This figure shows a system used in modern fermentors. The oil is metered by sequential timing of solenoid valves with an expansion chamber. When a shot is called for, the inlet valve opens so that pressurized oil fills the metering chamber, next the inlet closes and the discharge opens to permit the chamber air pressure to deliver the oil. Both valves are closed between shots. A source of trouble, with this system, is valve leaks or sticking which may allow full flow through the system when the other valve is actuated. The calibrated oil reservoir is used to obtain accurate usage data for development and scale-up purposes.