Wednesday, April 3, 2019
Calibrating a pressure gauge using an air-operated dead-weight pressure gauge tester for air gauges
Calibrating a drive opine using an send out-operated dead- slant coerce forecast try outer for production line gaugesINTRODUCTIONCalibrationis the set of operations that tack the relationship among the measure outs of quantities indicated by a measuring creature and the synonymic values realized by standards. The result of a normalization consent tos for the determination of corrections to be made with regards to the indicated values. It may also overhaul in determining other metrological properties much(pre nominative) as the forcefulness of influence quantities.The results of a calibration be usually documented and referred to as calibration corroboration or a calibration report. Necessary adjustments argon made to the instrument by and by calibration so that it always indicates readings agree to given values of the measure measured.When the instrument is made to give a null indication corresponding to a null value of the quantity to be measured, the set of oper ation is called zero adjustment .The Calibration workThe first thing to consider in calibrating an instrument is its design. In frame to be able to calibrate an instrument, the design of the instrument has to be adapted of bills that argon withinengineering tolerance when utilize within certain conditions and over a h unmatchedst period of time. The criteria used for assigning tolerance values substitute according to regions and according to type of industry. Manufacturers of instruments assign a general measurement tolerance and suggest the calibration interval as well as the optimum environment for use and storage of the instrument. The user of the instrument on the other hand assigns the actual calibration interval, on the instruments likely impost level. For example, if a manufacturer states that an instrument needs to be calibrated after usage for 8-12 hours of use 5 days per week is six months, that same instrument in 24/7 usage would generally rifle a shorter interv al. The assignment of calibration intervals evict be a testis process based on the results of previous calibrations.Calibration process versus bellGenerally, the process of calibrating an instrument is a difficult and big-ticket(prenominal) whiz. As a rule of thumb, the cost for ordinary equipment support is generally about 10% of the purchasing cost of the instrument on a yearly basis. foreign devices such asscanning electron microscopes,gas chromatographsystems andlaserinterferometerdevices can be even more expensive to calibrate.When the instruments being calibrated argon integrated with computers, the integrated computer programs and both calibration corrections are also under control.The calibration enigmaSuccessful calibration has to be consistent and systematic. At the same time, the complexity of some instruments requires that only key functions be identified and calibrated. Under those conditions, a degree of randomness is needed to find unexpected deficiencies. Eve n the just about routine calibration requires a ordainingness to investigate any unexpected observation.Theoretically, anyone who can read and follow the directions of a calibration procedure can bring to pass the work. It is recognizing and dealing with the exceptions that is the most challenging aspect of the work. This is where experience and judgement are called for and where most of the resources are consumed.THEORYPrinciples of Operation Of perfectly Weight Testers closet is defined as force per building block of measurement of measurement discipline i.e. P=F/AF=M x g (product of mass and the speedup acting on that mass)This simple principle is used by Dead weight printing press sensation testers to generate a very s put off and correct insistency. A series of weights are stiff on to a plumbers helper unit which is housed inside a diver chamber. In principle, the components of the above equations are as followsA is the effective cross-sectional area of the pl umbers helper unitM is the mass of the weights loaded on top of the plumbers helper unit in addition to the mass of the plunger unit itself.g is the gravitational acceleration acting on the plumbers helper and mass set.For example, if a piston of area A = in2 (0.18cm2) weighing M = 12.5lb (5.67kg) is supported by a liquified in a cylinder, the pressure in the fluid is 12.5lb=100lb/in2 (7kg/cm2). The piston- cylinder and the weights are called a dead-weight balance.The effective area of the piston and cylinder unit is an propinquity of the average of the areas of the piston and of the cylinder. The performance of a tester depends largely on the accuracy with which the piston and cylinder are manufactured. These should be straight and round and have a good finish. They are usually made from pugnacious and stabilised tool marques, however, on the air operated type, high-pitched chromium brand name is used to prevent corrosion. They are protected from high pressure such that th e piston would not leave its cylinder and if the weights are supplied without air- pressure, the piston give not be in compression. The accuracy can be illustrated by stating that a variation of 0.1m on the effective diameter of a piston/cylinder unit would result in an area carriage of 63ppm.The area of the piston-cylinder units are compared with NPL Standards. Two units can be compared by connecting them hydraulicly (or on gas) under pressure when they were in balance, the area vocalise AD to be determined was found from the known area of an NPL unit say Ak, cover the weights apply to each say WD and Wk from the equation. When instrument accuracies are calculated, salary is made for the fact that effective area of the piston/cylinder unit increases with pressure. These is negligible on low pressure testers but becomes significant on testers such as type 380D (600 barrier) and 380H (1200 bar). For the 4000 bar type the weights for equal increments of pressure are greater as the pressure increases up to 4000 bar and weight must be employ in the correct sequence. The accuracy certificate of a tester takes into account the buoyancy of the piston immersed in liquid.When examination gauges on liquid it may also be necessary to allow for liquid head (1 cm corresponds to 1 mb). The datum levels of the hydraulic piston/cylinder units are marked with a groove on the outermost diameter of the unit. The effect of heads could normally be ignored on air testers. The certificate also gives details of the corrections to be made for transmute in temperature of the unit from 20oC delinquent to expansion of the piston/cylinder unit and also of corrections due to g varying from standard gravity. The hydraulic