Gas Laws PDF - Advanced Instruments

Art Williams and Harry Wendt Aeronautical Engineering School 1 ISO 9001:2008...

Art Williams and Harry Wendt Aeronautical Engineering School 1 ISO 9001:2008 Certified QMS GAS LAWS All pressure and temperature values are absolute. Absolute pressure = atmospheric pressure + gauge pressure. Equivalent units of Standard Atmospheric Pressure P.S.I. mBar BAR KN/M2 in. Hg mm. Hg 14.69 1013.25 1.013 101.3 29.92 760 Absolute temperature = (°Celsius + 273)K Gas Laws Boyle‟s Law Charles‟ Law Combined Gas Laws Boyle’s Law The volume of a fixed mass of gas is inversely proportional to the pressure, providing the temperature remains constant. V1 INITIAL STATE ABSOLUTE PRESSURE PV = CONSTANT 1 V α P P1 With temperature constant: P1 V1 = C P2 V2 = C Therefore P1 V1 = P2 V2 FINAL STATE P2 V2 VOLUME Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 2 ISO 9001:2008 Certified QMS Examples of Boyles Law 1) 4 cubic ft. of air at a pressure of 120 P.S.I. expands at constant temperature until the volume is 20 cubic ft. Find the final pressure. 2) If a mass of gas has a volume of 2m3, at 99 KN/m2 gauge pressure, what volume will it occupy at 299KN/m2 if the temperature remains constant. Charles’ Law 1 Charles found that for a gas at constant pressure, its volume decreases per °C fall in 273 temperature from 0°C. In theory all gasses have zero volume at –273 °C, but since they become liquid before this temperature is reached, it cannot occur in practice. 0°C Temperature Gas Liquified –273 °C Volume Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 3 ISO 9001:2008 Certified QMS Charles’ Law The volume of a given mass of gas, whose pressure is maintained constant, is directly proportional to its absolute temperature. V V V   Constant  1  2 T T1 T2 Examples of Charles‟ Law 1. 5 cubic feet of gas at 25K receives heat at constant pressure so that the temperature is raised by 75K. Find the final volume. 2. 3 cubic ft of gas at 27°C receives heat at constant pressure until the volume is 5 cubic ft. Find the final temperature. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 4 ISO 9001:2008 Certified QMS Combined Gas Equation If pressure and temperature (at a constant volume) are plotted against each other they produce a P pressure law that states that is a constant if the volume is constant. T Pressure Law – The pressure of a fixed mass of gas at constant volume is proportional to its absolute temperature. From The Gas Laws: Boyles Law PV = Constant V Charles Law = Constant T P Pressure Law = Constant T PV A general gas equation can be derived as = constant. T P1V1 P2V2   T1 T2 Example: The volume of a fixed mass of gas at a pressure of 30 ins Hg, at 27°C is 200 cubic inches. Find its volume if the pressure changes to 25 ins Hg and temperature to 57°C. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 5 ISO 9001:2008 Certified QMS THE ATMOSPHERE WATER AIR AIR HEATING AIR TEMPERATURE GASES VAPOUR DENSITY PRESSURE STRATO- % OUNCES/ MASS/CUB. P.S.I. SUN 100% 105,000 PAUSE CUB. YARD FT. NITROGEN NON-LINEAR + 0.303C 78% /1000 STRATOPHERE RATE OF 40% PRESSURE DUST/CLOUDS OXYGEN FALL DUE 65,800 21% TO THE 60% DECREASE IN WEIGHT CONSTANT CARBON ABOVE – 56.5C 15% DIOXIDE, ABSORBED BY WATER ARGON, VAPOUR 45% 36,090 TROPOPAUSE 1 NEON, oz. 200 0.029 lb 4.36 P.S.I. 1% 30,000 TROPOSPHERE HELIUM, – 2.39 P.S.I. – 45C 10% LAPSE 6.75 P.S.I. REFLECTED XENON, 20,000 RATE – 3.45 P.S.I. – 25C – 1.98C/1000 WATER 10.20 P.S.I. 10,000 VAPOUR – 5C – 4.49 P.S.I. RADIATION ½ oz. 0.077 lb 14.69 P.S.I. 0 + 15C 35% ABSORBED EARTH Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 6 ISO 9001:2008 Certified QMS EARTH’S SURFACE THE I.C.A.O. STANDARD ATMOSPHERE 150 140 130 CHEMOSPHERE 120 110 STRATOPAUSE 105,000 FT. 100 – 44.656C 8.885 mb. 90 + 0.303C/1000 80  1000 70 STRATOPHERE 65,800 60 50 TROPOPAUSE 40 36,090 FT. – 56.5C 30 226.32 mb. TROPOSPHERE 20 TEMP. DECREASES AT 10 – 1.98C/1000 (LAPSE RATE) 0 0 100 200 300 400 500 600 700 800 900 1000 MILLIBARS – 60 – 50 – 40 – 30 – 20 – 10 0 10 20 30 40 C Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 7 ISO 9001:2008 Certified QMS The Effects of Temperature on the Atmosphere 4. As the temperature decreases, we could expect an increase in density, but this is more than offset by the decrease in pressure. 3. Lower pressure causes a further decrease in density and a decrease in temperature. 2. Air rises towards lower pressure levels. 1. Heat is conducted to the air in contact with the surface; the air expands causing a decrease in density. The air is assumed to be dry. Gravity is assumed to be constant at 9.80665 m/s2. The pressure at sea level will be 1013.25 mbars at a temperature of 15°C. The temperature will decrease at a rate of 1.98°C per 1000 feet from sea level up to an altitude of 36,090 feet, above which it will remain constant at –56.5°C to an altitude of 65,800 feet. 1963 extension – states that for altitudes above 65,800 feet, the temperature increases at a rate of 0.303°C per 1000 feet up to an altitude of 105,000 feet (– 44.656°C). Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 8 ISO 9001:2008 Certified QMS PRESSURE SENSING ELEMENTS Pressure sensing elements are used to convert the pressure energy into a mechanical movement for the operation of either mechanical pressure gauges, or the operation of an electrical transmitter. The Sensing elements in general use are: 1. Diaphragms This consists of a circular plate which is held securely around its edge, and when a pressure is applied to it, it will deflect to give a measure of the pressure difference. 2. Capsules These consist of two diaphragms sealed at their joining edges. There are two types: a. Pressure Capsule b. Aneroid Capsule P Pressure applied to the inside This Capsule is evacuated and causes an expansion of the sealed and senses the change of Capsule. the surrounding pressure. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 9 ISO 9001:2008 Certified QMS 3. Bellows These are of similar operation to the capsules but may be regarded as being used to sense higher pressures. A constructional difference is that the corrugations are on the sides of the bellows as well as the surface. Pressure Bellows Aneroid Bellows 4. Sensitivity of Diaphragms, Capsules and Bellows This depends upon: a. The type of material used: Phosphor Bronze or Beryllium Copper: the material must have the property of elasticity. b. The material thickness: 0.002" upwards. c. The surface area. d. The number and depth of the corrugations controls the linear pressure/deflection characteristics of the element. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 10 ISO 9001:2008 Certified QMS 5. Effects of changes in Ambient Temperature An increase in ambient temperature on the pressure sensing element will result in a decrease of the element‟s elasticity causing an increase in distension for a given applied pressure. 6. Temperature Compensation for Mechanical Gauges This is normally achieved by making part of the mechanism of a bi-metal material: The bending of the bi-metal strip will create a positional movement of the mechanism, thereby correcting the indication for a temperature change. Pressure Sensing Tube (Bourdon Tube) This pressure sensing element is normally used for sensing oil, or air pressures, and can be manufactured for either high or low pressures. The tube is constructed with an oval cross-section area, and one end of the tube is sealed while the other is open for the inlet pressure. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 11 ISO 9001:2008 Certified QMS FREE END If a pressure is applied to the inlet, then there will be a tendency for the tube to straighten and the movement of the free end will be taken to an indicating mechanism which could be either a mechanical indication, or the tube can be used to drive an electrical position indicating transmitter (i.e. Desynn or Synchro). When the pressure reduces, the tube will return to its original „C‟ shape, because of the elasticity of the tube material. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 12 ISO 9001:2008 Certified QMS INSTRUMENT MECHANISM CALIBRATION The form of presentation is usually a pointer moving over a calibrated scale. The ranges covered are an arc up to approx. 300º. The detector movement is usually very small and must be magnified. The normal magnifying components are: - 1. Link. 2. Quadrant Lever. 3. Pinion. Principles of Instrument Calibration Quadrant Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 13 ISO 9001:2008 Certified QMS Pinion (Small Gear) The pinion, which is a small gear wheel, is driven by movement of the quadrant. The pointer is normally attached to the pinion. By selection of the pinion size and the number of gear teeth, the manufacturer can magnify the pointer movement. Backlash Because there must always be a clearance between the faces of gear teeth, there is a tendency for the instrument pointer to be inaccurate due to lost motion – this is overcome by using a hairspring, connected between the pinion and frame to bias the pinion into permanent contact with the quadrant. Lever Length The diagram shows for the same amount of link input movement, a decrease in lever length will give an increase of output movement. Summary If 9 lb/ was applied to a pressure gauge and the pointer only indicates 7, the amount that the pointer moves could be magnified by decreasing the lever length. Lever Angle Movement magnification and calibration can take place at another point. That is the angle of link to the lever. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 14 ISO 9001:2008 Certified QMS Lever angle is not a constant value, as the link and lever move the angle between them changes. It can be seen that for equal input movement of the link, the lever/ link angle increases and the pointer movement is magnified. Pressure Deflection Errors When we calibrate an instrument, we plot the instrument output against the master on a graph. The perfect instrument would give a line at 45º. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 15 ISO 9001:2008 Certified QMS A typical example of instrument calibration is as follows: - Master Instrument under Test Error 0 0 0 1 0.8 - 0.2 2 1.6 - 0.4 3 2.4 - 0.6 4 3.2 - 0.8 5 4.0 - 1.0 When plotted on the graph: - We see a regularly increasing negative error. (For each unit on the master the instrument under test increases by only 0.8) This could be rectified by reducing the lever length If we had a regularly increasing positive error, we could increase the lever length. Constant Error Master Instrument under Test Error 0 0.3 + 0.3 1 1.3 + 0.3 2 2.3 + 0.3 3 3.3 + 0.3 4 4.3 + 0.3 5 5.3 + 0.3 Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 16 ISO 9001:2008 Certified QMS We now have a constant positive error. This is termed pointer error and can be rectified by removing and repositioning the pointer. Instrument under-reads with Strained sensing element – A% error increase – re- replace and re-calibrate. calibrate by increasing lever angle. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 17 ISO 9001:2008 Certified QMS (b) (a) a) Gear mechanism jammed Strained/ weak spot on sensing element – replace element and b) Stressed sensing element. re-calibrate. a) Inspect. b) Replace element. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 18 ISO 9001:2008 Certified QMS PRESSURE GAUGE CALIBRATION The accuracy of pressure gauges and systems may be checked by substituting the pressure source with known pressures and noting the indicator readings. Most pressure gauge calibrators use the Brahm‟s press principle for their operation. A simple pressure gauge calibrator is illustrated below. Consider that the piston assembly weight is 1 1b. and its cross-sectional area is 1 sq. in. Then, if the oil pressure is gradually increased by screwing in the plunger, the piston will begin to lift when the pressure reaches 1 lb. per sq. in. By adding a weight to the piston platform, making its total weight 2 lbs. the piston will now lift when the pressure reaches 2 lbs. per sq in, and so on. The pressure of oil is thus determined by the weight it will support and a gauge connected to the pressure is checked accordingly. In practice, calibrators have piston areas less than 1 sq. in, so the actual value of the weights used can be reduced. For example, the piston of the dead weight tester has an area of ⅛ sq. in; a pressure of 1 lb. per sq. in will therefore only support ⅛ lb. This is the actual weight of the piston assembly. But since it is equivalent to 1 lb. acting on 1 sq. in, it is marked 1 lb. The weights are similarly stamped with their equivalent values for calibration purposes. If weighed, however, they would be found to be one-eighth of the value marked. Portable Calibrator Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 19 ISO 9001:2008 Certified QMS The dead weight tester is supplied, complete with weights and accessories, in a fitted transit case. To prepare for use, mount the calibrator on top of the transit case. A special securing screw is built into the case lid for this purpose which screws into a boss on the base of the calibrator. Fill the calibrator as follows:- 1. Close the outlet valve and open the inlet valve. 2 Screw in the hand-wheel. 3. Fill the filling cup with oil and gradually unscrew the hand-wheel to draw oil into the calibrator, replenishing the cup as necessary to prevent air entering the system. 4. When the hand-wheel is completely unscrewed, close the inlet valve under the filling cup. Direct reading pressure gauges are removed from the aircraft and connected directly to the calibrator for testing. Capillary type instruments are tested in position on the aircraft by disconnecting the transmitter unit and uncleating sufficient capillary tubing to allow the transmitter to be connected to a conveniently placed calibrator. A special adaptor is supplied with the calibrator for this purpose. The following points must be observed when testing instruments. 1. After connecting the gauge (or transmitter unit) to the calibrator, apply the necessary weights for the first reading and open the outlet valve. In assessing the weights required, the weight of the piston assembly must be included (weights and piston are clearly marked with their equivalent weight value). 2. Slowly screw in the hand-wheel until the weights lift. The weight platform should be rotated during this operation to eliminate the effects of oil drag. Lightly tap the indicator and note the reading. 3. Similarly check the readings at three or four equidistant points going up the scale and again at the same points corning down. Any discrepancy in the two readings at any one point is usually due to friction in the indicator mechanism. 4. When taking readings down the scale, do not remove weights from the weight platform until the pressure has been suitably reduced. 5. The complete test should occupy about 20 minutes as a quicker rate of testing may introduce errors due to lag in the response of the indicator expanding member (bourdon tube or capsule). Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 20 ISO 9001:2008 Certified QMS Screw/piston assembly Piston/platform assembly Reservoir Isolation valves Gauge or system under test Area 1/8 sq. in Anti-freeze oil Typical test weight 20psi True weight value is 2.5 lbs. Marked value DEAD WEIGHT TESTER Calibrator Mark 2 The calibrator previously described will test gauges up to 300 lbs. per sq. in. The Mark 2 calibrator is designed to test gauges up to 4,000 lbs. per sq. in. It is in effect two calibrators mounted on a common base plate and sharing a common pressure plunger system. Either calibrator can be brought into use or isolated as required by operating the “on-off” cocks situated in the branch pipes connecting the calibrators with the pressure chamber. Referring to the illustration, the left-hand calibrator is for pressures from 1 to 400 lbs. per sq. in. and is basically the same as the calibrator previously described. The right-hand unit is the high pressure calibrator which handles pressures from 200 to 4,000 lbs. per sq. in. It employs the unusual piston assembly described in succeeding paragraphs. It will be apparent that to increase range it is necessary to either increase the weight loading of the piston or decrease the piston area. Heavier weights are cumbersome so the decreased area Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 21 ISO 9001:2008 Certified QMS WEIGHT method is used. To avoid the mechanical weakness of directly reducing the piston area, the same effect is obtained by two interconnected and vertically opposed pistons arranged as illustrated. 1 The area of the upper piston is sq. in, larger than the lower, i.e. pressure between them 80 1 will result in an upward thrust equivalent to that experienced by a single piston of sq. 80 in, in cross-sectional area. Since this effective area is exactly one-tenth of the area of the low pressure calibrator piston, it follows that 10 times the pressure is required by the low pressure unit. For this reason the high pressure calibrator is labelled "lb. × 10" to indicate that the weight values must be multiplied by 10 when this component is in use. Filling: Before use, the calibrator must be filled with anti-freeze oil as follows:- 1. Close the high and low pressure cocks and open the reservoir valve. 2. Fit the blanking plug to the gauge union, and screw in the plunger by rotating the hand-wheel clockwise. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 22 ISO 9001:2008 Certified QMS 3. Fill the reservoir with oil, and unscrew the plunger to its limit. 4. Close the reservoir valve and remove the blanking plug. ENGINE PRESSURE INSTRUMENTS Direct Reading Pressure Gauges Direct reading pressure gauges are connected directly to the source of pressure by pipelines and are usually of the bourdon tube type. They may be defined as those gauges which measure pressure direct from the source without using any form of transmitter or other form of intermediary device between the gauge and the pressure source. Changes in both positive and negative pressures may be detected and indicated and in the case of negative reading pressure gauges the bourdon tube is reversed in the case in order to maintain a clockwise rotation of the pointer with an increase in suction. Some gauges have outward relief valves in the rear of the case as a safety device in the event of failure of the bourdon tube. Connection between the gauge and pressure source is usually by small bore copper tubing called capillary tubing, the small bore acts as a choke and helps to damp out fluctuations in source pressure. Pressure enters the bourdon tube at the open, fixed, end and changes in pressure move the closed Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 23 ISO 9001:2008 Certified QMS end which is attached to a link the other end of which is connected to a quadrant arm. The quadrant is in mesh with the pointer carrying pinion. Also attached to the pointer pinion is a hair spring which takes up any backlash. Overload stops are fitted to prevent excessive movement of the bourdon tube. Operation: Pressure changes cause the free end of the bourdon tube to move and this movement is magnified and transmitted to the pointer through the linkage, sector and pinion. Testing: Direct reading pressure gauges are tested using a Dead Weight Tester. The tests consist of an overload test, a leak test, calibration check and a check on the lag or hysteresis of the movement. Transmitting Pressure Gauges A transmitting pressure gauge is used where the source of pressure is some distance from the indicator, and may involve a fire risk or engine failure if the line to the instrument panel ruptures. In this system pressure readings are transmitted by liquid in a sealed capillary tube from a transmitter attached to the engine. Bourdon Tube Capillary Tube Hollow Bolt Capsule with Transmitting Fluid System Fluid Pressure Inlet CAPILLARY SYSTEM SCHEMATIC Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 24 ISO 9001:2008 Certified QMS The Indicator is a normal bourdon tube indicator as used in the direct