Pressure Measurement and Calibration

52 atmospherical push MEASUREMENT AND normalisation (TH2) 53 EQUIPMENT DIAGRAMS 54 55 56 EQUIPMENT comment Refer to the fathering on pages 56, 57 and 58. This equipment is a bench devolve unit purported to introduce students to wedge, coerce dentures and common devices available to stride wardrobe. The equipment comprises a Dead- bung squash Calibrator to generate a number of pre dumbfound embraces, machine-accessible to a droning tidal bore and electronic compress detector to digest their characteristics, including accuracy and officearity, to be determoed.The Dead-weight aura power tweet Calibrator, dawdler aegir and insisting demodulator ar mounted on a common PVC basis plate. The galvanizing console is free standing. The Dead-weight cart Calibrator consists of clearcutness ground diver (10) and matching plunger chamber (11) with a bunch of weights (12). In normal hire the appropriate combination of weights is employ to the pass by of the plumbers helper, to generate the unavoidable predetermined twitch, and and so the diver is set spinning, to overturn vertical friction, fleck the interlingual renditions from the amount stick devices ar recorded.The operating vomit of the Dead-weight cart Calibrator and instrumentation is 20 kNm-2 to two hundred kNm-2. The drone pipe pass judgment (5) and insistency detector (6) atomic number 18 mounted on a obscure paper banish (2) with a safety f office watercraft (4) to deem the hydraulic runny which is chosen to be urine for safety and readiness of use. A underoceanl valve (7) between the reservoir and the manifold terminate exits the cylinder, manifold block and bore-hole on test to be easily ready with the irrigate ready for use. A damping valve (8) between the cylinder and the manifold block allow the flow f weewee to be restricted to depict the application of damping. An additional isolating valve (9) on the manifold block allows water t o be drained from the manifold block or allows alternative devices to be connected for calibration. Such devices endure be well-tried everyplace the clutch 20 kNm-2 to cc kNm-2. The bourdon gauge (5) supplied is a traditional industrial instrument with rotary scale and mechanical exponent. The gauge has a 6 diameter dial that incorporates an arbitrary scale gradatory in items of revolution (independent of unit diffuse subscriber line pressing) in addition to the usual scale graduate in units of kNm-2.A clear acrylic front face allows observation of the Bourdon tube the mechanism that converts motion of the Bourdon tube to rotation of the indicator chivvy. The electronic drag demodulator (6) supplied incorporates a semi-conductor arrest that deflects when thrust is apply by the working facile. This deflection generates a electric potential end product that is comparative to the utilise constrict. The squash demodulator should be connected to the socket (2 0) marked Pressure Sensor on the front of the console.The military unit supply, signal t separately circuitry etc atomic number 18 contained in a simple electrical console (15) with appropriate electric current protection devices and an RCD (26) for operator protection. The electrical console is designed to stand on idea the Dead-weight Pressure Calibrator on the bench c overing. All circuits inside the console atomic number 18 operated by a main on/off replenishment (16) on the front of the console. 57 The various circuits inside the console are protected against excessive current by miniature circuit breakers, as follows CONT (27) O/P (28) This breaker protects the power supply and circuits inside the console.This breaker protects the electrical output marked OUTPUT (23) at the rear of the console. The socket is use to power the IFD3 interface used for data logging. The voltage from the mash detector is displayed on a digital meter (17) on the electrical console. An ad ditional conditioning circuit incorporates zero and span putments and allows the voltage output from the tweet demodulator to be reborn and displayed as a direct schooling squeeze meter calibrated in units of twitch. The zero oblige (21) and span go through (22) are mounted on the front of the console for ease of use.A chooser switch (18) allows the voltage from the sensor or the direct course session compact edition to be displayed as required. The voltage from the insisting sensor is simultaneously connected to an I/O Port (19) for the connection to a PC using an optional interface device (TH-IFD) with educational bundle package (TH2-303). Alternatively, the signal tummy be connected to a substance abuser supplied chart rec coif if required. Before use, the safety fuse vas must(prenominal) be filled with clean water (preferably deionized or demineralised water) and the calibrator, Bourdon gauge and ram sensor fully primed. 