 0.00136 0.00133 1 0.00131 Pressure gauge selection criteria should include the measurement atm 14.696 1.0332 1.01325 760 1 accuracy required. The following guidance is derived from B40.1 and B40.7 contained in Standard ASME Figure 1. B40.100. Standard BS EN 837-1:1998 also addresses this issue. Full Grade 4A gauges offer the highest Vacuum Absolute Pressure (p ) a accuracy and are calibrated to ±0.1% of span over the entire range of the gauge. Absolute Pressure (p ) a The gauges are called laboratory Gauge Pressure (pg) precision test gauges. These high- -ve +ve accuracy test gauges may be

Vacuum Pressure (pvac) compensated. They must be handled carefully in order to retain accuracy. Grade 3A gauges are calibrated to an Pressure accuracy of ±0.25% of span over the Absolute Atmospheric entire range of the gauge. The gauges Zero Pressure (patm) are called test gauges but are generally not temperature compensated. Grade 2A gauges are calibrated to an both vented or pressurised vessels and accuracy of ±0.5% of span over the Figure 2. for pressure measurement on low- entire range of the gauge. These gauges pressure vessels where a dp are generally used for process pressure transmitter with the low-pressure side measurement. They are often referred to vented to atmosphere would give more as process gauges and are not accurate results than a pressure temperature compensated. transmitter. Grade 1A gauges are calibrated to an accuracy of ±1% of span over the entire range of the gauge. These gauges are C. Pressure Gauges high-quality general purpose industrial Pressure gauges, of the dial type shown gauges. in Figure 2, can be used for test Grade A gauges are calibrated to an purposes, pneumatic signals or local accuracy of ±1% of span over the middle process indication. The pressure- half of the scale and ±2% of span over measuring element can be a Bourdon the first and last quarters of the scale. primary flow elements such as an orifice tube, diaphragm or bellows which are These gauges are often referred to as plate, venturi or and across available in a wide variety of materials to industrial gauges. process equipment such as prime satisfy process fluid compatibility. Grade B gauges are calibrated to an movers, filters and process columns. Process measurement can be in the accuracy of ±2% of span over the middle Measurements of differential pressure range full vacuum to 2000 barg. half of the scale and ±3% of span over are also used to find other quantities by Pneumatic signal measurement use the first and last quarters of the scale. making use of known formula, for receiver gauges which have measuring These gauges are often referred to as example, level and density on ranges 3–15 psig or 0.2–1 barg. commercial or utility gauges.

242 Measurement and Control l October 2014 Vol 47 No 8

Downloaded from mac.sagepub.com by D W Otterson on February 19, 2015 Themed Paper

