Instrument Trade Theory PDF (N2/N3/N4 - 1st Edition)

 INSTRUMENT TRADE THEORY N2/N3/N4 First Edition P.Stelzner © All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means without permission in writing from the author. First edition: 1995 Second: impression 2001 Published by: Technical B...

 INSTRUMENT TRADE THEORY N2/N3/N4 First Edition P.Stelzner © All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means without permission in writing from the author. First edition: 1995 Second: impression 2001 Published by: Technical Bookpost P.O. Box 48122 HERCULES 0030 Tel/Fax (012) 335-7621 PRINTED BY: THABANG PRINTERS ISBN 0-9584045-4-2  PREFACE This book/study guide became necessary because of the increasing amount of students interested in Industrial Instrumentation and to make available a neat set of notes which could also be of use to Instrument apprentices. It is definitely not the Alpha and Omega of Instrumentation, which in itself is a very diverse field of study. It will mainly serve Technical College students N2, N3 and can be used to the N4 student too. It could also be of use to anyone interested in obtaining a basic idea of what Industrial Instrumentation is about. However I feel I should stress that even with the continual technical advances made in the Instrumentation field, BASICS REMAIN BASICS, in other words, the basic knowledge obtained in the foundation upon which a subject is built and is critical for further studies. This work could not be achieved without the help of other people. To name them all individually is impossible. I therefore would like to thank everyone who contributed. Special thanks to Mrs. C Stelzner for the typing as well as Mrs D Marais and Mr. T Eloff for the printing. This book is not without mistakes and I apologise for those which have possibly been overlooked. I will value any possible criticism or any advice so as to improve on this first edition. Please write to: Mr. P Stelzner P.O Box i15431 Verwoerdburg 0140 One morning a trainer approached his three apprentice brick layers and asked each one in turn. "What are you doing Mr. X?" "I am building bricks." He then asked the next one, "What are you doing Mr. Y?" "I am building a wall." He finally asked the last one. "What are you doing Mr. Z?” “I am building the greatest building of all time." The attitude if Mr. Z should be the attitude for any apprentice / artisan to adopt. So what if we work with our hands, we have the capability or talent of creating fantastic monuments, as in the case above or repairing and maintaining in which ever trade you chose (artisans keep the wheels turning). Have pride in being an artisan and remember: 1: This, is something my father taught me. "IF YOU WANT TO DO SOMETHING DO IT PROPERLY OR DON’T DO IT AT ALL" 2: Know your Lord and abide by his word P STELZNER  CONTENTS CHAPTER TITEL GRADE 1 NON ELECTRICAL PRESSURE N2, N3 AND N4 2 ELECTRICAL PRESSURE N3 AND N4 3 NON ELECTRICAL TEMPERATURE N2, N3 AND N4 4 ELECTRICAL TEMPERATURE N2, N3 AND N4 POTENTIOMETER , THERMISTOR , 5 N2, N3 AND N4 PYROMETER 6 TELEMETERING N3 AND N4 7 AUTOMATIC CONTROL AND VALVES N3 AND N4 8 FLOW N2 AND N4 9 LEVEL N2 AND N4 PLEASE NOTE THAT ALL CHAPTERS ARE FOR THE N4 STUDENTS.  CHAPTER 1 NON ELECTRICAL PRESSURE N2 / N3 / N4 INSTRUMENT TRADE THEORY CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.2  CONTENTS INTRODUCTION PRESSURE 1 UNITS OF MEASUREMENT EXERCISE 1. 2 UNITS OF PRESSURE 3 4 MAIN TYPES OF PRESSURE MEASUREMENT 4 DEFINITIONS 5 NON ELECTRICAL METHODS OF PRESSURE MEASUREMENT 5.1 MANOMETERS INTRODUCTION 5.2 ERRORS IN DIRECT READING MANOMETERS 5.3 PRINCIPLE OP OPERATION AND FORMULA 5.4 TYPES OF MANOMETERS 5.4.1 EQUAL LEG MANOMETER (DRY CALIBRATION) 5.4.2 EQUAL LEG MANOMETER (WET CALIBRATION) 5.4.3 UNEQUAL LEG MANOMETER 5.4.4 WELL TYPE MANOMETER 5.4.5 INCLINED MANOMETER EXERCISE 2 6 BOURDON TUBES 6.1 C TYPE 6.2 HELICAL TYPE 6.3 SPIRAL TYPE 7 DIAPHRAGM TYPE INSTRUMENTS 7.1 NON METALLIC TYPES 7.2 METALLIC TYPES 7.2.1 DIAPHRAGM STACK 7.2.2 BELLOWS 8 RINGBALANCE METERS 9 BELL TYPE INSTRUMENTS 10 McLEOD GAUGE 11 DEAD WEIGHT TESTER 12 THE PIRANI GAUGE EXERCISE 3 CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.3 1 UNITS OF MEASUREMENT Various units exist for various measurements. In South Africa we make use of the System International (SI) units for measurement. Listed below are the units most commonly used: Physical Quantity SI Unit Symbol Force, weight Newton N Work, energy Joule J Power Watt W Electrical potential Volt V Electrical current Ampere A Electrical resistance Ohm Ω Electrical charge Coulomb C Electrical capacitance Farad F Magnetic flux Weber b Inductance Henry h Temperature Kelvin k Time Seconds s Length Meter m Mass Kilogram kg Pressure Pascal Pa Multiples and Sub-multiples Prefix Symbol Factor of multiplication Tera T 10 ˉ12 Giga G 10 ˉ9 Mega M 10 ˉ6 Kilo K 10 ˉ3 Hecto H 10 ˉ2 deca da 10 deci d 10 ˉ1 centi cm 10 ˉ2 milli mm 10 ˉ3 micro μ 10 ˉ6 nano n 10 ˉ9 pico p 10 ˉ12 femto f 10 ˉ15 atto a 10 ˉ18 CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.4 2. UNITS OF PRESSURE DEFINITION OF PRESSURE Pressure is defined as the force per unit area. There are two systems used for units of measurement, namely; British System (Imperial) and SI System. The SI System is used in South Africa. Examples of the British Units are inch, feet, pound and P.S.I. (Pound per square inch). Examples of the SJ Units are Metre, Gram and Pascal. For pressure measurement South Africa has standardised on the Pascal and the Bar (1 Bar = I00 kPa). It is necessary to know how to convert from one system to the other. South African Units or SI Units for pressure are Pascal and Bar. British Units are PSI (Pounds per square inch), inches mercury and inch water or water gauge (W.C.). Example 1. (refer to table) Convert 5 PSI to Bar 1 PSI = 0,0689 Bar therefore 5 PSI = 5 x 0,0689 5 PSI = 0,344 Bar Example 2a Convert 3 PSI to kPa 1 PSI = 6,895 kPa therefore 3 PSI = 3 x 6,893 3 PSI = 20,685 kPa Example 2b Convert 15 PSI to kPa 5 PSI = 6,895 kPa therefore 15 PSI = 6,895 x 15 15 PSI = 103,425 Kpa Example 3 200 Inches WC to mm WC 1 inch WC = 25,4mm WC therefore 200 inches WC = 25,4 x 200 200 inches WC = 5080mm WC EXERCISE 1 1. Convert 0,53 Bar to metre WC (5,4113 metre WC ) 2. Convert 8,6m WC to Bar (0,8428 Bar) 3. Convert 2 inches WC to mm WC (50.8mm WC) 4. Convert 508mm WC to inches