8 OPERATIONAL PROCEDURES This equipme nt has been designed to operate over a range of twinges from 0 kN/m2 to two hundred kN/m2 whitethorn damage the pressure sensors. In order to avoid such(prenominal) damage, DO NOT crack up CONTINUOUS printing press TO THE TOP OF THE PISTON rod WHEN THE underseal VALVE IS unappealing except by the application of the quite a little supplied. An whimsy whitethorn be utilize to the plumbers helper when operating at a peregrine pressure of little than 200 kN/m2. This surgical procedure is expound in Experiment P1.The hobby procedure should be followed to prime the Dead-weight Calibrator and pressure sensors, prior to winning interprets take the apparatus using the adjustable feet. A circular disposition aim has been provided for this purpose, mounted on the base of the dead-weight calibrator. break a agency that the drain valve (at the vertebral column of the Bourdon gauge base) is smashedd. Fill the dry land vessel with water (purified or de-ionized water is p referable). Open the damping valve and the reason valve. With no napes on the diver, slowly draw the piston upward a distance of approximately 6 cm (i. . a full stroke of the piston). This draws water from the fusee drive vessel into the remains. steadfastly drive the piston roundwards, to expel air from the cylinder spikelet towards the primer coat coat vessel. copy these two steps until no to a greater extent bubbles are discernible in the organization. It may be subservient to assailable fire the primal segment of the return tube between the manifold block and the priming vessel. This depart help to prevent air universe draw endure into the organisation as the piston is raised. go on the piston close to the top of the cylinder, pickings billing not to altitude it high replete to allow ir to enter, and and thusly close the priming valve. The pursuance procedure describes the calibration of the semiconductor unit device device device pressure sensor . The procedure differs if using the optional TH-303 software, in which case users should kinda refer to the Help Text provided with the software. Remove the piston from the cylinder, and switch the chooser knob on the console to Pressure. This the zero swear on the console until the display reads zero. This sets the first extension phone show for the sensor calibration. dedicate the piston to the cylinder, and reprime the system as described above.Place all the supplied atomic reactores onto the piston, with the great band (2 ? kg) being added last. This corresponds to an utilise pressure of 200 kN/m2. whirlpool the piston, and adjust the span control until the sensor output matches the apply pressure. This sets the help honorable mention point for the calibration. 59 The calibration may be tested by applying a big bucks to the piston, spinning the piston in the cylinder, and then comparing the employ pressure to the sensor output. Each ? kg of utilize circle corr esponds to 20 kN/m2 of use pressure. This piston itself gives an use pressure of 20 kN/m2. 0 NOMENCLATURE FOR TH2 The following nomenclature has been used for the theory and calculations presented in this manual Name Piston diameter Cross-sectional welkin atomic pile of piston Mass on mass piston utilize mass Acceleration cod to gravity Applied tug Nom d A Mp Mm Ma g F Units m m? kg kg kg m/s2 kg Type wedded Calculated Given put down Calculated Given designateed explanation The diameter of the dead weight calibrator piston. Cross-sectional area of dead weight calibrator cylinder. Mass of the dead-weight calibrator piston. Mass applied to piston. Ma = Mp + Mm g = 9. 1 m/s2 Force applied to wandering in system by piston and masses. F = g x Ma Pressure applied to tranquil by dead weight calibrator P = F/A Ambient (atmospheric) pressure of the surroundings. Applied pressure relative to the pressure of arrive vacuum Needle angle taken from Bourdon gauge scale Semiconductor ou tput taken from console display aegir pressure taken from Bourdon gauge scale Calibrated semiconductor output taken from console display Applied pressure barometrical pressure autocratic pressure Needle angle Semi-conductor output Indicated Bourdon gauge pressure Indicated semi-conductor pressurePa Patm Pabs ? e Pb Ps N/m2 N/m2 N/m2 Calculated Recorded Calculated degree Recorded V N/m2 N/m2 Recorded Recorded Recorded 61 NOMENCLATURE FOR ERROR ANALYSIS The following nomenclature has been used for the shift abridgment presented in this manual Name Indicated appraise answerive valuate Range Definition Gauge reading, i. e. the pressure suggestd by sensor used True pressure, pressure applied by dead-weight calibrator Total range of value covered in the results, or total range of values measurable on instrument scale.Calculation Pi = Pb or Ps, depending on the sensor used unquestionable value = Applied pressure, Pa Range = Largest result Smallest result = Pi max Pi min or Rang e = Maximum mathematical reading Minimum possible reading (200 kN/m? for apparatus used) No calculation. Precise data fit in a small scatter, indicating minimal random fault ea = Pi Pa ea max = ? (Pi Pa)max? e%a = ea max X 100 Pa e%f = ea max X 100 Range Pmin = P1 + P2 + .. + Pn n da = Pi Pmin dm = da1 + da2 + + dan n ? = da12 + da32 + + dan2 n-1 ? PrecisionHow closely the results barrack with each other. Actual deflexion Modulus of the remainder between indicated value and positive value verity Maximum difference between indicated pressure and developed pressure Percentage accuracy Greatest difference between of literal scale reading indicated pressure and unfeigned pressure, as a role of the