Thermal mass flow controller AFC series

for gasprecisionstainless steel

rigid metallic construction leak integrity 1 x 10-9 smL/sec of helium accuracy of ±1% full scale totalizer option circuit protection Description Model AFC analog mass flow controllers are designed to indicate flow rates and control set flow rates of gases. Each of these units incorporates an advanced straight tube sensor in conjunction with flow passage elements constructed of stainless steel. LED readouts of command modules are supplied with 0 to 100 percent calibrations. Zero and span adjustments are conveniently accessible from outside of the transmitters. Principles of Operation Metered gases are divided into two laminar flow paths, one through the primary flow conduit, and the other through a capillary sensor tube. Both flow conduits are designed to ensure laminar flows and therefore the ratio of their flow rates is constant. Two precision temperature sensing windings on the sensor tube are heated, and when flow takes place, gas carries heat from the upstream to the downstream windings. The resultant temperature differential is proportional to the change in resistance of the sensor windings. A Wheatstone bridge design is used to monitor the temperature dependent resistance gradient on the sensor windings which is linearly proportional to the instantaneous rate of flow. In AFC mass flow controllers the combined gas streams flow through a proportionating electromagnetic valve with an appropriately selected orifice. The closed loop control circuit continuously monitors the mass flow output and maintains it at the set flow rate. Flow rates are unaffected by temperature and pressure variations within stated limitations.