Division 2.5.2 Critical flow venturi (CFV)

Calibration of the CFV is based upon the flow equation for a critical venturi. Gas flow is a function of venturi inlet pressure and temperature.

To determine the range of critical flow, K _V shall be plotted as a function of venturi inlet pressure. For critical (choked) flow, K _V will have a relatively constant value. As pressure decreases (vacuum increases), the venturi becomes unchoked and K _V decreases, which indicates that the CFV is operated outside the permissible range.

The airflow rate (q_{V CVS}) at each restriction setting (minimum 8 settings) shall be calculated in standard m³/s from the flowmeter data using the manufacturer's prescribed method. The calibration coefficient K _V $\left[\left(\sqrt{K}\times m^4\times s\right)∕\mathrm{kg}\right]$ shall be calculated from the calibration data for each setting by means of equation (7-72):

Where:

q_{V SSV}

air flow rate at standard conditions (101,325 kPa, 273,15 K) [m³/s]

T

temperature at the venturi inlet [K]

p _p

absolute pressure at venturi inlet [kPa]

The average K _V and the standard deviation shall be calculated. The standard deviation shall not exceed ± 0,3 % of the average K _V.