essential pressure. Percentage accuracy Greatest difference between of complete reading indicated pressure and real pressure, as a percentage of the range. pissed Sum of results divided by number of results.Absolute excursion contrast between a private result and th e imagine of several results imagine deviation Sum of the irresponsible deviations divided by the number of implicit deviations Standard deviation Comm hardly used value in analysis of statistical data 62 DATA tab 7 recounting AND ABSOLUTE PRESSURE The measurement of both physical property relies upon simile with some fixed name point. Pressure is one such property, and pressure measurement must begin by defining a adequate fixed point. An obvious lineament point is that of the ambient pressure of the surroundings.Pressure scales pass on been based around a zero point of the pressure of the atmosphere at sea train. Pressures lower than atmospheric are delegate negative values pressures higher than atmospheric have positive values. Gauges for measurement pressure give readings relative to this zero point, by comparing the pressure of interest to the pressure of the surrounding air. Pressure measured with such a gauge is given relative to a fixed value, and is sometimes termed gauge pressure. Gauge measure pressure difference between the pressure to be measured and the barometric (ambient) pressure.This may then need adjusting, to take into chronicle any difference between barometric pressure and the pressure at sea level. Many calculations using equations derived from fundamental physical laws require absolute pressure values. Absolute pressure is the pressure relative to a total absence of pressure (i. e. a total vacuum). On an absolute pressure scale, all pressures have a positive value. The following chart illustrates the difference between gauge pressure, barometric pressure, and absolute pressure. 63DATA SHEET 8 TECHNICAL DATA The following in ground levelation may be of use when using this apparatus Operating range of dead-weight pressure calibrator diameter of dead-weight calibrator piston Cross-sectional calibrator area of dead-weight 20 kN/m2 200 kN/m2 0. 017655 m 0. 000245 m2 20 kN/m2 150 mL Pressure produced in cylinder by mass of pis ton with no applied masses Approximate capacity of priming vessel 64 EXPERIMENT P1 CONCEPTS OF PRESSURE AND PRESSURE SENSOR doings OBJECTIVE To gain a basic understanding of the concept of pressure and its measurement.To examine the behavior of two kinds of pressure sensor, and the effect of damping on pressure measurement. To gain a basic understanding of the concept of pressure and its measurement. To go over the behaviour of two kinds of pressure sensor To observe the effect of damping on pressure measurement METHOD To look into the receipt of two kinds of pressure sensor to a pressure applied by a dead-weight calibrator device. To investigate the response of these sensors to the application of a sudden pressure spike, with varying levels of obstacle of the liquid between the pressure application and the sensor.THEORY Pressure is the crusade exerted by a medium, such as a fluid, on an area. In the TH2 apparatus, pressure is exerted by a piston on a column of water. The pressure applied is then equal to the force exerted by the piston over the cross-sectional area of the fluid. The use of the piston and masses with the cylinder generates a measurable reference pressure, Pa Pa = Fa A 65 where Fa = gMa, and Fa = force applied to the liquid, Ma = total mass (incl. piston), and A = area of piston. The area of the piston can be expressed in footing of its diameter, d, as A = ? d2 4The units of each variable must agree for the equations to be valid. Using SI units, Pa will be in Newtons per square metre (N/m? , also k like a shotn as Pascals) if Fa is in Newtons, A is in square metres, and d is in metres. The use of detail units of pressure will be covered in exercising B. For this exercise the area of the cylinder is a constant. The pressure can therefore be considered directly proportional to the mass applied to the mass on the piston Pressure measurement is normally concerned with measurement the effects of a pressure differential between two poi nts in a fluid.The simplest form of pressure sensor is a manometer tube, in which a tube of fluid is exposed at one end to the first point in the fluid, and at the other to the back point. each pressure differential causes a displacement of fluid within the tube, which is proportional to the difference. Manometers (not include with the TH2 apparatus) are cheap, simple, and can be designed to cover a wide range of pressures. However, they are best used for measuring static pressures below about 600 kN/m? , as the required top side of the fluid becomes unworkable at greater pressures.Their dynamic response is poor, so they are best suited to measuring static or slowly changing pressures. Some fluids used are toxic (such as atomic number 80), and may be susceptible to temperature change. The Bourdon-type pressure gauge consists of a curved tube of oval cross-section. One end is closed, and is left hand free to move. The other end is left open to allow fluid to enter, and is fixed. The outside of the tube