reading instruments. The Transmitter unit is attached to the particular system by a hollow bolt and contains a capsule unit connected to the capillary tube. When pressure is applied a force is exerted on the capsule to cause a displacement of the transmitting fluid and movement of the bourdon tube. The Capillary tube is a small bore copper tube which is filled with Heptane (a paraffin hydro- carbon) a liquid which has a low freezing point and very little viscosity change. Testing This is carried out in the same manner as a direct reading pressure gauge using the dead weight tester fitted with a suitable adaptor to the transmitters. Care should be taken to ensure that the Dead Weight Tester is level with the point to where the transmitters are normally fitted to minimise 'Head effects'. Installation The installation should start at the instrument panel by passing the transmitter unit through the appropriate instrument panel cut-out. The capillary should then be laid along its route before fixing the transmitter unit or indicator. The indicator should then be fixed into position and the capillary should then be cleated to the aircraft‟s structure at intervals not exceeding 9 inches. Sharp bends should be avoided and when it is not possible to avoid them they should not be less than ⅝ inch. Any spare lengths of capillary tube should be formed into a coil of not less than 4 inches in diameter secured by not less than three cleats. On no account should the capillary be broken or disconnected from the transmitter unit or indicator. The capillary should be inspected before and after installation for any abrasions or denting and for security of capillary and connections. Pressure Transmitter System This system operates in a similar way to the capillary instrument the pressure of the liquid is applied to a pressure sensitive capsule in a transmitting unit; the pressure is then relayed through capillary tubing to a Bourdon tube instrument. The units are separate independent units and are easily replaced should any of the units fail. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 25 ISO 9001:2008 Certified QMS Transmitter Unit The transmitter unit consists of two flanged circular shaped castings and a neoprene diaphragm. The diaphragm is interposed between the casting, which are bolted together to form two separate chambers. The liquid whose pressure is to be measured is admitted to the smaller of the two chambers, while the pressure outlet is at the second chamber, connections to the pressure source and capillary tubing are made by high pressure unions. A light alloy disc, known as a centralising disc, is provided in the inlet housing to ensure that the diaphragm remains in a central position and is not over stretched during filling and priming operations. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 26 ISO 9001:2008 Certified QMS Pipelines The pipelines are small bore copper or tungum tubing of approximately 1/16" bore, the ends belled to accept the nipples of the high pressure unions of transmitter and indicator. Some pipelines may incorporate a 'T' piece near to the indicator to facilitate bleeding the system. Pressure Switches It is often important that a pilot learn immediately of a dangerous condition. In these situations a pressure switch may be used to initiate a warning device when a definite low or high pressure is reached. Lights on the instrument panel are the most commonly used warning devices, but audible signals may also be used. Figure 1 shows a typical fuel pressure warning switch. The pressure port is attached to the fuel pressure inlet of the fuel control unit (or carburettor) and the vent port to the air inlet. Fuel pressure applied below the diaphragm moves it over, and through the actuating arms opens the normally closed micro-switch. A disc spring behind the diaphragm exerts a force in the direction to close the switch and is opposed by the helical spring which tends to open it. The compression of the helical spring is adjustable to cause the switch to close at the pressure desired. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 27 ISO 9001:2008 Certified QMS ELECTRICAL PRESSURE SWITCH FOR SENSING DIFFERENTIAL PRESSURE Manifold Pressure Gauge The purpose of this type of gauge is to measure the pressure of the fuel/air mixture entering the induction chamber of an internal combustion engine (propeller type aircraft). The power of such an engine is dependent on the density of the fuel/air mixture entering the cylinders on the induction stroke of the engine. As the piston moves down on the induction stroke, the inlet valve opens and the fuel/air mixture enters the cylinder due to the pressure difference between cylinder and induction chamber. At sea level with a standard pressure of 29.92 in. Hg. the cylinder pressure could be 6 in. Hg. The pressure difference is therefore 23.92 in. Hg. and it is this pressure which forces the fuel/air mixture into the cylinder. This type of engine is said to be normally aspirated. This characteristic causes