**Instrumentation Fundamentals**Module 1 – Pressure Scales Units of Pressure Pressure Scales & Conversions Atmospheric, PSIG, PSIA, PSID, Bar Manometers**Pressure**This module will cover: • The physics of pressure • Units of measure (SI, Metric, Imperial) • Pressure scales and conversions • How pressure is measured • Elastic elements (bourdon, bellows diaphragm) • Electrical elements (strain gauge, piezoelectric) • Sensors, Switches and Transmitters**What is it?**Pressure is an operating parameter that is relevant in many applications. (similar to voltage in an electric circuit) • Pressure applied over a given area can be used for useful work. • Steam pressure, Water pressure • Pressures can be measured to infer the condition of other process parameters. • Flow, level**Force**Area Pressure = The Physics of Pressure Pressure is defined as “force per unit area” Therefore any object or material having a weight will exert a pressure over the area the force is acting on.**Force**Area Pressure = The Units of Pressure Pressure is defined as “force per unit area” Pound force, Kilogram force Newton, dyne Square Inches, Square feet Square Centimeters, Square Meters • Common units include: • Pounds per Square Inch (psi) • KiloPascals (kPa)**550 lbs**708 lbs 62.4 lbs 849 lbs Example of pressure from a 1 cubic foot pound force acting on a surface Each base has an area of 144 in2 1 cubic foot of copper 550 lbs 144 in2 = 3.8 psi 1 cubic foot of lead 708 lbs 144 in2 = 4.9 psi 1 cubic foot of water 62.4 lbs 144 in2 = 0.43 psi 1 cubic foot of mercury 849 lbs 144 in2 = 5.9 psi**More Pressure Scales**PSI and kPa are the most common pressure scales but there a few more: • Inches* of water • Inches* of mercury • Bar • Atmos • Torr (vacuum) * or millimeters when using metric**Example of the various pressure scales**The same process pressure is being applied to each gauge. Each gauge has a different scale calibration. 27.6806 “H20 2.03602 “Hg 0.068947 Bar 0.068046 Atmos Applied process pressure is 1 psi or 6.89 kPa • The choice of scales will depend on • the amount of pressure being measured (high pressure = psi/kPa, low pressure = inches H20) • The type of application ( flow = inches H20, blood pressure = inches of Hg.)**Conversion Factors**Need to Know:psiand kPaconversion Imperial vs Metric vs SI • 1 cubic foot of water that weighs 62.4 lbs acting over an area of 144 in2 produces a pressure of 0.433 pound per square inch (psi) • The same volume of water weighs 28.3 Kilograms over an area of 929 cm2, therefore the pressure is 0.03 kilograms per square centimeter. (30.46 g/cm2) • SI use Newton per sq. meter and call it the Pascal 1 psi = 0.006894757 Pascals = 6.895 kiloPascals**Need to Know**Ball Parking: 1 psi ≈ 7 kPa 3 psi ≈ 21 kPa 15 psi ≈ 105 kPa 20 psi ≈ 140 kPa 3 to 15 psi is a common pressure range 20 kPa to 100 kPa is also a common pressure range**12 inches of water exerts a pressure of 0.433 psi**Inches of Water Scale This scale is used to measure small pressures. The properties of water are known and constant and can be used as a primary standard. Pressure is proportion to the height of the water column (hydrostatic head pressure)**Water Column**The hydrostatic head produced by a column of liquid is proportional to the height and density of the liquid. P = height x Density (Density = Mass/Volume) Density of water is 0.0361 lbs/in3 P = 12 x 0.0361 = 0.433 lbs/in2 12 “ H20 0.433 psi**Water Column**The greater the height the greater the hydrostatic head. P = height x Density (Density = Mass/Volume) 24 “ H20 Density of water is 0.0361 lbs/in3 P = 24 x 0.0361 = 0.866 lbs/in2 0.866 psi**Applied Process Pressure**Atmospheric Press Atmospheric Pressure h Height (h) of displaced water = applied pressure U-Tube Manometer Manometers Manometers can be used as a primary standard to measure small pressures**Applied Process Pressure**Atmospheric Press h Height (h) of displaced water = applied pressure Reading pressure with a U-tube Manometer If the total displacement h = 3“ the applied pressure would be 3”H20 = 3”WC = 0.108 psi Using Mercury as a filling liquid increases the pressure range by 13.6 times.