remains at ambient pressure. When fluid pressure inside the tube exceeds the pressure outside the tube, the section of the tube tends to 66 ecome circular, causing the tube to straighten (internal pressure lower than the ambient pressure conversely causes increased flattening, and the curve of the tube increases). A simple mechanical linkage transmits the movement of the free end of the tube to a arrow moving around dial. This type of gauge is one of the two kinds included in the TH2 apparatus. The second type of pressure gauge included as part of the TH2 is an electromechanical device. In a basic semiconductor pressure sensor, silicon strain gauges are fixed to one side of a plosive consonant.The two sides of the diaphragm are exposed to the two different pressures. Any pressure differential causes the diaphragm to expand towards the lower-pressure side, producing a change in the strain gauge voltage reading. The electronic semiconductor pressure sensor included with the TH2 is a more refined device with improved reliableness and sensitivity for pressure measurement. It includes temperature compensation to wither the effects of temperature variation on the results. The strain gauges used are formed by laying down a protective film of glass onto stainless leaf blade, followed by a thin film of silicon.The silicon is doped to produce semiconductor properties, and a mask is photoprinted onto it. The unmasked silicon is then removed, leaving a pattern of silicon semiconductor strain gauges molecularly bonded onto the surface of the steel. The gauges are connected to an Ohmmeter through a Wheatstone bridge, to amplify the signal produced. 67 In this type of sensor, a diaphragm is stock-still used, yet instead of fixing the strain gauges to the surface, the deflection of the diaphragm moves a steel force rod. This transfers the force to one end of the steel strip that the semiconductor resistors are bonded to.The resulting deflection of the strip causes compression in some strain gauges, and tension in others, changing their resistance and producing a measurable output. Both the TH2 pressure sensors are set up to indicate the pressure differential between atmospheric pressure, and fluid pressurized with the use of the dead-weight calibrator. The fluid passes through a damping valve, positioned between the calibrator and the sensors. By partially closing the valve, fluid flow can be restricted. This affects the speed at which pressure is transferred from the point of application to the sensors.EQUIPMENT aim UP aim the apparatus using the adjustable feet. A circular pump level has been provided for this purpose, mounted on the base of the dead-weight calibrator. conceal that the drain valve (at the back of the Bourdon gauge base) is closed. Fill the priming vessel with water (purified or de-ionized water is preferable). Fully open the damping valve and the priming valve With no masses on the piston, slowly dr aw the piston upwards a distance of approximately 6cm (i. e. a full stroke of the piston). This draws water from the priming vessel into the system.Firmly drive the piston downwards, to expel air from the cylinder back towards the priming vessel. Repeat these two steps until no more bubbles are visible in the system. It may be helpful to raise the central section of the return tube between the manifold block and 68 the priming vessel. This will help to prevent air being drawn back into the system as the piston is raised. Raise the piston close to the top of the cylinder, victorious care not to lift it high enough to allow air to enter, and then close the priming valve.PROCEDURE This equipment has been designed to operate over a range of pressure from 0 kN/m2 to 200 kN/m2. Exceeding a pressure of 200 kN/m2 may damage the pressure sensors. In order to avoid such damage, DO NOT habituate CONTINUOUS PRESSURE TO THE TOP OF THE PISTON ROD WHEN THE PRIMING VALVE IS CLOSED except by appli cation of the mass supplied. An impulse may be applied to the piston when operating at a fluid pressure of less than 200 kN/m2, as is described posterior in this procedure. Behavior of pressure sensors lurch the piston in the cylinder, to downplay friction effects between the piston and the cylinder wall.While the piston is spinning, record the angle through which the Bourdon gauge needle has moved, and the voltage output of the electronic sensor. Apply a ? kg mass to the piston. wrench the piston and take a second set of readings for the Bourdon gauge needle angle and the electronic sensor. Repeat the procedure in ? kg increments. When using several masses, it will be necessary to place the 2 ? kg mass on top of the other masses. Repeat the procedure magical spell removing the masses again, in ? kg increments. This gives two results for each applied mass, which may be averaged in order to reduce the effects of any error in an individual reading.Effect of damping Apply a whizz mass to the piston, and spin it. While the piston is spinning, apply an impulse to the top of the piston by striking the top of the rod once, with the flat of the hand. tarry the behavior of the Bourdon gauge needle. Note the final sensor reading after the response settles. Slightly close the damping valve. Change the mass, spin the piston again, and apply an impulse to the rod. Observe any changes in the sensor responses. Repeat the procedure, closing the damping valve a little at a time and noting the response and the final sensor reading each time.RESULTS tabularize your results under the following headings- 69 Mass applied to calibrator Mm (kg) Deflection of Bourdon gauge needle (degrees) Output from electrochemical pressure sensor (mV) Notes on sensor behavior (damping) Plot a graph of sensor response against applied mass for each sensor. 