A Technique for Construction of Predictable Low-Capacity Critical Orifices

Pressure change effects on hypodermic needle critical orifice air flow rates.

“…Consequently, the adjust-Figure 3. Comparison of measured and calculated maximum critical orifice flow rates as a function of the upstream (I.e., ambient) pressure ment of calibrated critical flow rates for pressure differences, becomes, through Equation 6, a simple matter of (1) selecting the proper slope correction factor for the size needle being used (Cs from fourth column, Table I); (2) multiplying the correction factor by the pressure difference between the pressure at calibration and the actual pressure expressed in mmHg; and (3) subtracting, or adding for higher pressures, the product from the calibrated flow rate.…”

Ambient Aerodynamic Ionization Source for Remote Analyte Sampling and Mass Spectrometric Analysis

“…The induced (secondary) flow is in contact with the primary flow and is ejected simultaneously at the exit of the device . From fluid dynamics, there is a limit to the maximum volumetric flow rate that can be achieved in a critical orifice based on its internal diameter and the source of induced vacuum. – The largest inner diameter of the vacuum collection tube for the current RASTIR design was 1 mm. The induced vacuum results in pickup of the powder or laser desorbed species, which are entrained as they flow through the device and into the ESI plume (Figure A).…”

A theory for critical flow through hypodermic needles

“…in which wmax is the critical mass flow rate (kg/s), CD is the empirical discharge coefficient, A is the cross-sectional area of the needle (m2), p0 is the upstream total pressure (Pa), and T0 is the upstream temperature (K). Corn and Bell (1) found that the discharge coefficient varied from 0.30 to 0.66 for the needles that they tested. On the basis of Fliegner's formula, they predicted that the critical mass flow rate should vary directly with the upstream pressure.…”