The Tylan General mass flow controllers (mass flow valves) require periodic cleaning and calibration (section 5.7, Tylan General Mass Flow Controller Instruction Manual ). The first step in this process is to check the operation of the instruments and perform further work as necessary. This procedure should be planned and carried out whenever 1) there appears to be a problem with the operation of a flow controller, or 2) there is a lengthy break in the Hall-A program that would allow the gas system to be taken off-line for several weeks. If absolutely necessary the needle-valve/rotameter combinations plumbed in parallel with the mass flow controllers could be used to allow interim operation of the gas system while one or more flow controllers is removed for maintenance.