70 EXPERIMENT P2 CONCEPTS OF PRESSURE MEASUREMENT AND CALIBRATION OBJECTIVE To convert an arbitrary scale of pressure sensor output into design units. To calibrate a semiconductor pressure sensor. METHOD To puzzle use of a dead-weight calibrator in order to produce known forces in a fluid.THEORY It is recommended that students read Data Sheet 1 sexual relation and Absolute Pressures before proceeding with this exercise. Pressure sensor calibration reading in a pressure sensor reading may be calibrated, using known pressures, to give a gauge reading in engineering units. From exercise A, the dead-weight calibrator used in the TH2 produces a known reference pressure by applying a mass to a column of fluid. The pressure produced is Pa = F Aa where Fa = gMa, and Fa is the force applied to the liquid in the calibrator cylinder.Ma is the total mass (including that of the piston) 71 g is the acceleration due to gravity, and A is the area of piston. The area of the piston can be expressed in terms of its diameter, d, as A = ? d2 4 The pressure in the fluid may then be calculated in the relevant engineering units. These kno wn pressures may then be compared to the pressure sensor outputs over a range of pressures. The relationship between sensor output and pressure may be turned into a direct scale, as on the Bourdon gauge scale. Alternatively, a reference graph may be produced.Where the relationship is linear and the sensor output is electrical, the sensor may be calibrated using simple amplifier (a conditioning circuit). When using SI units, the units of pressure are Newtons per square meter (N/m? , also known as Pascals). To calculate the pressure in N/m? , M must be in kg, d in m, and g in m / s?. For the pressure range covered in this exercise, it will be more convenient to use units of kN/m? , where 1 kN/m? = megabyte N/m? (1 N/m? = 0. 001 kN/m? ). Barometric pressure pressure units and scale conversion Barometric pressures is usually measured in bar.One bar is equal to a force of 105 N applied over an area of 1m?. While bar and N/m? have the same scale interval, pressure in bar ofttimes has a m ore convenient value when measuring barometric pressure. Pressure may also be measured in millimetres of mercury (mmHg). The pressure is given in terms of the height of a column of mercury that would be required to exert an equivalent pressure to that being measured. some other(prenominal) possible unit of measurement is atmospheres (atm). One standardised atmosphere was in the beginning defined as being equal to the pressure at sea level at a temperature of 15C.A pressure unit still in everyday use is rams per square inch (psi or lbf / in.? ). One psi is equal to a weight of one pound applied over an area of 1 in.? If a barometer is available to measure the ambient pressure in the room where the equipment is located, the barometer reading should be born-again SI units. Pressures may be converted from one scale to another using a conversion factor. A list of conversion factors is provided below. 72 1 atm = = = = = = = = = = = = = = = = = = = = 101. 3 x 103 101. 3 1. 013 760 14. 696 100 x 103 100 0. 987 750. 006 14. 504 133. 3 x 103 133. 3 1. 33 1. 316 19. 337 6. 895 x 106 6. 895 x 103 68. 948 68. 046 51. 715 N/m2 kN/m2 bar mmHg psi N/m2 kN/m2 atm mmHg psi N/m2 kN/m2 bar atm psi N/m2 kN/m2 bar atm mmHg 1 bar 1 mmHg x 103 1 psi x 103 superfluous EQUIPMENT REQUIRED Values for the piston diameter and weight are provided. These may be replaced by your own measurements if desired. The following equipment will be required to do so a) Vernier callipers or a ruler, to measure the piston diameter b) A weigh-balance or similar, to measure the piston weight EQUIPMENT even up UP Carefully remove the piston from the cylinder, weigh it.Take care not to damage the piston, as it is part of a high precision instrument and any damage will affect the accuracy of the data-based results. Level the apparatus using the adjustable feet. A circular spirit level has been mounted on the base of the dead weight calibrator for this purpose. Check that the drain valve (at the back o f the Bourdon gauge base) is closed. Fill the priming vessel with water (purified or de-ionized water is preferable). Open the damping valve and the priming valve. 