testers can have accuracies of 0.01% on 1/16 in2 piston/cylinder unit, 0.015% on 1/80 in2 and 0.02% on 1/160 in2 units. apparatus1. A pressure gauge that could measure up to 100 lb/in2bar2. A Budenberg an air-operated pressure gauge calibrator Made with levellin g screws at its base which is used to mount it on a bench, a 0.5 square inch piston-cylinder unit, twain control valves, one 0.5 inch B.S.P gauge connection, some weights (each marked with corresponding pressures they exert). The apparatus can basically be divided into three elements The piston and cylinder units The weights The testers.The Piston-Cylinder unitThe effective area of the piston and cylinder unit is an approximation of the average of the areas of the piston and of the cylinder and is 0.5 in2. The weight exerted by the unit is 0.1 kg/cm2 or 0.1bar. The Piston-Cylinder unit is made from high chromium steel is used to prevent corrosion. It is also fitted with mechanical stops to prevent the piston leaving the cylinder housing if the applied pressure is excessive, and if the weights are supplied without air- pressure, the piston give not be in compression. There is a minor(ip) gap between the piston and the cylinder so that when the piston rotates in the cylinder the p ressure medium forms a bearing eliminating friction and aluminiferous contact any viscous forces are circumferential and so do not act in a vertical direction and so do not affect the accuracy of the balance.If the gap between the piston and the cylinder is too small, the piston will not rotate freely at low pressure long enough for a certain balanced pressure to be attained. If the gap is too large, there will be a leakage between the two and the piston will fall in the cylinder. The piston will spin for a reasonable length of time at low pressure and will confront in its floating position for several minutes at high pressures. The WeightsThe weights used are DH-Budenberg, manufactured from series 300 austenitic untainted steel, which makes them highly resistant to corrosion and magnetic permeability. They are marked with the nominal pressure value that they will generate (in bar) when used with the piston-cylinder unit they are designed for. These weights have been manufacture d to specific set of tolerances and according to guinea pig (NPL) standards to give an accuracy of 0.015% under all nominal conditions. They give the remove force when subjected to a gravitational acceleration of 9.80665m/s2(International Standard, g) and in an air of stringency 1.2 kg/m3. The TesterThis is the last element of the dead-weight tester. This unit is generally called the pneumatic dead weight tester base. It is the unit that generates the pressure which is thus applied to the piston-cylinder unit and the instrument under calibration. It is supplied with an incoming port where a cull dry non-corrosive source of gas is connected. The type 240 air-operated tester has two valves one valve to admit air from a H.P. supply to raise the pressure and one to spare air to the atmosphere.PROCEDURE1. The gauge to be calibrated was right on cleaned to remove any dirt or chemical contamination that could infect the tester. Using a bonded seal at the joint the Gauge was screwed on to the calibration equipment. 2. Using the conversion table given, (see table 2) the weight unavoidable in bar to test a pressure indicated by the gauge (the one being calibrated) was checked and the dead-weight piston was loaded with weight equivalent weight to the desired pressure less the pressure of the Piston-Cylinder unit. For example, when it was unavoidable to test the 10 lb/in2 reading on the gauge scale, the amount of weight infallible was 0.69 bar (from the table). But the piston already weighed 0.1 bar so this was subtracted from 0.69 bar to get 0.59 bar. So only 0.59 bar of weight equivalent was loaded onto the piston. 3. Next, the left-hand valve which releases pressure from the tester was closed.4. Then, to test for rising pressure, the right-hand valve which admits pressure to the tester was opened carefully. This admitted pressure into the tester and the rate of pressure nobble was watched on the gauge under test. As the pressure approached the desired value to be tested, the weights were spun carefully, and as soon as the piston began to float half(a) way between the two stops, the reading of the pressure gauge was taken. The release valve was opened and the admitting valve was closed.5. Next, to test for falling pressure, the release valve was closed and the admitting valve was opened. As the pressure rose beyond the desired pressure, the admitting valve closed and the release valve was opened slow to enable the pressure drop in the tester. As the pressure approached the required pressure, the weight was spun carefully and ss soon the piton began to float half way between the two stops, the reading on the gauge was taken. All the pressure was then released.6. A new set of weights were loaded on the piston to test the next pressure reading. These steps were carried out for pressure readings of from 10 lb/in2 to 100 lb/in2 at intervals of 10 lb/in2. The readings obtained were tabulated in table 2.RESULTSThe results obtained were tabu lated as in belowPressure being tested (lbf/in2) apply LoadMinus 0.1 (bar)Actual reading Up pressure (lbf/in2) mound Pressure (lbf/in2)100.6910.59.50201.3819.5019.00302.0729.5029.00402.7639.0039.00503.4549.5049.00604.1459.5059.00704.8369.0069.00805.5279.0078.50906.2189.0088.501006.999.0099.50 Table 1 showing readings from calibration exercise. CONCLUSIONThe calibration of the pressure gauge using a dead weight tester was carried outBased on the data-based results obtained a deviation in the calibrated reading was compared to the theoretical values. thus the pressure gauge on the masswards pressure was observed to be not appropriate for very low pressure levels Especially when the supplied air pressure is low incapable of lifting the applied load this can be express mathematically as Psa = W/Pd were W = Psa x Pd W = weight/loadPd = downwards pressurePsa = supplied air pressureTherefore applied load/weight is directly proportional to the obtainable pressure gauge calibration meter readings.Sources of Errors Possible air leakage from the valves. Error due to parallax when reading the half way level mark. Possible pushing down on the piston while spinning the weight. Possible loss of pressure in the piston hydraulics.REFERENCE1. N. E. Connor, Gas Quality Measuring Devices on Gas Measurement University of Salford, 19692. DH- Budenberg, An-Introduction-to-Dead-Weight-Testers http//www.scribd.com/doc/18933664 (25th Nov, 2009)3. Wikipedia Encyclopaedia (www.wikipedia.com)4. Practical Manual on pressure gauge calibration, 2009.