**Well Type Manometers**The well type uses one measuring arm. Gives a larger pressure range Mercury filled well type manometers can measure up to 30 psi and more. (6 footer) Can be used as a primary standard.**Inclined Plane Manometer**Used for very small pressure measurements. Very sensitive, often used to measure room pressures.**psi inatmosphere**Gauge Pressure (psig) The standard pressure measurement is referenced to atmospheric pressure and is called gauge pressure. The scale units on the manometer could be calibrated in • inches of water (gauge) • inches of mercury (gauge) • psig And all measurements would be relative to atmospheric pressure 14.7 psi (varies slightly with elevation and weather)**Gauge, Absolute and Atmospheric Pressure**Any pressure above atmosphere is called gauge pressure (psig) Any pressure below atmosphere is a vacuum (negative gauge pressure) Absolute pressure (psia) is measured from a perfect vacuum Differential Pressure (psid) has no reference to either absolute vacuum or atmospheric pressure**Gauge Pressure (psig)**Applying 1 psi would produce a displacement of about 2 in. Hg or 30 in. H2O Since the reference side of the manometer is open to atmosphere, the applied pressure would be read as gauge pressure i.e. 1psig or just 1 psi 1 psi Atmos h**Standard Gauge**• When a gauge has no input applied, it will read 0 psig • The pressure range for this gauge is 0 – 100 psi • What is the range in kPa? Some gauges may not include the “g” after psi, some will.**Pressure Range & Scale**This gauge has a pressure range of 0 to 30 in. H2O The pressure being measured is still gauge pressure. What is the maximum psig that can be applied? kPa?**Small pressure measurements**Dwyer differential pressure gauge registers a differential of 0 - 2 psi, 1/8" npt. High and low pressure input ports on side and back. Manual set point. Max 15 psi and 140ºF. 4-3/4" diameter x 2" high.**Examples of psig, psia and vacuum**20 psig = 20 + 14.7 = 34.7 psia 60 psia = 60 – 14.7 = 45.3 psig 10 psia = 10 – 14.7 =-4.7 psig =-4.7=- 9.6 ”Hg 0.0361 x13.6**PSIA – Absolute Pressure**A gauge with a psia scale will indicate 14.7 when no pressure is applied. The compound gauge is more common than psia, it measures vacuum and gauge pressure. This gauge has a range of 0 – 30 in. Hg vacuum and 0 to 15 psig**6 psi2 psi**4 0.0361 x 13.6 h = = 8.15 inches of H20 differential Differential Pressure (psid) Differential pressure = 4 psid**Differential Pressure Gauge (psid)**Requires 2 inputs. Must observe pressure polarity, i.e. hi side / lo side**Differential Pressure Cell Transmitter**The d/p cell is often used to measure level and flow. What is the maximum allowable input pressure in psi? 4 – 20 mA output 2 wire transmitter Differential Input 0 – 200 in. H20 Typical input range**Pressure Conversion Chart**• Ball Parking • 1 psi = 7 kPa • 1 inch Hg = 0.5 psi • 100 inch H20 = 3.5 psi • 1 Bar = 1 Atmos = 14.7 psi • Accurate • 1 psi = 6.89 kPa • 1 inch Hg = 0.49 psi • 100 inch H20 = 3.61 psi • 1 Bar = 14.5 psi = 100 kPa**Exercise (ball park is fine)**What is this in psi, kpa, inches of H20? What is this in psig, psia, inches of Hg?