WC (20 inches WC  CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.5 KPa Bar PSI Atmos- Metre mm inch Metre mm inch phere Hg Hg Hg H2O H2O H2O 1 0.01 0.145 0.00986 0.0075 7.5 0.2953 0.10213 102.13 4.021 100 1 14.50 0.986 0.750 750.062 29.53 10.21 10210 401.8 6.895 0.0689 1 0.0684 0.0517 51.71 2.036 0.703 703.6 27.7 101.03 1.01325 14.689 1 0.76 760 29.92 10.34 10340 407.14 3.386 0.0338 0.491 0.334 0.0254 25.4 1 0.3457 345.7 13.61 9.79 0.0979 1.421 0.0967 0.0734 73.48 2.893 1 1000 39.37 0.243 0.00243 0.0361 0.00245 0.001867 1.867 0.073 0.025 25.4 1 0.0098 0.000098 0.00143 0.000097 0.000735 0.0735 0.0029 0.001 1 0.039 3 4 MAIN TYPES OF PRESSURE MEASUREMENT A ABSOLUTE PRESSURE (or Total Pressure) The "absolute pressure of a fluid" is the difference between the pressure of the fluid and the "absolute zero of pressure". Absolute pressure is the total pressure measured from zero pressure line (see graph). When its value exceeds the local atmospheric pressure (101,3kPa) it is then the sum of the local atmospheric pressure and gauge pressure. It is thus the pressure that is measured with a vacuum as reference. Aneroid Barometer This measurement is seldom used in industry. It is mainly used for measurement of atmospheric pressure (the Barometer).  CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.6 B GAUGE PRESSURE This is the pressure measured with atmospheric pressure as its reference point. This is the most commonly used pressure measurement in industry. When a tyre is inflated to 200kPa the gauge pressure will then be 200kPa, P gauge = P absolute - P atmospheric. This is tie pressure measured above the local atmospheric pressure line (see graph). C DIFFERENTIAL PRESSURE This is the difference between two pressures. This pressure is used for flow measurement and any differential pressure measurement.  CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.7 D VACUUM PRESSURE This pressure uses the local atmospheric pressure as reference but unlike gauge pressure, it is always less than atmospheric pressure ie: it is any lower than atmospheric pressure. Formula to remember Pg = gauge pressure Pa = atmospheric pressure Pabs = absolute pressure (total pressure) a. Pg = Pabs - Pa b. Pabs = Pg + Pa 4. DEFINITIONS ATMOSPHERIC PRESSURE It is the pressure exerted by atmosphere. At sea level atmospheric pressure is equal to 101,3 kPa. The pressure is different from place to place and is relative to the height above sea level. The instrument used to measure atmospheric pressure is the Barometer. DENSITY (p) Density may be defined as the mass of a unit volume of a substance. In SI units it is the mass in kilograms of 1cm³ (cubic metre) of a substance. The density of mercury = 13,6 x 10³ kg/m³ and water is 1000kg/m³ CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.8 HOW TO CONVERT g/cm³ TO Kg/cm³ Example: Convert 1, 6 g/cm³ to Kg/m³ 1) First convert gram to kg 1, 6 g = 1, 6 ÷ 1000 = 0, 0016 Kg 2) Convert cm³ to m³ Therefore 1, 6 m³ = 1,6 x 1000 000 = 1600 000 cm³ 3) (1, 6 x 1000000) ÷ 1000 = 1600Kg/m³ SPECIFIC GRAVITY (RELATIVE DENSITY) SG is the number of times the mass of any volume of a substance is greater than the Mass of the same volume of water under identical conditions. therefore SG = mass of any volume of substance ÷ mass of the same volume of water The relative density or SG of water is 1 and the SG of mercury is 13, 6. Density is expressed by the Greek letter rho (ρ) STATIC PRESSURE The term "static pressure" is widely used in the measurement of pressure, liquid level and rate of flow. Where fluid in a process is still or "static" the meaning of the phrase is reasonably clear. When fluid in motion is encountered the meaning is not so clear. Consider a pipe full of a fluid in motion. If a hole is drilled in the wall of the pipe and a pressure measuring instrument connected to it, the pressure indicated on the instrument is the static pressure at the tapping. BOYLE'S LAW The volume of any mass of gas is inversely proportional to the absolute pressure if the temperature stays constant. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.9 5. NON ELECTRICAL METHODS OF PRESSURE MEASUREMENT 5.1 MANOMETERS - INTRODUCTION The manometer is a simple and accurate instrument. They are normally limited to differential pressure (Δ P) less than 2 bar. A manometer consists of two vertical lubes connected together at the bottom. The tubes are filled with sealing fluid. The pressure is then a function of the difference in height between the levels of the two tubes. The density is a very important factor. The common sealing fluids are transformer oil, mercury and water. 5.2 ERRORS TN DIRECT READING MANOMETERS a. The Capillary Effect In a fine tube the liquid will creep up the narrow lube and make it difficult to read the level. b. Parallax Effect If the reading of a level is not taken from the correct place a error reading will result. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.10 5.3 THE PRINCIPLE OF OPERATION AND FORMULA The pressure exerted by a column of liquid depends on its: 1. Height (h) in metres 2. Density (p) in kg/m³ 3. Gravitational force (g) in m/s² 4. Pressure (P) in Pascals therefore ΔP = ρ g h ΔP = PI - P2 5.4 TYPES OF MANOMETERS 5.4.1. Equal leg manometer (dry calibration) 5.4.2. Wet leg calibration 5.4.3. Unequal leg manometer 5.4.4. Well type manometer 5.4.5. Inclined manometer 5.4.1 Equal leg manometer (Dry calibration) This manometer is rarely used for pressure measurement on plants. However, it is used in Instrument workshops for calibrating other pressure measuring instruments. A scale is fixed between the limbs. It is a differential instrument in that it measures the difference between the two pressures, PI and P2 (Δ P). In sketch a) there is no pressure difference but in sketch b) PI is higher than P2 and this forces the liquid downwards in limb A and the level in limb B will rise. PI can be the pressure to be measured and P2 can be atmospheric pressure. (We will now obtain gauge pressure). The sealing fluid has a density of ρ (or SG). CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.11 Pressure at A = Pressure at B PI = P2 ΔP = P2 + h ρ g Therefore ΔP = h ρ g The disadvantage of the equal leg manometer is that it can only be used for small differential pressures. Where larger differential pressures must be measured we use the unequal leg manometer. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.12 5.4.2 WET LEG CALIBRATION When working, with a manometer which is open to atmosphere one only has consider the density of the sealing liquid, Suppose we have a U tube manometer across an orifice so as to measure Δ P and the pipeline is filled with water. We now have water exerting a pressure on the sealing fluid. We thus have two densities to consider. This is an example of using a pressure measuring device to measure flow in a pipe line. Proof PRESSURE AT A = PRESSURE AT B P1 + ρ1 x h1 x g = P2 + ρ1 x L x g + ρ2 x h x g BUT h1 = L + h P1 + ρ1 x (L + h) x g = P2 + ρ1 x L x g + ρ2 x h x g P1 + ρ1 x L x g + ρ1 x h x g = P2 + ρ1 x L x g + ρ2 x h x g BUT ρ1 x L x g = ρ1 x L x g P1 + ρ1 x h x g = P2 + ρ2 x h x g P1 – P2 = ρ2 x h x g – ρ1 x g x h ΔP = g x h (ρ2 – ρ1) h = ΔP ÷ g(ρ2 – ρ1)  CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.13 WET AND DRY CALIBRATlON DRY CALIBRATION When a manometer type instrument is calibrated with air. WET CALIBRATION When a manometer type instrument is used to measure liquid flow it must be calibrated to a higher differential pressure to compensate for the apparent density loss of the sealing fluid. When measuring flow a liquid is placed on top of the sealing fluid. This causes the sealing fluid to undergo a loss of density. TO CALCULATE THE HEIGHT OF THE MERCURY OF A MANOMETER DRY CONDITIONS WET CONDITIONS ΔP = ρ g h ΔP = g h ( ρ2 – ρ1 ) EXAMPLE 1 a) To what height will the mercury rise when a differential pressure of 2 Bar (200 Kpa) is applied, to a manometer type instrument, under DRY conditions? b) Calculate the height of the mercury when the same differential pressure is applied (200 Kpa) when using the manometer to measure water flow (WET conditions) DRY CONDITIONS WET CONDITIONS ΔP =ρ g h ΔP = (ρ2 – ρ1) g h h = ΔP / ρ g h = ΔP / (ρ2 – ρ1) g h = 200 x 10 ³ / 13,6 x 10 ³ x 9,8 h = 200 x 10 ³ / (13,6 x 10 ³ - 1000) x h = 1,5 meter 9,8 h = 1,619 meter NOTE: It can be seen that the two answers differ for the same differential pressure. If the instrument is to be used to measure water flow it must be "DRY" calibrated in the instrument workshop to a higher value so that when it is installed in the plant, on a wet application, it will measure and record the correct flow in the pipeline. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.14 TO CALCULATE THE RANGE AN INSTRUMENT MUST BE CALIBRATED TO h (dry) = (P1 –P2) / ρ g h (wet) = (p1 – P2) / g (ρ2 – ρ1) WET / DRY = [ (P1 – P2) / g (ρ2 – ρ1) ] ÷ [ (P1- P2) / ρ g ] h (WET) = [ ρ2 / (ρ2 – ρ1) ] x h (DRY) h (DRY) = [ (ρ2 – ρ1) / ρ2 ] x h (WET) EXAMPLE 2 To what range must a mercury manometer be calibrated if it is to give a full scale reading, when measuring water flow, with a differential pressure of 200 Kpa. The density of the mercury is 13, 6 x 10 ³ Kg/m³. h (WET) = [ ρ2 / (ρ2 – ρ1) ] x h (DRY) h (WET) = [ 13,6 x 10³ / ( 13,6 x 10³ - 1000) ] x 200 x 10³ h (WET) = 215,873 Kpa CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.15 5.4.3 UNEQUAL LEG U TUBE MANOMETER To compensate for the disadvantage of the equal leg manometer and to be able to measure greater differential pressures we enlarge the cross sectional area of the one leg. By doing this we increase the measuring range of the manometer. When a differential pressure is applied to the manometer the thin leg (high pressure) will drop by distance h and rise in the wide leg (lower pressure) by distance d. Proof ΔP=ρgh But H = ( d + h ) P1 – P2 = ρ g ( d + h } Volume Risen = Volume Displaced A1 x d = A2 x h h = ( A1 x d ) / A2 d = ( A2 x h ) / A1 Substitute d → P – P = ρ g [ {( A x h ) / A1} + h ] P1 – P2 = ρ g h [ ( A2 / A1) + 1 ] CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.16 5.4.4 THE WELL OR CISTERN MANOMETER Proof ΔP=ρgh But H = ( d + h ) P1 – P2 = ρ g ( d + h } Volume Risen = Volume Displaced A1 x d = A2 x h h = ( A1 x d ) / A2 d = ( A2 x h ) / A1 Substitute d → P – P = ρ g [ {( A x h ) / A1} + h ] P1 – P2 = ρ g h [ ( A2 / A1) + 1 ] CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.17 5.4.5 THE INCLINED TUBE MANOMETER The inclined manometer is used to measure low pressure or measure differential pressures very accurately. The formula of this manometer is as follows: P1 - P2 = ρ g L ( Sin θ + A2/A1 ) PROOF Pressure at A = Pressure at B P1 = P2 + ρ g h + ρ g d……………………1 Volume Displaced = Volume Risen A1 x d = A2 x L d = ( A2 / a1 ) x L…………………………2 Substitute d into Equation 1 P1 - P2 = ρ g h + ρ g ( A2 x L / A1 )…………...3 Sin θ = h / L h = L sin θ P1 – P2 = ρ g L Sin θ + ρ g ( A2 / A1 ) x L…….4 P1 – P2 = ρ g L [ Sin θ + ( A2 / A1 ) ] A2 = Area of narrow tube A1 = Area of well θ = Angle of tube p = density L = Distance the liquid in the inclined rube has moved from zero NOTE: In this proof an "L” was used in the formula it is not uncommon to use a "d" or even an "H", CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.18 EXERCISE 2 Use g - 10 m/s EQUAL LEG U-TUBE MANOMETER 1. Calculate the height of water displaced in an equal leg manometer if the high pressure is 30kPa, the low pressure is 10kPa. (2m) 2. Calculate the height of mercury displaced in an equal leg manometer if the high pressure is 200kPa and the low pressure is 150kPa. (0,368m) 3. Calculate the height of water displaced in an equal Leg manometer if the applied differential pressure is 0,0056 Bar. (0,056m) 4. Calculate the high pressure applied to an equal leg manometer which is filled with mercury. A reading of 0,01m is given if the low pressure is open to atmosphere. (102,66kPa) 5. Prove from the basic pressure formula that Pa = N/m². (P = Force/area = N/m²) 6. The height between the sealing fluid levels in a mercury filled U-tube manometer is 20cm, Calculate the applied gauge pressure in kPa. (27,2kPa) 7. Calculate the height between the sealing fluid levels in a U-tube manometer if the differential pressure 80kPa, The manometer is filled with mercury. (0,588m) UNEQUAL LEG U-TUBE MANOMETER 1. Calculate the height of mercury displaced in the narrow limb of an unequal leg manometer if the following information is given: Cross sectional area of wide limb = 10cm² Crass sectional area of narrow limb = 3cm² High pressure = 230kPa Low pressure = 125kPa (0,598m) 2. Calculate the height that the mercury will rise in the narrow limb of an unequal -leg manometer if the following