problems however when the aircraft is flown at increasing heights above sea level. Air pressure decreases with increase of height and thus at 30,000 feet the air pressure is approximately 8.85 in. Hg. The pressure difference between induction chamber and cylinder is thus reduced to 2.85 in. Hg. The charge of fuel/air mixture is thus reduced by a large Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 28 ISO 9001:2008 Certified QMS amount and therefore there is a corresponding power loss. To overcome this problem a centrifugal air pump is connected between the carburettor and cylinders, which is driven from the engine crankshaft. The air pressure is thus increased to equate to the air pressure at sea level, and the pressure difference between the induction chamber and the cylinder is maintained. The pressure sensing element consists of two metallic bellows mounted in tandem. The rear bellows is connected by a pipeline to the engine induction manifold and the front bellows is evacuated and sealed and is spring-loaded by an internal spring. The outer ends of each bellows are secured to the instrument frame and the inner ends are connected to a common distance piece. The distance piece is connected to a gear type pointer mechanism via an arm, rocker shaft and lever mechanism. Gauges are fitted with a lubber mark which may be pre-adjusted to indicate the maximum manifold pressure permitted for engines with which they are associated. When the engine is stopped and the pressure is at standard conditions, the evacuated bellows assumes a position where its tendency to collapse is balanced by the internal spring. The bellows therefore provides an atmospheric pressure datum against which induction manifold pressure is referenced. Modern instruments are calibrated in inches of mercury. Under engine operating conditions variations in manifold and atmospheric pressures cause relative displacements of the bellows which are transmitted to the pointer via the distance piece, rocker shaft and lever mechanism. Gauge calibrated to Absolute Pressure indicating in inches of mercury; Absolute Pressure = Gauge Pressure + Atmospheric Pressure Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 29 ISO 9001:2008 Certified QMS Testing/Checks 1. Visual, security of attachment, discolouration of pointers, moisture on inside of instrument face. 2. Static engine conditions - the gauge should correspond to the prevailing atmospheric pressure within prescribed tolerances. 3. Engine running - ensure that readings on gauges correspond to thrust settings, RPM indications etc. in accordance with the relevant Maintenance Manuals. Calibration Positive and negative pressures are applied to the gauge and the readings checked against a boost gauge calibrator or a suitably calibrated manometer. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 30 ISO 9001:2008 Certified QMS Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 31 ISO 9001:2008 Certified QMS Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 32 ISO 9001:2008 Certified QMS The boost gauge calibrator consists of a dead weight tester for positive pressure and connections for a vacuum system for negative pressures. A separator is fitted to prevent oil from the dead weight tester getting into the vacuum out pipes. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 33 ISO 9001:2008 Certified QMS Torque Meters The torque meter is a supplement to the engine power indications which are obtained from tachometers and manifold pressure gauges. They are fitted on supercharged piston and turbo- prop aircraft. There are three (3) basic types of torque pressure systems as indicated below:- 1. Torque meter system – mechanical 2. Desynn Torque pressure system 3. Synchro Torque pressure system Torque Meter System The basic operation of the torque meter is shown below: Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 34 ISO 9001:2008 Certified QMS This is fitted on propeller type aircraft and is a purely mechanical type system. Between the crankshaft of the engine and the propeller shaft is fitted a reduction gear assembly and the torque meter itself forms part of the gear assembly. The crankshaft of the engine drives two (2) planet gears which attempt to turn the external ring gear. Attached to the external ring gear is a beam pivoted at each end and connected to a piston. An opposing force is fed to the piston face by a special torque meter pump which delivers engine oil pressure (modified). The effect of the piston movement as shown in the diagram compresses the oil and thus pressure is shown on the torque pressure indicator. Force Pressure = where the Force is that delivered to the oil by the torque of the planet and Area external gears of the reduction gear assembly The indicator is calibrated in pounds per square inch [lbs/sq" (psi)] and usually takes the principle of the Bourdon tube for the pressure measurement. