A pinhole orifice with a known conductance can be used as the basis of a flowmeter. Commercially available laser-drilled pinhole orifices with diameters ranging from 1.0 μm to 50 μm can have conductances ranging from about 0.1 μL/s to 200 μL/s in the molecular-flow regime for N2 at 23 °C. Gas flows of 10-11 moles/s to 10-6 moles/s can easily be produced by applying an upstream pressure in the range of 1 Pa to 105 Pa. Accurate measurements of the orifice conductance as a function of pressure are required to use the pinhole orifice as a basis of a flowmeter. We use a constant-pressure flowmeter to make precise and accurate measurements of the conductance of a pinhole orifice as a function of pressure for gas flows of Ar or N2 into vacuum. We present results of these conductance measurements for an orifice of nominal diameter 20 μm. The N2 conductance of this orifice ranged from 30 μL/s to 60 μL/s over the range of pressures investigated, and we achieved an uncertainty of better than 0.2% (k = 2) for an upstream pressure greater than 10 Pa.
Pub Type: Journals
Low gas flow, flow standard, rarefied gas flow, conductance, cirular orifice