MASS FLOW CALIBRATION SYSTEM

A high accuracy mass flow calibration system has been developed which incorporates two Paroscientific transducers as the key pressure sensing elements.¹

The molbloc/molbox™ laminar mass flow calibration system uses a new patented laminar flow element design and two Digiquartz® pressure transducers to provide accuracy of ± 0.2% of reading for flows from less than 1 sccm to 30 slm with one-year reproducibility of ± 0.1% of reading and resolution of 0.001% FS. The mass flow is directly proportional to the differential pressure drop across a precisely machined and polished (0.15 micron finish) laminar flow element (piston-cylinder-annular gap arrangement) which has been designed for exceptional mechanical and thermal stability. The gas comes into isothermal equilibrium with the flow element and its temperature is measured with platinum resistance thermometers to account for thermal effects on gas density, compressibility, and dynamic viscosity.

The uncertainty on the mass flow measurement is highly dependent on the measurement accuracy of the pressure drop across the laminar flow element and of the average absolute gas pressure in the element used for the calculation of gas density and viscosity. Two Digiquartz® absolute transducers are used to measure upstream and downstream pressures. There are several advantages to using two absolute transducers rather than one absolute and one differential transducer. Using two identical sensors allows them to be regularly compared against each other to check drift. Using the average of two completely independent upstream and downstream measurements reduces the uncertainty of the absolute pressure of the gas. Whenever static pressure is changed, the two units are compared at zero flow and tared so subsequent readings will not be subject to full scale zero and span errors. The static line pressure may be ten times greater than the differential pressure drop, thus requiring pressure resolution of 1 ppm FS, and linearity, repeatability, and stability after taring of 10 ppm FS. DH Instruments has had extensive experience with the Paroscientific transducers used in their pressure calibration products and achieve resolution better than 1 ppm FS, zero stability better than 100 ppm/year and span stability better than 50 ppm/year.

The mass flow calibrators are directly traceable to NIST and other national standards laboratories. A gravimetric method is used for primary calibration and characterization. Calibration is normally performed using nitrogen gas. The geometric constant thus determined can be used to calculate the flow of any other gas whose density and viscosity characteristics are known

The Digiquartz® transducers have played a central role in bringing the high performance molbloc/molbox™ mass flow products to market.

1. "A High Accuracy, Portable Calibration Standard for Low Mass Flow", Presented September 6, 1994 at IMEKO World Congress of Metrology, Torino, Italy, by: Pierre Delajoud (CalTechnix S.A.-France- Tel.(1) 46.40.37.08) and Martin Girard (DH Instruments, Inc. USA Tel: 602/967-1555).

2. molbloc and molbox are trademarks of DH Instruments.

©2007 Paroscientific, Inc.