The use of diaphragm bellows to construct a constant pressure gas flowmeter for the flow rate range 10⁻⁷ Pa m³ s⁻¹to 10⁻¹ Pa m³ s⁻¹

Abstract

A constant pressure gas flowmeter using a directly driven diaphragm bellows as a volume displacer was studied. This flowmeter is perfectly vacuum-sealed, does not contain elastomers and liquids that would prevent outgassing at elevated temperatures and can achieve a smaller ratio of the final volume to the displaced volume than when bellows are used, so that the uncertainty in generating small flow rates can be small. As, when a bellows is used in the volume displacer, the displaced volume cannot be calculated from the geometric dimensions and its dependence on the displacement is non-linear, a sensitive method had to be developed for measuring this dependence. The uncertainty in measuring the displaced volume by this method and the achieved uncertainty of the flowmeter were determined.

Method of measuring the change in volume of a diaphragm bellows used in a volume displacer of a constant-pressure gas flowmeter (with a practical guide)

“…Computer control also ensures maintenance of all the necessary pauses for stabilization of the system. (2) No surface freshly exposed to the surrounding atmosphere may be immersed in the chamber during the measurement of the entire dependence (i.e. one curve for a given P D value).…”

“…Constructions of constant-pressure gas flowmeter employing directly driven bellows and/or diaphragm bellows as volume displacers were described in recent decades [1,2]. Practical advantages of this solution are obvious: difficulties with a dynamic seal [3,4] or with liquid [5] can be avoided.…”

Constant pressure primary flow standard for gas flows from 0.01cm3/min to 100cm3/min (0.007–74μmol/s)

“…Jousten [15] s Calibrated bellows 12 5 Â 10 À 7 13 0.2 Jousten [16] s Calibrated bellows 12 5 Â 10 À 7 13 0.14 Calcatelli [17] s Pistonþ cylinderþ custom PTFE seal 5, 20 1 Â 10 À 5 0.7 0.2 Berg [18] s/r Pistonþ bellows þ oil-filled cylinder 102 1 Â 10 À 1 270 0.02 Cignolo [19] s/r Pistonþ lubricated o-ring 120 9 Â 10 À 2 1900 0.01 Kramer [20] s/r Double piston 16 1 Â 10 À 3 1.3 0.03 Cheng [21] s Pistonþ bellows þ oil-filled cylinder ? 5 Â 10 À 6 0.05 0.57 Gronych [22] s Calibrated bellows 9,30 3 Â 10 À 4 55 1. This work s/r Pistonþ FEP encapsulated o-ring 25 1 Â 10 À 2 100 0.03 [23]).…”

“…The third type of flow standard, calibrated bellows [12,15,16,22], also has no sliding seal, and it eliminates the complications due to the oil by driving the bellows directly. Its chief disadvantage is that the bellows volume V(x) must be calibrated as a function of its linear displacement x. Jousten et al [12] found that V(x) was more linear for a formed bellows than for a welded diaphragm bellows, although Gronych et al [22] later were able to use a welded diaphragm bellows despite its nonlinearity.…”

“…Its chief disadvantage is that the bellows volume V(x) must be calibrated as a function of its linear displacement x. Jousten et al [12] found that V(x) was more linear for a formed bellows than for a welded diaphragm bellows, although Gronych et al [22] later were able to use a welded diaphragm bellows despite its nonlinearity. Most of the devices in Table 1 were not flow receivers that measured an unknown flow directly but instead were flow sources that produced an accurately known gas flow.…”