Tech Talk: (4) Pressure Measurement Basics
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MAC0010.1177/0020294014551627Tech Talk (4) Pressure Measurement BasicsTech Talk (4) Pressure Measurement 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 vacuum, and gauge 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 atmospheric pressure p giving component of pressure in a moving safe operation of most industrial atm (dynamic) fluid 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 pressures; 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 dynamic pressure is referenced to signal, typically the standard 4–20 mA, to as p giving static pressure, 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 bar 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, force 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 calibration 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 Pascal 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) temperature 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 gas 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 Pitot tube 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, liquid 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.