is given: Pi = 800kPa P2 = 78OkPa Area of wide leg = 4cm² Diameter of narrow leg = 1,128379cm (5117647m) 3. Calculate the height of the mercury displaced in the wide leg of an unequal leg manometer if the following particulars are given: Area of wide leg = 60 mm² Area of narrow leg = 25 mm² Differential Pressure = 89 kPa (0,184 mm) CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.19 4. a. Calculate the height of mercury displaced in the narrow limb of an unequal leg manometer if the following information is given: Area of wide limb = 50mm² Area of narrow limb = 20mm² Pressure applied to narrow limb = 220kPa Pressure applied to wide limb = 150kPa (0,368m) b. Calculate the height of mercury displaced in the wide limb in 4a, (0,147m) 5 Calculate the height that the mercury is displaced in the thin leg of an unequal leg U- tube manometer if the following is known: Diameter of wide leg = 0,276m Diameter of thin leg = 0,178m ΔP = 36kPa (0,447m) 6. Calculate the height that the mercury is displaced in the thin leg of an unequal leg U-tube manometer if the following is known: PI = 800kPa P2 = 780kPa, Area of wide leg = 0,0004m² Diameter of thin leg = 1,128cm (0,118m) INCLINED TUBE MANOMETER I. Calculate the distance reached on the scale of an inclined tube manometer filled with water. The applied differential pressure is 0,15kPa and the range tube is inclined at an angle of 45° with the horizontal. The area of the cistern is 100mm² and that of the range tube is 10mm². (0,0186m) 2. Calculate the angle which the narrow leg of a water filled inclined tube manometer must be if the following information was given: Differential pressure = 0,85kPa Scale reading = 100mm H20 Diameter of cistern = 190mm Diameter of tube = 10mm (57,779°) 3. Calculate die distance on the scale of an inclined tube manometer filled with Hg, The applied pressure difference is 20kPa and the tube is at a 35° angle with the horizontal. The area of the well is 115mm² and the rube is 20mm². (0,196m)  CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.20 6 BOURDON TUBES The following Bourdon tubes will be dealt with here: 6.1 C-type 6.2 Helical type 6.3 Spiral type Metals that Bourdon tubes are made from: 1. Bronze 2. Stainless steel 3. Brass 4. Steel Factors that influence the free end movement of the Bourdon tube. 1. Temperature 2. Radium of bend 3. Total tube length 4. Wall thickness of the tube 5. Material of the tube Calibration Errors: 1. Zero error 2. Span error 3. Angularity 4. Hysteresis (metal fatigue) Hysteresis is the difference between the value which is indicated (measured value) and the value obtained from a reliable instrument (true value)  CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.21 6.1 C-TYPE The Bourdon tube is the most commonly used pressure measuring device used in industry today. It consists of a metal tube of elliptical shape in the form of a C. The one side is connected to a fixed block and the other end is made to move. The free end is connected by means of a connecting link to a sector gear and a pinion to which the pointer is connected. When pressure is applied to the bourdon tube it will tend to straighten out and thus by using different gears a reading on a scale is obtained. The bourdon tube is a most common pneumatic instrument and can be used for positive and negative ranges. 6.2 HELICAL TYPE The Helical type is used for high pressures, One advantage is that this type takes less space (it is smaller). More of these types can be installed in one instrument. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.22 6.3 SPIRAL TYPE 7 DIAPHRAGM TYPE INSTRUMENTS Two types will be discussed: 7.1 Non metallic (slack diaphragm) 7.2 Metalic diaphragm 7.1 NON METALIC DIAPHRAGM OR SLACK DIAPHRAGM It consists of an extremely flexible diaphragm and made of such materials as leather, nylon or rubber. The instrument basically consists of a diaphragm and connected to the one side is a spring. When pressure is applied to the instrument the pressure works against the area of the diaphragm and this causes a movement against the spring (the springs will compress). This movement is then made by means of a linkage system in order to move a pointer. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.23 7.2 M ETALIC TYPES 2 Types: 7.2.1 Diaphragm Stack 7.2.2 Bellows 7.2.1 DIAPHRAGM STACK. This diaphragm consists of flexible metal diaphragms which are stacked upon one another, which are normally made from copper or Beryllium copper. When pressure is applied to the instrument the diaphragms will move away from applied pressure. This movement causes a pointer to be moved. A differential pressure can also be measured by this instrument. 7.2.2 BELLOWS This resembles the diaphragm stack. The diaphragm stack consists of separate plates joined together, where the bellows is manufactured from a single tube. Materials of which it is made are Stainless steel and copper. Bellows are found in recorders and pneumatic controllers. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.24 8 RING BALANCE METER The instrument consists of a hollow ring, divided into two sections by a partition C on either side of the partition are flexible connections for pressure connections. The ring is pivoted at its centre on a knife edge. When pressure is applied a rotation is set up away from the high pressure, this continues until the opposite motion created by the counterbalance balances the rotating movement. The rotating moment = ( P1 - P2) A x r1 P1 - P1 = Differential pressure A = Tube cross sectional area R1 = Radius of ring The restoring moment = r2 M g Sin θ M = The mass g = Gravitational acceleration r2 = The radius of counterweigh! θ = Angle of rotation therefore Rotating moment = Restoring moment ( P1 – P2) A x r1 = r2 x M x g x Sin θ For balance P1 – P2 = ( r2 M g Sin θ ) ÷ A r1 CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.25 NOTE: It will be noted that there is no density factor included in the equation so the instrument is not affected by any density variations that might result from temperature changes. The most common sealing fluid is paraffin. Example Calculate the angle "movement” of a ring balance meter the applied pressure difference is 1 Kpa The mass of the counterweight is 0,8 kg, radius of the ring is 30 cm radius of counterweights 40 cm, area of tube is 0,00785 m² and g is 