VERIFICATION OF GAS FLOW TRACEABILITY FROM 0.1 SCCM TO 1 SCCM USING A PISTON GAUGE

摘要

Fluke Calibration is accredited for gas flow measurements in the range of 0. sccm to 6000 slm in nitrogen and air. Traceability is maintained directly through a gravimetric flow standard but only recently from 1 to 10 sccm. The traceability of flow in the range of 0.1 to 1 sccm is based on extrapolation of the use of laminar flow elements below 1 sccm. This part of the range has never been completely verified through interlaboratory comparisons, proficiency testing or other means of measurement assurance. In an internal document from DH Instruments in the early 1990s it was suggested that a piston gauge could be used to gain traceability for very lows gas flows. In order to prove out traceability in this range an attempt was made to use a piston gauge using a piston-cylinder size of 35 mm diameter as a reference. One of the reasons for choosing a piston gauge as a reference is its ability to control pressure. This is crucial when measuring gas flow through a laminar flow element (LFE) in this design and range. In addition, the effective area is known to within 0.001% leaving the vertical displacement of the piston to dominate the uncertainty of the dimensional part of the flow test. This was a challenge because the measurements needed to be made in absolute mode and the internal piston position sensor supplied with the piston gauge did not have sufficient precision. This paper describes the theory and design of the gas flow measurement system, the current results, and improvements needed or suggested. Two different designs are discussed, one with a single piston gauge as a reference and the possibility of two piston gauges measuring flow on either side of the laminar flow element. Note: sccm (standard cubic centimeters per minute) is an industry accepted alternative to kg/s [1]. It is used out of convenience to normalize flow rates of gases with significant differences in density.