The ÆV method of space pressurization control utilizes analog or digital electronic controls to measure the real-time variables and solve the dynamic air balance equation. A typical ÆV system is shown in Fig. 2. The exhaust volume is either measured after Page 3

Offsetdesign = 2 e S Fmax Equation 1.
Where:
e = instrument error in % full scale or % of reading
S = safety factor: depends on tightness of envelope,
amount of unmeasured duct leakage and
degree of laboratory hazard present;
recommended range: 0.5 -2.0
Fmax = design maximum exhaust or supply flowrate,
whichever is greater

This will assure (if the safety factor is greater than 1) that under worst case conditions you still have some actual offset in the desired direction of flow. For example, in a negatively pressurized lab, the exhaust flow rate will be higher than the supply flow rate, so the Fmax is the maximum exhaust volume. If Fmax is 5000 CFM, e is 5% (0.05), and S is 110% (1.1) then the Offsetdesign = (2)(.05)(1.1)(5000) = 550 CFM. Therefore, the worst case scenario would be a system where the exhaust volume reading is 5% below actual, and the supply volume reading is 5% above actual, giving a total airflow error of +500 CFM. If the design offset is -550 CFM then the actual offset will be -50