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 35 ISO 9001:2008 Certified QMS Functional tests/checks 1. Visual, security, discolouration of pointers, cleanliness, leakage 2. Ensure that with the engine running the power settings (R.P.M.) correspond to appropriate dial markings (arcs and radial lines) NOTE: Red radial line: Maximum permissible torque pressure Green arc: Normal range of operation (minimum to max. range) Yellow Arc: Maximum torque pressure to maximum permissible torque pressure Desynn Torque Pressure System This system is commonly found on Turbo prop engines and it consists of mechanical and electrical sections. The transmitter is a combination of a Bourdon tube and Slab Desynn transmitter, while the indicator is composed of a star wound stator and a moveable permanent magnet rotor, to which the instrument pointer is attached. The input pipeline of the transmitter is connected to the oil pressure line of the mechanical torque meter arrangement previously explained for propeller type aircraft. The oil pressure causes the free end of the Bourdon Tube to move and connected to this end are the brushes of the slab Desynn transmitter. The transmitter windings are fed with 28V DC and rotation of the brushes across the windings alter the resistance legs and thus the current flow through the 3 transmitter tappings is varied. The magnitude and direction of current flow is transmitted to the stator windings of the indicator and resultant magnetic fields are produced. The indicator rotor aligns itself with the stator magnetic field and thus the pointer registers the torque pressure of that engine. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 36 ISO 9001:2008 Certified QMS Synchro Torque Pressure System This system is very similar to that of the Slab Desynn System except that it utilises an A.C. synchro transmitter and A.C. synchro receiver for the presentation of torque pressure. The Bourdon Tube operates through a sector gear and pinion as before, but the output pinion controls the position of the rotor of a CX synchro. The supply to the CX rotor is usually 115V 400Hz single phase A.C. The CX stator has voltages induced into it by the rotor field and causes current to flow from the CX stator to the CT stator in the Torque Pressure Indicator. The rotor of the CT forms the input circuit to a 2 stage amplifier. The rotor signal is amplified before being applied to a 2 phase motor which drives through a gearbox to position the pointer of the Torque pressure indicator. At the same time the gearbox provides an output shaft movement to null the CT rotor‟s signal, by moving the rotor into alignment with the CT stator field. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 37 ISO 9001:2008 Certified QMS The Engine Pressure Ratio Gauge (E.P.R) Earlier type gas turbine engines used R.P.M. as the only engine monitoring method to establish the thrust of an engine. The R.P.M. gauge has been superseded on gas turbine engines by the E.P.R. gauge due to the fact that temperature on a hot day will give a higher R.P.M. for a given thrust than it will on a cold day. Engine pressure ratio is used because it varies directly with thrust. E.P.R. is the ratio of the total air pressure at the front of the compressor to the total gas pressure at the rear of the turbine. The sections of the turbine selected for deriving these pressures from, are Pt2 and Pt7 as shown below. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 38 ISO 9001:2008 Certified QMS Absolute Pressure Relating to E.P.R. Gauge Absolute Pressure = Atmospheric Pressure + Gauge Pressure Atmospheric Pressure - that pressure above zero or vacuum pressure Gauge Pressure - that pressure above atmospheric pressure In turbo-jet engines, Pt2 pressure = air intake pressure = air pressure + atmospheric pressure In turbo-jet engines Pt7 pressure = thrust or exhaust gas pressure + atmospheric pressure outlet gas pressure (Pt 7 ) The E.P.R. = inlet air pressure (Pt 2 ) Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 39 ISO 9001:2008 Certified QMS Turbo Engine Pressure Sections and Pressure Variations As altitude increases air pressure decreases from atmospheric at sea level. Therefore inlet pressure is being measured from a vacuum pressure, as is outlet thrust pressure. The ratio between the two pressure sources of Pt2 and Pt7 is the Engine Pressure Ratio, and is a measure of the absolute pressure within the exhaust section Pt7 of the engine. Absolute Pressure is that pressure which is measured from a zero or a vacuum. The E.P.R. gauge is an electrically operated instrument which obtains its signal from an A.C. transmitter unit. The transmitter itself contains two (2) opposing pressure-fed bellows as well as Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 40 ISO 9001:2008 Certified QMS an A.C. synchro transmitter. The ratio of the air pressures between Pt2 and Pt7 cause movement of the 2 bellows, this movement being transferred to a mechanical shaft output. The shaft output turns the rotor of the transmitter synchro which causes a distortion of the magnetic field within the transmitter. The stator windings of the transmitter have an emf induced into them by the changing rotor field, and current flows to the stator windings of the receiver synchro within the E.P.R. gauge. The rotor of the receiver synchro is connected to the pointer of the gauge and is free to turn. The rotor will turn to align itself with the stator field of the receiver synchro and thus a different E.P.R. value will be recorded on the gauge. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 41 ISO 9001:2008 Certified QMS The E.P.R. gauge is a differential pressure gauge which measures the Absolute pressure in terms of engine thrust. The gauge is usually calibrated from 1 to approx. 4 and has no units of measurement; it simply indicates the pressure ratio between input and output stages of the gas turbine engine. The setting knob allows a predetermined pressure ratio to be selected which is displayed on the instrument by a digital type counter and a cursor on the instrument scale which corresponds to the counter setting. The E.P.R. pointer displays the actual E.P.R. of the engine. WHY USE E.P.R. INDICATORS? The thrust of a centrifugal compressor jet engine, approximately proportional to the R.P.M., therefore the tachometer and gas temperature indications can be used to determine the engine thrust. The thrust of an axial compressor jet engine does not vary in direct proportion to the engine speed. For a given R.P.M. the temperature on a hot day will give a lower thrust than on a cold day. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 42 ISO 9001:2008 Certified QMS 1) An increase in temperature leads to 2) a decrease in density which means 3) less air per cubic foot and therefore 4) a decrease in thrust The thrust developed can therefore be more accurately determined by measuring the ratio between the intake and discharge gas pressures. Thrust-meters The Percentage Thrust Indicator These types of engine monitoring instruments are sensitive differential pressure gauges which display the percentage thrust output of a turbojet engine in terms of absolute pressure. The thrust output is displayed over a range of 50 to 100% and the indicator can comprise of either a pressure capsule and an aneroid capsule, or one pressure capsule only and a mechanical compensation facility for varying atmospheric conditions. Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 43 ISO 9001:2008 Certified QMS Gauges fitted with the aneroid capsule are automatically compensated for altitude and thus atmospheric pressure changes, while those with mechanical compensation are fitted with a setting knob on the front of the instrument. Movement of the knob adjusts the complete assembly within the instrument, plus a digital counter and the instrument pointer. The three (3) digit counter displays the appropriate atmospheric condition obtained from the performance curve relating to the specific aircraft/engine combination. With compensation applied by the setting knob the instrument will indicate 100% thrust as a minimum take off value (under those conditions which are least favourable to the performance of the engine). Under more ideal conditions, the engine performance will result in an instrument reading in excess of 100% thrust. Testing At specified check periods, or whenever indications are suspect the following should be carried out: 1. Application of recommended air pressure to the system and checking that i.a.w. the relevant aircraft Maintenance Manual, the readings obtained are within the prescribed tolerances. 2. Check of water drain taps. 3. Disconnection of pressure and static lines for pressure testing of pipelines. 4. Inspection and cleanliness of any filters fitted. (Cleaning agent INHIBISOL). 5. Under static and dynamic conditions (engines running) gauges should be checked for correct readings as given in the Maintenance Manual. Power Loss Indicator This is another type of pressure sensing indicator which is designed to measure the exhaust pressure of a turbojet engine against prevailing static pressure. The gauge is a sensitive differential pressure gauge which measures absolute pressure and is thus calibrated in inches of mercury. Two capsules within the instrument respond to exhaust unit pressure and static pressure. The static pressure acts on both capsules one of which is evacuated and sealed. This capsule effectively acts as a height/pressure correction device. The exhaust unit pressure enters the pressure capsule, while static pressure is admitted to the case of the instrument. The displacement of the two capsules is therefore a measure of differential pressure which is equal to exhaust - static pressure. Mechanical coupling conveys the resultant Issue 1, Revision: 1 Advanced Instruments January 3, 2011  Art Williams and Harry Wendt Aeronautical Engineering School 44 ISO 9001:2008 Certified QMS capsule movements to the indicator pointer thereby displaying total exhaust unit pressure (= exhaust pressure - static pressure + static pressure). Testing As per percentage thrust indicators. Issue 1, Revision: 1 Advanced Instruments January 3, 2011