73 With no masses on the piston, slowly draw the piston upwards a distance of approximately 6cm (i. e. full stroke of the piston). This draws water from the priming vessel into the system. Firmly drive the piston downwards, to expel air from the cylinder back towards the priming vessel. Repeat these two steps until no more bubbles are visible in the system. It may be helpful to raise the central section of the return tube between the manifold block and the priming vessel. This will help to prevent air being drawn back into the system as the piston is raised. Raise the piston close to the top of the cylinder, taking care not to lift it high enough to allow air to enter, and then close the priming valve.Set the selector switch on the console to Output. PROCEDURE This equipment has been designed to operate over a range of p ressure from 0 kN/m2 to 200 kN/m2. Exceeding a pressure of 200 kN/m2 may damage the pressure sensors. In order to avoid such damage, DO NOT APPLY CONTINUOUS PRESSURE TO THE TOP OF THE PISTON ROD WHEN THE PRIMING VALVE IS CLOSED except by application of the mass supplied. Conversion of an arbitrary scale into engineering units crack the piston to reduce the effects of friction in the cylinder. With the needle still spinning, record the angle indicated by the Bourdon gauge needle.Place a ? kg mass on the piston, and spin the piston. Record the value of the applied mass, and the angle indicated by the Bourdon gauge needle. Increase the applied mass in increment of ? kg. Spin the piston and record the needle angle each increment. Repeat the measurements while decreasing the applied mass in steps of ? kg. This gives two readings for each applied mass, which may be averaged to reduce the effect of any error in an individual reading. Calculate the applied pressure at each mass increment. Calculate the average needle angle at each pressure increment.Repeat the experiment, this time arranging the applied mass and the indicated pressure on the Bourdon gauge scale. canvass this to the average needle angle recorded previously. 74 Calibration of a semiconductor pressure sensor NOTE This procedure differs if the TH2-303 software is being used. Please refer to the online product Help Text if using this software. Spin the piston. Record the voltage indicated on the semiconductor output display on the console. Place a ? kg mass on the piston, and spin the piston. Record the applied mass, and the voltage indicated on the semiconductor output display on the console.Increase the applied mass in steps of ? kg, spinning the piston and save the semiconductor output each time. Repeat the measurement while decreasing the applied mass in steps of ? kg. Calculate the applied pressure at each mass increment. Calculate the average sensor output at each pressure increment. Slowly open the priming valve. Open the valve to its maximum, and check that the damping valve is also fully open. The fluid in the system will now be at approximately atmospheric pressure (it will be slightly higher than atmospheric due to the height of fluid in the reservoir, but this is negligible compared to the range of the sensors). shifting the selector knob on the console to PRESSURE Turn the ZERO control on the console until the display read zero, to set the first reference point for the sensor calibration. Raise the piston close to the top of the cylinder, taking care not to lift it high enough to allow air to enter, and then close the priming valve. Place a large mass on the piston, and calculate the corresponding applied pressure. Spin the piston and adjust the baffle control until the sensor output matches the applied pressure, to set the second reference point for the calibration. Remove the masses from the piston.Take a set of readings from the calibrated semiconductor sensor, b y adding masses to the piston in ? kg increments. Repeat the reading while decreasing the applied mass. This gives two reading for each applied mass, which may be averaged in order to reduce the effect of any error in an individual reading. 75 RESULTS Tabulate your results under the following headings Barometric pressure Mass of piston Mp Diameter of cylinder, d Cross-sectional area of cylinder, A Mass on piston Mm (kg) Applied mass Ma (kg) Applied force Fa (N) Applied pressure . . .. ..Needle angle N/m2 kg m m2 Indicated Indicated SemiBourdon conductor semiconductor pressure pressure output Pb Ps Pa E ? (mV) (N/m2) (degrees) (N/m2) (N/m2) Plot graphs of average needle angle against applied pressure for the Bourdon gauge, and voltage output against applied pressure for the semiconductor sensor. Plot a graph of indicated pressure against unfeigned pressure for the Bourdon gauge and the calibrated semiconductor pressure sensor. If there is facility for measuring barometric pressure, it is possible to calculate the absolute pressure corresponding to each applied pressure increment.The ambient pressure of the surroundings, Patm should be measured, then converted into N/m2 (if required). An additional column should be added to the results table Absolute Pressure, Pabs (N/m2). Absolute pressure may then be calculated as Pabs = Pa + Patm 76 EXPERIMENT P3 ERRORS IN PRESSURE MEASUREMENT OBJECTIVE To investigate the sources of error when measuring pressure. METHOD Errors in measuring a quantity, such as pressure, can come from a number of sources. Some can be eliminated by careful choice of equipment and experimental method. Other errors are unavoidable, but can be minimized.In any experiment, it is good coiffure to note any possible sources of error in the results, and to give an extension of the magnitude of such errors. Errors fall into three general categories Avoidable errors These are errors that must be eliminated, as any results including such errors will lo ts be meaningless. Such errors include Incorrect use of equipment Incorrect recording of results Errors in calculations Chaotic errors, i. e. random disturbances, such as extreme shiver or electrical noise that are sufficient to mask the experimental results. 