Valve leakage

• Class I, or 'dust-tight' valves, are intended to work but have not been tested
• Class II valves have no more than 0.5% leakage with 50psi of air pressure at the operating temperature
• Class III valves have no more than 0.1% leakage under those conditions; this may require soft valve seats, or lapped metal surfaces
• Class IV valves have no more than 0.01% leakage under those conditions; this tends to require multiple graphite piston rings or a single Teflon piston ring, and lapped metal seats.
• Class V valves leak less than 5*10^-12 cubic metres, per second, per bar of pressure differential, per millimetre of port diameter, of water when tested at the service pressure.