Enter the inside diameter and length of a pipe or cylindrical vessel to get its internal volume, then the nitrogen volume needed to bring the oxygen concentration from its starting value down to the target. Space that dimensions miss, such as valves and vessel heads, goes in the extra volume field, and adding a supply flow gives the time required.
The method assumes exponential dilution, where mixed gas leaves at the same rate purge gas enters. The natural logarithm of the concentration ratio gives the number of volume changes, and multiplying by the internal volume gives the nitrogen required. The formula comes from conservation of mass and geometry rather than from a standard document, and it is printed with the result so it can be checked against another method. Figures current as of October 2026.
The calculation rests on perfect mixing inside the equipment and on supply nitrogen containing no oxygen. Dead legs or lower purity nitrogen both push the real requirement higher, and correction factors for either are not confirmed by an authoritative source, so none are applied. Confined space ventilation rates, oxygen based entry decisions and the residual concentration after each cycle of pressure or vacuum purging are not covered. This calculator is for reference and does not replace design. Review by a qualified engineer is required.
Frequently Asked Questions
In exponential dilution the number of internal volumes that must pass through follows a natural logarithm. Going from 20.9% down to 1% takes about 3.04 volumes, and rounding to a whole number overshoots or undershoots.
Exponential dilution approaches zero without reaching it, so a target of zero would demand an infinite purge volume. Values at or below zero therefore stop the calculation with a notice.
No. This page works out only the gas volume needed to displace the atmosphere inside equipment. Confined space ventilation rates and entry decisions are not covered.