10m/s². P1 – P2 = ( r2 M g Sin θ ) ÷ A r1 1 x 10 ³ = ( 40 x 10 ˉ ² X 0,8 x 10 x sin θ ) ÷ ( 0,00785 x 30 x 10 ˉ ² ) 1 x 10 ³ = ( 3,2 x sin θ ) ÷ 0,002355 sin θ = ( 1 x 10 ³ x 0,002355 ) ÷ 3,2 sin θ = 0,7359 θ = sin ˉ ¹ x 0,7359 θ = 47,386 ° 9 BELL TYPE INSTRUMENTS 3 TYPES: 9.1 Single Bell Thick wall 9.2 Single Bell Thin wall 9.3 Double Bell differential pressure gauge 9.1 SINGLE BELL THICK WALL This instrument consists of a bell which has its open side placed in a liquid inside a sealed container which is usually made from cast iron. The sealing fluid is usually mercury or oil. The liquid covers the open end of the bell and acts as a seal, so forming two chambers. In this type of instrument where gravity provides the controlling force, the higher pressure is connected to the inside and the lower pressure to the outside of the bell. The resulting force causes the bell to rise until equilibrium is reached between the upward force and the weight of the bell. As the bell rises there is less of it immersed in the sealing fluid, so that the upward force on it due to buoyancy is reduced. The apparent weight will therefore increase. The thickness and density of the material from which the bell is made, its cross- sections area and the density of the sealing liquid will determine the range for which the instrument is used. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.26 9.2 SINGLE BELL THIN WALL In this type of instrument the bell is made of a thin material and the controlling force is obtained by means of a spring. If the bell is made of a thin material the areas over which the pressures act will be equal on the inside and the outside. In this type of instrument the high pressure is applied to the outside of the bell and the low pressure to the inside, CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.27 9.3 DOUBLE BELL DIFFERENTIAL PRESSURE GAUGE Proof ROTATING MOMENT = RESTRAINING MOMENT AREA OF BELL 1 = AREA OF BELL 2 ([ P1 x A1 ] – [ P2 x A2 ]) x L = M g Z BUT Z = r Sin θ ([ P1 x A1 ] – [ P2 x A2 ]) x L = M g r Sin θ A1 = A2 (P1 – P2) A x L = M g r Sin θ P1 – P2 = (M g r Sin θ) ÷ A x L P = differential pressure m = weight g = gravitational constant r = weight radius θ = angle of weight from Vertical A = areas of bells (Al - A2) L = distance from pivot CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.28 10 McLEOD GAUGE The McLeod gauge is mainly used for: 1. The measurement of very low absolute pressure 2. Compressibility of gasses This gauge works according to Boyle's Law which states that the volume of any given mass of gas is inversely proportional to its absolute pressure provided the temperature stays constant. In other words if the absolute pressure on a gas is doubled its volume is halved provided the temperature remains constant. Operation Mercury is initially at some level (D). All three limbs are connected to the gas to be measured at point (f). All three limbs are thus initially filled with gas. The mercury level is raised so that it cuts off the gas in the bulb (V) and limb (C) and then compresses it in limb (B). Then mercury in (C) and (B) is brought to the same level as the top of limb (A) which is the zero of the scale. The pressure can now be read off the scale. Formula P = (A h² ρ g) ÷ (v - A h) A = cross-sectional area of limb A h = difference in levels between A and B p = density g = gravitational acceleration V = volume of bulb and limb A above level W Disadvantages 1 Unsuitable for continuous measurement 2 If the gas whose pressure is being measured contains condensable vapours, Boyle's Law does not apply and errors occur. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.29 11 DEAD WEIGHT TESTER The dead weight tester is used to calibrate pressure instruments. Precautions when using a dead weight tester 1. To overcome friction the piston must be rotated during operation. 2. Make sure there is no leakage on system, 3. Calibration gauges must not be disconnected while system is under pressure. 4. Insure correct mass of weights are used. 5. Oxygen pressure gauges may not be calibrated on an oil filled dead weight tester. PRESSURE = FORCE ÷ AREA BUT FORCE = mass x g AREA = π d² ÷ 4 P=mxg ÷A CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.30 P = Pascal ( Pa ) A = Area ( m² ) m = Mass ( Kg ) g = Gravitational acceleration ( m/s² ) Example 1 Calculate the mass required on a dead weight tester to provide a pressure of 40 Kpa if the diameter of the piston is 60mm. P=mxg÷A A = π d² ÷ 4 m=PxA÷g A = [ 3,142 x ( 60 x 10 ˉ³ ) ² ] ÷ 4 m =( 40 x 10 ³ x 2,827 x 10 ˉ³ ) ÷ 9,8 A = 0,00287 m² m = 11,54 kg Example 2 A dead weight tester contains a piston with a diameter of 3cm. The mass of the platform and the piston together is 400g. Calculate the mass that has to be placed on the platform to calibrate a gauge to l00kPa. P=mxg÷A A = π d² ÷ 4 m=PxA÷g A = [ 3,142 x ( 3 x 10 ˉ² ) ² ] ÷ 4 m =( 100 x 10 ³ x 0,007068 ) ÷ 9,8 A = 0,007068 m² m = 7,212 kg MASS OF PLATFORM + PISTON = 400 g MASS ON PLATFORM = TOTAL MASS - MASS OF PLATFORM + PISTON MASS ON PLATFORM = 7,212x10 ³ - 400 MASS ON PLATFORM = 6812 gram  CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.31 12 THE PIRANI GAUGE PRESSURE The gauge consists of a pressure measuring cell with a metal wire inside it. The wire forms one arm of a Wheatstone bridge circuit. A reference cell which is identical to the measuring cell makes up another arm of the cell. The reference cell is sealed at a low pressure of 1mm Hg. The reason for the reference cell is that if there is a change in ambient temperature the reference cell will compensate so that there is no false reading. The circuit is connected to a regulated power supply. The current flowing through the measuring and reference cells will cause their elements to heat up. This increase in temperature will cause the resistance of the elements to increase. Since these elements are in different arms of the bridge their increase in resistance will cancel out and the bridge will remain in balance. An indicating instrument (milliampmeter) is connected across the bridge and calibrated directly in vacuum units. When a vacuum is connected to the measuring cell the gas decreases. Less heat is conducted away from the element causing the temperature to increase and thus an increase in resistance. This increase will unbalance the bridge and be displayed on the milliampmeter. Advantages of Pirani gauge 1. Low cost 2. Remote indication 3. Interchangeability of sensing element 4. Sturdy Disadvantages 1. Reading is dependant upon the type of gas being measured. 