Tech Talk: (4) Pressure Measurement Basics

Note that wide ambient temperature Figure 3. Figure 4. excursions from that at which a non- temperature-compensated Bourdon gauge has been calibrated can lead to significant reading errors, typically ±0.4% of span for each 10°C change (rising or falling) from a reference temperature of 20°C. If pulsation is present in the process, the maximum operating gauge pressure should not exceed 50% of the full-scale range. A safety blow out panel is normally specified for gas pressure gauges. Other considerations in gauge selection include the following: to the gauge or remote mounted using a filled capillary connecting tube. Figure 5. •• Gauge mounting: direct, rear or front Pressure gauges are high-maintenance flange; items requiring frequent replacement to •• Orientation of pressure connection; ensure correct service. It is worth •• Pressure connection thread type; considering the more robust and expensive gauges of 100 mm diameter and pressure transmitter for critical applications. above would normally have 0.5″- or Programmable digital pressure gauges 15-mm thread connections for (Figure 4) are available, capable of strength and stability; measuring gauge, absolute and •• Liquid filling (typically glycerine or compound ranges. (A compound Fluorolube® for oxygen service) – pressure gauge is scaled from full used where vibration or severe vacuum through zero pressure up to the pulsation is present and for wet full-scale pressure.) environment including under water These instruments allow a selection of applications; measurement units and are available with •• Hermetic sealing – used where 4–20 mA output and alarm switches. ambient conditions are corrosive, very dusty humid or wet. Ingress III. Pressure Transmitters available which allow optimum design for protection (IP) rating to be specified flanged, wafer, flush mounting or hygienic accordingly; Pressure transmitters are available with applications for use in the food and •• Case material – typically, brass, pneumatic or electronic transmission and pharmaceutical industries (Figure 6). stainless steel, phenolic or can be specified to measure gauge When a compact pressure transducer polycarbonate; compound or absolute pressures is required, such as on a compressor •• Oxygen service cleaning; (Figure 5). (A compound pressure lubricating system, a range of miniature •• Maximum pressure red pointer; transmitter provides a linear output signal diaphragm sealed devices is available •• Maximum pressure follower pointer; from full vacuum through zero pressure (Figure 7). •• Alarm electrical contacts; up to the full-scale pressure.) Sensing Two-, three- and five-valve manifolds •• Pulsation dampener (snubber); element–measuring principles include (valves manufactured as a composite •• ‘Pigtail’ – a coiled pipe used to capacitive, piezoelectric and force block) are available, providing safe introduce a condensate cooling balance methods. Measured pressures pressure transmitter process isolation barrier between the process and the up to 2000 barg, with accuracies as low and on-line calibration facilities. Smart instrument in steam applications; as ±0.06%, and span turn-downs of up communications and signal transmission •• Flow restrictor in-line coupling – for to 10:1 are available. capability with 4–20 mA, 4–20 mA/HART, high-pressure gas applications. Remote seals are used when the 4–20 mA/FoxCom, FOUNDATION process fluid is corrosive, viscous, Fieldbus, Profibus or Low Power 1–5 V For dirty, viscous process such subject to extreme temperature, toxic, in output electronic modules are available. as heavy oil or slurries, the measuring sanitary applications or tends to collect Certifications can be obtained to meet element must be protected from the fluid and solidify. The seals are available to hazardous area requirements, for example, using diaphragm seals (Figure 3). The suit all standard flange fitting types. A Flameproof or Intrinsically Safe and also diaphragm seal may be directly mounted wide range of remote seal types are Safety Integrity Level (SIL) standards.

October 2014 Vol 47 No 8 l Measurement and Control 243

Downloaded from mac.sagepub.com by D W Otterson on February 19, 2015 Themed Paper

Tech Talk: (4) Pressure Measurement Basics

Figure 6. Table 2. General guidance on selection

Parameter Bourdon tube Diaphragm Piston Diaphragm/ Solid state piston

Range 3–1200 barg Full vacuum 1–850 barg Full vacuum to Full vacuum to 10 barg 70 barg to 70 barg

Accuracy ±0.5% ±0.5% ±2% ±2% ±0.25%

Cycle rate ≤25 cycles/min ≤25 cycles/ ≤50 cycles/ ≤50 cycles/min >50 cycles/ min min min

Life cycles 106 cycles 106 cycles 106 cycles 2.5 × 106 100 × 106 cycles cycles

instrument for the application. Most applied force (pressure) over an area. ground rules used for the specification of Figure 7. pressure gauges and transmitters apply Bourdon tubes. Bourdon tubes are for the specification of pressure switch circular-shaped tubes with oval cross wetted parts materials, process sections (Figure 8). The process pressure connections, housing materials, IP, acts on the inside of the tube. The hazardous area and functional safety outward pressure on the oval cross certification. section it to become more In addition to the above, the engineer rounded. must understand and specify the Because of the curvature of the tube requirements for the following: ring, the bourdon tube then bends with the resulting movement being •• Operating pressure range; transmitted to the gauge pointer by IV. Pressure Switches •• Design pressure limits; gearing. •• Accuracy and hysteresis limits; A pressure switch is a device capable of •• Dead-band limits; Bellows. Bellows or capsule-type detecting a pressure change, and at a •• Cycle speed, number of operations elements are constructed of tubular predetermined level, opening or closing and life expectancy; membranes that are convoluted around an electrical or pneumatic contact. •• Number of switching points; the circumference (Figure 9). The Pressure switches find application in many •• Fixed or adjustable switching point/s; membrane is attached at one end to the areas ranging from engine oil pressure •• Electrical load rating and source and at the other end to an monitoring and electrohydraulic control configuration of switch contacts; indicating device or instrument. through to the monitoring of process fluid •• Ambient temperature limits. For differential pressure transmitters, pressure for alarm and switching duties. the capsule is constructed with two Both pressure and differential pressure General guidance in selecting the type diaphragms forming an outer case. The switches are available. of primary element to be used is shown inter-space is usually filled with a process Much of the primary element in Table 2. The data are extracted from a compatible fluid. Pressure is applied to technology to be found in pressure 1 manufacturer’s published catalogue and both sides of the diaphragm, and it will gauges and pressure transmitters can be thus may vary between manufacturers. deflect towards the lower pressure. To found in the construction of pressure catalogue data should always be provide over-pressurised protection, a switches. This includes the Bourdon checked. solid plate with matching convolutions is tube, bellows, diaphragm and solid-state mounted in the centre of the capsule. designs. To these may be added piston and diaphragm–piston-type pressure V. Pressure-Sensing Technology switches. Diaphragms. A diaphragm is a circular- A. Force Collector Types Faced with a huge choice of shaped convoluted membrane that is instrument types, the engineer may either These types of pressure sensors generally attached to the pressure fixture around specify the duty and leave the selection use a force collector (such as a diaphragm, the circumference (Figure 10). The to the manufacturer or stockist or apply piston, bourdon tube or bellows) to pressure medium is on one side and the engineering know-how to select the best measure strain (or deflection) due to indication medium is on the other.

