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Title: Optimizing Instrument Accuracy
Author: Philip J. Mckibbin
Source: American Gas Association 1985
Year Published: 1985
Abstract: To many gas industry measurement personnel, the word measurement accuracy is equated to meter accuracy. This concept of measurement accuracy applies logically to standard pressure and fixed factor measurement stations (0.25 psig and constant pressure respectively) but falls somewhat short of the mark when highly variable metering conditions are involved. Measuring gas at variable line pressure and temperature conditions may be accomplished by using meter-driven instruments whose function is to take an uncorrected output from the meter and convert the uncorrected volume to a direct read volume corrected to base pressure and/or temperature. In line pressure measurement stations, the meter acts as a prime mover to power the integrator (instrument) input shaft in proportion to actual flow through the meter. The integrating instrument, or Base Volume Index (BVl), concurrently corrects the existing pies sure and temperature conditions at the meter to standard conditions of base pressure and temperature (typically 14.73 psig and 60CF). The integrating mechanics utilized by instruments to correct line metered volumes to standard volumes is subject tO accuracy degradation if operated in harsh environments. This change in accuracy relative to time is typical for any finely tuned mechanical device and is of considerable concern to gas companies thai depend on BVl instrumentation to calculate billing volumes. Gas meters are inherently less prone to accuracy degradation over time, relative to integrating instruments, because of the heavy duty design of gas meters. Additionally, instruments are more sensitive than meters to changing (or bad) environmental conditions, vibration, pulsation, tampering, etc., and thus will require more accuracy monitoring than the meter that drives the instrument. Typically. small deviations in measurement accuracy can result in a large corrected (billing) volume variance due to the large hourly flow rates at stations where BVl instrumentation is utilized. Because of this sensitivity (or overall effect) instrumentation has on customer billing volume, gas companies are presently approaching instrument calibration frequency in a more scientific manner than was utilized when gas was less costly. The following discussion will outline current instrument calibration practices of companies affiliated with the A.G.A. Distribution Measurement Committee and the novel use of billing computers to aid in forecasting when an instrument has lost calibration.

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