2. Contamination of the element leads to errors in reading. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.32 EXERCISE 3 QUESTION 1 The back pressure of an air purge level measurement meter is 0,5 x 10 5 kPa. If the density of the liquid is 1,2 g/cm³, calculate the depth of the purge point of the tube below the surface. QUESTION 2 State the mathematical expression to determine the differential pressure using the following manometers: a. inclined tube manometer b, unequal leg manometer c. ringbalance manometer QUESTION 3 Sketch the following manometers: a. unequal leg manometer b. ringbalance differential pressure gauge QUESTION 4 Sketch the following pressure gauges: a. C-type Bourdon tube b. Aneroid barometer c. Absolute pressure gauge QUESTION 5 State 6 factors that will influence the free end movement of a Bourdon tube. QUESTION 6 a. Make a neat, labelled sketch of a Pirani vacuum meter and its associated circuit, b. State the 4 major advantages of a Pirani vacuum meter. c. State 2 disadvantages of a Pirani vacuum meter. QUESTION 7 Calculate the pressure that must be indicated on a pressure gauge if the mass of 20kg placed on the piston of a dead-mass tester having an area of 0,1m2. QUESTION 8 Make a neat, labelled sketch of a dead-mass tester. QUESTION 9 State 2 precautions to be considered when calibrating instruments with a dead-mass tester. QUESTION 10 Make a neat, labelled sketch of a McLeod gauge. CHAPTER 1 NON - ELECTRICAL PRESSURE N2 and N3 1.33 QUESTION 11 Define the following terms; a. absolute pressure b. gauge pressure QUESTION 12 Define a. vacuum pressure b. static pressure c. atmospheric pressure d. density e. Boyles law QUESTION 13 Name 2 errors which occur in manometers. QUESTION 14 Name two types of diaphragms. QUESTION 15 Make a neat labelled sketch of a double bell differential pressure gauge. CHAPTER 2 ELECTRICAL PRESSURE N3 ONLY INSTRUMENT TRADE THEORY  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.2 CONTENTS 1 INTRODUCTION ELECTRICAL PRESSURE MEASUREMENT 2 STRAINGAUGES 2.1 BONDED STRAIN GAUGES 2.1.1 GRID TYPE 2.1.2 ROSETTE TYPE 2.1.3 HELICAL TYPE 2.1.4 FOIL TYPE 2.2 EXAMPLES OF BONDED STRAIN GAUGES 2.2.1 LOW PRESSURE MEASUREMENTS 2.2.2 LOW DIFFERENTIAL PRESSURES 2.2.3 WELDABLE TYPE 2.3 NON BONDED STRAIN GAUGES 2.3.1 LOAD CELL 2.3.2 LOAD CELL USED FOR WEIGHT MEASUREMENT 2.3.3 FOUR STRAIN GAUGES USED FOR WEIGHT MEASUREMENT 2.3.4 WEIGH-BRIDGE 2.3.5 HIGH PRESSURE MEASUREMENTS (NON BONDED) 2.4 FORMULA AND EXAMPLE 2.5 CHARACTERISTICS OF STRAIN WIRE 2.6 STRAIN GAUGE ADHESIVES 3 TRANSDUCERS 3.1 MAGNETIC TRANSDUCERS 3.1.1 INDUCTANCE TYPES and RELUCTANCE TYPES 3.1.2 DIFFERENTIAL TRANSDUCER 3.2 CAPACITIVE TRANSDUCER 3.3 PIEZO-ELECTRIC TRANSDUCER 3.4 RARE EARTH TRANSDUCER EXERCISE  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.3 1 INTRODUCTION An Instrumentation system consists mainly of three sections which are used together for measuring and controlling. a. Input b. Output c. Process a. The input part receives the amount being measured (level, temperature, pressure etc) and provides a signal in a form which is proportional to the measured variable. This quantity is now convened to another type of energy mainly electrical (4-20 mA) example, mechanical to electrical, pressure to electrical etc b. The output can be any of the following instruments namely a gauge or a recorder for visual displays and a permanent apparatus for storing information. Normally with instrumentatian the measured variable signal is in a non electrical form and by means of electrical and electronic methods (transducer) and is convened to a proportional quantity, so that it can be transmitted. A transducer can thus be defined as an apparatus which will convert one form of energy to another. c. The process is the physical substance to be controlled example flow in a pipeline. 2 STRAIN GAUGES The principle upon which this transducer works can be explained as follows: the electrical resistance will change when a tension or a pressure is applied to it. The formula which is used: R= (pXL)÷a When we, for example, take a length of wire with a certain resistance as shown When this piece of wire is heated basically two things will simultaneously happen.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.4 The piece of wire expanded and caused a change in the internal resistance. Therefore the gauge or sensitivity factor can be formulated as follows: K = ( ΔR ÷ R ) ÷ ( ΔL ÷ L ) ΔR = Change of resistance ΔL = Change of length L = Original length R = Original Resistance K = Sensitivity Factor TYPES OF STRAIN GAUGES Bonded types: These types are physically cemented to the structure whose strain is to be monitored. Adhesives which are used are fendhals and Araldite. Unbonded types or Nonbonded types: These types are strain gauges that are not cemented to a structure but consist of wire which is wound around cores and structures. 2.1 BONDED STRAIN GAUGES 2.1.1 Grid type With this type a thin wire is cemented to a carrier sheet in the shape of a grid. This gauge is now cemented onto the structure whose expansion or strain is to be measured. If expansion now takes place the paper as well as the wire will both be lengthened. The diameter of the wire will get slightly smaller which will also slightly increase the resistance of the wire. This change in resistance is measured b.m.o. a Wheatstone bridge and by using a scale an exact indication of the expansion can be displayed. Contraction can also be measured by using the same gauge provided the wire is correctly cemented onto the carrier sheet.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.5 In order to obtain accurate readings we must compensate for a change in surrounding temperatures. This is done by means of cementing a compensating gauge (m2) next to the measuring gauge or active gauge (m1). Both are exposed to the same temperature changes and therefore the effect of temperature can be compensated for. 2.1.2 Rosette Type This type is very much the same as the grid type but is cemented 90° over each other, so that gauge (A) forms the active gauge and (B) the compensating gauge.