Calibrating a pressure gauge using an air-operated dead-weight pressure gauge tester for air gauges

Calibrating a pressure gauge using an air-operated dead-weight pressure gauge tester for air gauges INTRODUCTION: Calibrationis the set of operations that establish the relationship between the values of quantities indicated by a measuring instrument and the corresponding values realized by standards. The result of a calibration allows for the determination of corrections to be made with regards to the indicated values. It may also help in determining other metrological properties such as the effect of influence quantities. The results of a calibration are usually documented and referred to as calibration certificate or a calibration report. Necessary adjustments are made to the instrument after calibration so that it always indicates readings corresponding to given values of the quantity 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 operation is called zero adjustment . The Calibration Process The first thing to consider in calibrating an instrument is its design. In order to be able to cal ibrate an instrument, the design of the instrument has to be capable of measurements that are â€Å"withinengineering tolerance† when used within certain conditions and over a reasonable period of time. The criteria used for assigning tolerance values vary 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 instrument’s likely usage 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 get a shorter interval. The assignment of calibration intervals can be a formal process based on the results of previous calibrations. Calibration process versus cost G enerally, the process of calibrating an instrument is a difficult and expensive one. 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. Exotic devices such asscanning electron microscopes,gas chromatographsystems andlaserinterferometerdevices can be even more expensive to calibrate. When the instruments being calibrated are integrated with computers, the integrated computer programs and any calibration corrections are also under control. The calibration paradox Successful 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. Even the most routine calibration requires a willingness to investigate any unexpected observation. Theoretically, anyone who can read and follow the directions of a calibra tion procedure can perform 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. THEORY Principles of Operation Of Dead Weight Testers Pressure is defined as force per unit area i.e.