This valve is opened by supplying air to a diaphragm which is connected to the valve stem. The valve is closed by spring pressure on the valve stem. The temperature changes in the retort cause the Bourdon tube to expand and contract. As the temperature in the retort rises, the supply of air to the valve diaphragm would be reduced, the spring would force the valve partly closed and less steam would be supplied to the retort. As the temperature in the retort starts to fall, more air is supplied to the valve diaphragm and the valve is forced open, with more steam being supplied to the retort.
Pneumatic air controlled valves are also common in many advanced computer controlled retort systems.
In these systems the air supply to the steam control valve is usually controlled through an electronic solenoid. With any retort using air for control of the system it is very important that the air supply to the instruments and control valves be kept clean and free of oil, water, rust and other contaminants. It is ideal for the air supply to the instruments to be separate from the air supply to the rest of the plant. This air supply should be equipped with devices for keeping the air clean, such as air filters, condensate traps, water drains and oil traps. Maintenance of the air supply system on a routine basis is essential to the continued accurate operation of pneumatic air controls.