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.6 2.1.3 Helical Type 2.1.4 Foil Type By replacing the thin wire with a thin foil a bigger contact area is obtained, therefore the foil type stain gauge has a number of advantages above all other strain gauges. Advantages 1. They follow the surface to which it is cemented very well. 2. Because of the high contact area foil gauges dissipate heat easily. 3. They have a high gauge factor. 4. They have relatively small "creep" and hysteresis effects. 5. They can' he prepared within a few minutes with the proper cement and equipment, 6. They are easily made waterproof. 7. Special patterns are easily produced.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.7 2.2 EXAMPLES OF BONDED STRAIN GAUGES Bonded strain gauges can be cemented as follows so as to measure low pressures. 2.2.1 Low Pressure Measurements Strain gauges A and B are cemented on both sides of a cantilever C The cantilever is fixed at one side D, The pressure in the bellows F changes the tension between the two strain gauges and therefore pressure can be measured. To measure small differential pressures the following is obtained: 2.2.2 Low differential Pressure Measurements  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.8 2.2.3 Welded Types C is a metal tube. S is a strain element. The middle part of S is made thinner but remains one length from T1 to T2. Weldable types are spot welded (W) directly on the object to be measured (O). Surfaces can be flat or curved. 2.3 NON BONDED STRAIN GAUGES With these types the wire is not cemented but rather wound around structures. 2.3.1 Load Cell  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.9 2.3.2 Load Cells used to determine mass 3.3.3 Four Strain Gauges used to measure mass  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.10 2.3.4 Weigh-bridge 2.3.5 Non-bonded strain gauges to measure high pressures  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.11 2.4 FORMULA AND EXAMPLE Practical examples of where 3 strain gauge can be used is to weigh the amount of ore in a truck pulled by a locomotive (weigh bridge), or to determine the weight of any material (load cells). Simple calculations can be done to calculate the effect of the applied stress on the resistance change of a strain gauge. Hook's law gives the ratio of stress and strain for a linear strain curve in terms of the modules of elasticity of the material under stress and defines stress as the applied force per unit area and strain as the increase in length per unit length. Hook's law can also be written as follows: HOOK'S LAW E=α/ε WHERE α = STRESS ε = STRAIN E = YOUNGS MODULES STRESS α=F/A WHERE α = STRESS (Pa] F = FORCE (Newton] A = Area (m2) STRAIN ε=ΔL/L WHERE ΔL = Change of length L = Original length ε = STRAIN GAUGE FACTOR K = ( ΔR / R ) ÷ ( ΔL / L ) R = Change of resistance ΔL = Change of length L = Original length R = Original Resistance K = Sensitivity Factor ε = STRAIN THEN ALSO K = ( ΔR / R ) ÷ ε Example A load cell makes use of a strain gauge which has a nominal resistance of 187 Ω and a gauge factor of 1,75. The load cell is then subjected to a mass which results in a compressive stress of 27 MPa. Calculate the change in resistance of the strain gauge if the module of elasticity of the load cell beam (to which the strain gauge is attached) is 120 GPa.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.12 GIVEN: R = 187 ii K = 1.75 STRESS = 27 MPa ΔR = ? E = 120 GPa E=α/ε K = ( ΔR / R ) ÷ ε ε=α/E ΔR / R = K x ε ε = 27 x 10 ˉ6 / 120 x 10 ˉ9 ΔR = K x ε x R ε = 2,25 x 10 ˉ4 ΔR = 1,75 x 2,25 x10 ˉ4 x 187 ΔR = 0,0736 Ω 2.5 CHARACTERISTICS OF STRAIN GAUGE WIRE The choice of wire from which the strain gauge is manufactured is influenced by the Following: Maximum resistance per unit length Maximum gauge factor K Maximum melting point Minimum resistance temperature coefficient Minimum coefficient of linear expansion Minimum thermal-electrical effect at connections Optimum flexibility and easily soldered 2.6 STRAIN GAUGE ADHESIVES To ensure that the strain gauge will respond and in so doing obtain an accurate reading the strain gauge cement used to paste the strain gauge to the object under lest must be of a special type. "Araldite" is one type certain manufacturers recommend. 2.7 STRAINGAUGE CIRCUIT (WHEATSTONE BRIDGE) Circuit for a straingauge CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.13 The straingauge is used together with a Wheatstone bridge. The bridge is balanced when the voltmeter reads zero. The straingauge will then be in the unstrained position. Straingauge with one active element This circuit which has a straingauge A in one arm of the bridge.The only unbalance is created by the change of resistance of the active meter, caused by the strain applied to the meter. This change is measured by means of a voltmeter which is calibrated in terms of mass of pressure units Straingauge with temperature compensation The second straingauge B is connected to an unstressed pan of the object which is to be measured. They are mounted as close as possible together. Gauge A is the active gauge and gauge B is the compensating gauge.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.14 3. TRANSDUCERS 3.1 Magnetic pressure transducers 3.2 Capacitive transducers 3.1.1 Inductance type and Reluctance type 3.1.2 Differential transformer 3.3 Piezo – Electric transducers 3.4 Rare earth transducers 3.1 MAGNETIC PRESSURE TRANSDUCERS Magnetic transducers are classified into two main groups: Inductance type and Reluctance type. The inductance type operates on the principle of a change in inductance in an alternating current circuit and the Reluctance type operates on the principle of changing the reluctance (obtained from a change in the air gap between a magnet and a core). The primary measuring elements which are used to obtain movement are the same for all instruments namely; the bourdon tube, bellows, diaphragm etc. The only difference is that instruments which operate from a primary measuring element changes the inductance or the reluctance of the magnetic circuit. 