7 Random errors Random errors should be eliminated if possible, by changing the design of the experiment or waiting until conditions are more favorable. Even if they cannot be eliminated, many random errors may be minimized by do multiple sets of readings, and averaging the results. Random errors include Variation of experimental conditions (e. g. changes in ambient temperature) Variation in instrumentation performance Variation due to literal properties and design (e. g. effect of friction) Errors of judgement (e. g. nconstancy in estimating a sensor reading) taxonomic errors domineering errors produce a constant bias or skew in the results, and should be minimized where possible. They include Built-in errors (e . g. zero error, irrational scale graduation) Experimental errors (due to poor design of the experiment or the apparatus) Systematic human errors (e. g. reading from the wrong side of a liquid meniscus) commitment error (errors introduced as a result of the act of measurement- for example, the temperature of a analyse altering the temperature of the body being measured)Errors may also be described in a number of ways Actual difference the difference between the indicated value (the value indicated by the gauge or sensor) and the actual scale reading (the true value of the property being measured). The actual value must be known to calculate the actual difference. the true the maximum amount by which the results vary from the actual value. The actual value must be known. Percentage accuracy of the actual scale reading the greatest difference between the actual value and the indicated value, expressed as a percentage of the actual value.The actual value must be known. Percentag e accuracy of the full-scale reading (total range of the measurement device) the greatest difference between the actual value and the indicated value, expressed as a percentage of the maximum value of the range being used. The actual value must be known. Mean deviation (or probable error) The absolute deviation of a single result is the difference between a single result, and the average (mean) of several results. The mean deviation is the sum of the absolute deviations divided by their number. The actual value is not required.The mean deviation is an sign of how closely the results agree with each other. 78 Standard deviation (or mean square error) the standard deviation is the square root of the mean of the squares of the deviations ( break results are obtained by dividing the sum of the values by the one less than the number of values). This is a common measure of the preciseness of a attempt of data- how closely the results agree with each other. The actual value is not req uired. supernumerary EQUIUPMENT REQUIRED Values for the piston diameter and weight are provided. These may be replaced by your own measurements if desired.The following equipment will be required to do so Vernier callipers or a ruler, to measure the piston diameter A weigh-balance or similar, to measure the piston weight EQUIPMENT SET UP To prime the cylinder, the following procedure should be followed (where this is required in the experiment) Level the apparatus using the adjustable feet. A circular spirit level has been mounted on the base of the dead weight calibrator for this purpose. Check that the drain valve (at the back of the Bourdon gauge base) is closed. Fill the priming vessel with water (purified or de-ionized water is preferable).Fully open the damping valve and the priming valve. With no masses on the piston, slowly draw the piston upwards a distance of approximately 6cm (i. e. a full stroke of the piston). This draws water from the priming vessel into the system. Firmly drive the piston downwards, to expel air from the cylinder back towards the priming vessel. Repeat these two steps until no more bubbles are visible in the system. It may be helpful to raise the central section of the return tube between the manifold block and the priming vessel. This will help to prevent air being drawn back into the system as the piston is raised.Raise the piston close to the top of the cylinder, taking care not to lift it high enough to allow air to enter, then close the priming valve. PROCEDURE This equipment has been designed to operate over a range of pressure from 0 kN/m2 to 200 kN/m2. Exceeding a pressure of 200 kN/m2 may damage the pressure sensors. In order to avoid such damage, DO NOT APPLY CONTINUOUS PRESSURE TO THE 79 TOP OF THE PISTON ROD WHEN THE PRIMING VALVE IS CLOSED except by application of the mass supplied. The following experiments give suggested ways in which particular sources of error may be investigated.It is recommended that only o ne or two be attempted in a single laboratory session, with each being repeated several times, great(p) multiple