RATIONAL DESIGN REQUIREMENTS FOR A SET OF OVERLAPPING PRODUCTION GAS FLOW STANDARDS TO COVER THE FLOW RANGE 3*10~(-1) TO 3*10~5 SCCM.

摘要

Progress in thermal mass flow controller technology (multigas, multirange, digital, userconfigurable MFC's) has advanced to the point where production calibration of each MFC is required over a 40:1 flow turndown to an accuracy specified as a fixed percentage (~ 0.25%) of flowrate. To meet today's requirements and provide for future MFC improvements requires production calibration standards with much increased flow range and improved accuracy. The re-calibration burden of keeping numerous production flow standards self-consistent and accurate must be balanced against the metrological advantage of substantial overlap in the ranges of the production standards. Some overlap is essential to demonstrate end-to-end consistency and avoid coverage gaps, yet too much overlap can add significant costs of ownership and operation because the recalibration cost scales as the product of the total number of production standards required and the recalibration frequency. This paper will explore a rational approach to setting of production flow standard design requirements by developing the quantitative relationships between flow turndown of individual flow standards, degree of overlap of adjacent standards, total number of standards required to cover the necessary range, and estimated cost of maintaining the production standards in calibration.