3.1 1 Inductance type transducers In its simplest form this pressure transducer consists of a coil in which an iron core moves. CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.15 The iron core is connected to a primary measuring element far example, a bourdon lube or bellows. Any movement which lakes place will cause the iron core to move inwards and thereby changing the inductance of the coil. This change of the inductance is proportional to the amount of movement of the core which is also then a function of the movement which the primary element moved and thus an indication of the applied pressure. 3.1.2 Differential transformers The differential transformer is commonly used for displacement measurements, which is directly proportional to the measured variable and is converted to an electrical signal. The differential transformer consists basically of a transformer with a movable core with two secondary windings which are electrically neutral and connected in series opposition. When the core is in the middle between the two coils on the secondary side then the emf’s E1 and E2 are equal to one another. When the core moves away from the middle point then one emf will be greater than the other. If the displacement is made proportional to the value of the measured variable, then the ratio between the two emf's will be an electrical signal proportional to the measured variable (pressure).  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.16 3.2 CAPACITIVE PRESSURE TRANSDUCER In this case the distance between the plates changes If gas flow in a pipe is to be measured then it can be done by using the following example as shown in the above sketch. If the flow tempo changes the pressure on the diaphragm will also change and thus move the plates X and Y closer or further apart. As the distance changes between the two plates so the capacitance will change. This change in capacitance will thus indicate flow tempo. Capacitance in a parallel plate capacitor is determined as follows: C=(KAE)/d K = di-electric constant d = distance of plates A = area of each plate E = 9,85 x 10 ˉ12 3.3 PIEZOELECTRICAL TRANSDUCER If force or pressure is applied to certain crystals, such as quartz, Rochelle salt and barium titanate, the crystal generates an emf proportional to the pressure or force applied to it. This is called the Piezo-electric effect. Piezo-electric crystals are used in record players and ceramic microphones. Because of its high output voltage, and frequency response, this type of transducer is suitable for use in high-frequency accelerometers. This transducer does not use an external supply due to the fact that it generates it's own.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.17 The main advantages of this instrument: 1. It can generate emf. 2. Suitable for dynamic response. 3. Small size. 4. Rugged. 5. Linear output. Disadvantages: 1. High impedance output, signal-conditioning systems 2. Affected by temperature variations 3. With long leads, error signals are generated 34 RARE EARTH TRANSDUCERS CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.18 Certain combinations of the lanthanide series of elements are called rare-earth materials. If these are mixed with Zirconium tetrachloride the resulting mixture undergoes large changes in electrical resistance, when pressure is applied to it. The resistance is then proportional to the applied pressure. Thus the mixture acts as a pressure or force transducer. The mixture can be applied as a paste or paint that can be sprayed onto a base surface. After the paint has dried, it is covered with a conducting surface which forms an electrode. Thus blood-pressure variations can be measured by a spot with a diameter of +- 0,1cm the resistance of which varies in operation by something like 20 000 Ohms during the measured blood-pressure variations. Also, a layer of strain-sensitive paint, 0,25cm thick between two metal discs which are 1,9cm in diameter can be used for measuring pressure up to 500 p.s.i. (+-345 Bar). The main advantages of these transducers are: 1. Negligible hysteresis effects 2. They can withstand high pressures and overload 3. Temperature effects are very small 4. They have very good frequency response 5. High accuracy and repeatability.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.19 EXERCISE QUESTION 1 Make a neat, fully labelled sketch of the following types of strain gauges: a. grid type b, weldable type c. spiral pattern foil type d. unbonded type QUESTION 2 Explain what is meant by the term "bonded" strain gauge. QUESTION 3 Explain the term "gauge factor" as applicable to strain gauges. QUESTION 4 State 3 requirements which must be met by the resistance wire from which a strain gauge is manufactured. QUESTION 5 Transducers are used to alter mechanical signals to electrical signals. Explain how this can be done with the aid of annotated sketches of the following: a. a piezo-electric transducer b. a rare earth transducer c. a capacitive transducer d. an inductive transducer QUESTION 6 Briefly describe, with the aid of sketches, how one or more strain gauges can be used to: a. measure the expansion and contraction of flat surfaces b. measure pressures which vary from very low to very high c. measure low pressures d. measure small differential pressures e. measure very large masses QUESTION 7 Sketch and describe a transducer which converts low pressure into electrical signals and Contains two strain gauges. QUESTION 8 Give the names of any 4 transducers which can convert mechanical displacement into electrical signals. Stale in each case how the signal is generated.  CHAPTER 2 ELECTRICAL PRESSURE N3 only 2.20 QUESTION 9 Strain gauges are used to obtain an electrical signal from a mechanical signal. Show with the aid of sketches, the construction of: a, an unbonded strain gauge b. a grid type strain gauge QUESTION 10 Strain gauges can be used to determine mass. a. Sketch the arrangement of strain gauges for this purpose. b. Sketch a diagram of the electric circuit required, together with the strain gauges, to determine mass. c. How would you compensate for a change of ambient temperature and length of lead wires? QUESTION 11 A differential transformer can be used to change mechanical movements into electrical signals. a. Sketch such a transformer circuit b. Describe the operation of the circuit  CHAPTER 3 NON ELECTRICAL TEMPERATURE N2 AND N3 INSTRUMENT TRADE THEORY  CHAPTER 3 NON - ELECTRICAL TEMPERATURE N2 and N3 3.2 CONTENTS INTRODUCTION NON ELECTRICAL TEMPERATURE INSTRUMENTS 1. TEMPERATURE SCALES AND CONVERSIONS 1.1 THE CELSIUS SCALE (°C) 1.2 THE KELVIN SCALE (K) 1.3 THE FAHRENHEIT SCALE