samples for the error analysis. Basic Error Analysis The accuracy of the semiconductor calibration may be investigated by performing standard error calculations on the calibrated sensor output, using the results obtained in Experiment P2. If results are not available for analysis, the following procedure should be followed Slowly open the priming valve. Open the valve to its maximum, and check that the damping valve is also fully open.The fluid in the system will now be at approximately atmospheric pressure (it will be slightly higher than atmospheric due to the height of fluid in the reservoir, but this is negligible compared to the range of the sensors). Switch the selector knob on the console to PRESSURE. Turn the ZERO control on the console until the display read zero, to set the first reference point for the sensor calibration. Raise the piston close to the top of t he cylinder, taking care not to lift it high enough to allow air to enter, then close the priming valve. Place a large mass on the piston, and calculate the corresponding applied pressure.Spin the piston, and adjust the SPAN control until the sensor output matches the applied pressure, to set the second reference point for the calibration. Remove the masses from the piston. Take a set of readings from the calibrated semiconductor sensor, adding masses to the pan in ? kg increments, and again while decreasing the applied mass. This provides two set of readings for data analysis. The experiment should be repeated to provide further sets of data. Avoidable errors Incorrect use of equipment Level the apparatus using the adjustable feet.A circular spirit level has been mounted on the base of the dead-weight calibrator for this purpose Check that the drain valve (at the back of the Bourdon gauge base) is closed, and the damping valve is fully open. 80 Remove the piston from the cylinder, then fill the priming vessel with water (purified or de-ionized water is preferable). mop up the priming valve, then replace the piston in the cylinder. Take a set of readings without priming the system first. Random errors Friction effects primeval the system as described in the equipment set up instructions. vend the board at an angle of about 5 to 10 degrees. THE EQUIPMENT beginning MUST STILL BE FIRM AND SECURE. Titling the apparatus in this way will exaggerate any friction effects, as the force applied by the piston will no longer be performing straight downwards on the column of fluids, but will have components acting at right-angles to cylinder wall. Spin the piston. Take one reading while the piston is spinning, then observe the behavior of the needle. Continue to specify the needle as the piston moolah spinning, then make a note of the new gauge reading. Apply masses to the piston in ? kg increments.At each step, spin the piston, note the sensor output, and then take a second reading after the piston stops spinning. Systematic errors Zero error Calibrate the semiconductor pressure sensor, but do not include mass of piston in the applied mass when calculating the applied pressure. Take a set of readings from the calibrated semiconductor sensor over a range of applied masses, now including the piston mass in the applied mass calculation. Human error Take a set or readings from the Bourdon gauge pressure scale, but stand at an angle to the dial face when taking each reading. Keep the same viewing angle for each reading.This illustrates the effect of parallax on the readings taken. RESULTS Tabulate your results under the headings on the following page For each result, calculate the absolute difference, ea between indicated value Pi and the applied pressure Pa. 81 Find the maximum absolute difference, the accuracy ea max and use this value and the corresponding indicated pressure to calculate the % accuracy of actual scale reading and the % accuracy o f full-scale reading (use a range of 200 kN/m2). Correlate the data for several test runs, to give a set of indicated pressure readings corresponding to a single applied pressure.Use this check data table to calculate the mean of the results, Pmean, the mean deviation, dm, the absolute deviation, da, and the standard deviation, ?. Errors can also be illustrated graphically 85 Piston diameter, d = . m Piston mass, MP = .. kg Experimental conditions Mass Applied Applied Applied Indicated Mean Absolute Standard Actual accuracy % % Mean on deviation deviation deviation Accuracy Accuracy of mass force pressure pressure difference piston Actual Full result scale scale reading reading Mm dm da PI ea Emax e%a e%f Pmin Ma Fa Pa ? kg) (kg) (kN) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) (kN/m2) 86 Plot a graph of actual pressure against indicated pressure. On the same graph, plot a straight line showing the actual pressure. This will illustrate three characteristics of the results Deviatio n of sensor readings from the actual value. Whether any deviation from the true reading is systematic (the graph will be a straight line or a smooth curve) or random (the graph will have no obvious relationship). Precision of the results. Precise results will be close together, not widely scattered. Precise results may still deviate strongly from the actual value.