244 Measurement and Control l October 2014 Vol 47 No 8

Downloaded from mac.sagepub.com by D W Otterson on February 19, 2015 Themed Paper

Tech Talk: (4) Pressure Measurement Basics

systems (MEMS). Generally, this Figure 8. Figure 10. technology is considered to provide very stable readings over time.

Thermal. The changes in of a gas due to density changes are used to measure pressure. A common example of this type is the Pirani gauge which is a robust gauge used for the measurement of the pressures in vacuum systems.

Ionisation. It measures the flow of technologies are mostly applied to low charged gas particles () which varies pressures. due to density changes to measure Figure 9. pressure. Common examples are the hot Electromagnetic transducer. The and cold gauges. displacement of a diaphragm is measured by means of changes in (reluctance), linear voltage VI. Conclusion displacement transducer, or The instrument engineer is faced with a by an principle. bewildering choice when specifying pressure-measuring instruments. It is Piezoelectric transducer. The recommended that the potential vendors piezoelectric effect in certain materials are supplied with full data covering the such as quartz is used to measure the physical properties of the process, including strain upon the sensing mechanism due the normal measurement operating range to pressure. and extremes, the operating environment, output signal form, hazardous area and Piezoresistive functional safety requirements. The vendor’s transducer. The is Potentiometric transducer. The motion data sheet should always be cross- based on bonded or formed strain of a wiper along a resistive mechanism is checked against the above requirements. gauges to detect strain due to applied used to detect the strain caused by pressure using various thin-film applied pressure. Funding technologies. The strain gauges are This research received no specific grant from connected to form a Wheatstone bridge B. Other Types Resonant. The changes in resonant any funding agency in the public, commercial circuit to maximise the output of the or not-for-profit sectors. sensor and to reduce sensitivity to frequency in a sensing mechanism are used to measure the , or changes in errors. This is the most commonly References employed sensing technology for gas density, caused by applied pressure. 1. Selection Guides. Barksdale Control Products, Inc. general purpose pressure This technology may be used in 2. Edwards JE. Process Measurement & Control conjunction with a force collector, such as in Practice. 1st ed. Thornaby: P&I Design Ltd, measurement. 2011. those in the category above. Alternatively, 3. Morris AS. Principles of Measurement and resonant technology may be employed by Instrumentation. Englewood Cliffs, NJ: Prentice Capacitive transducer. A diaphragm exposing the resonating element itself to Hall, 1993. 4. Rangan CS, Sharma GR, Mani VSV. and a pressure cavity are used to create the media, whereby the resonant Instrumentation Devices and Systems. New Delhi, a variable to detect strain due frequency is dependent upon the density India: Tata McGraw Hill, 1993. to applied pressure. Common of the media. Sensors have been made 5. Liptak BG. (ed.). Instrument Engineers Handbook – Process Measurement and Analysis. 3rd ed. technologies use metal, ceramic and out of vibrating wire, vibrating cylinders, Radnor, PA: Chilton Book Company, 1995. silicon diaphragms. Generally, these quartz and silicon micro-electromechanical

October 2014 Vol 47 No 8 l Measurement and Control 245

Downloaded from mac.sagepub.com by D W Otterson on February 19, 2015