An existing accelerator ion source has been modified to allow gas flow control from ground potential as opposed to control at high voltage as had been used. The mode of achieving this objective was to use a pressure reduction device at the gas entrance of the ion source so that the gas feed line could be maintained at a high pressure so that no arcing would occur. A pressure reduction device was developed which allows reproducible gas flows through and a pressure differential across the device. The development and production of the calibrated variable leak is discussed in detail. The components used an inexpensive and readily available, as are the tools necessary for construction and testing. A simple vacuum system was utilized for testing and calibrating the leak which had a dynamic range of more than 200 for a nominal pressure ratio across the leak of 50. A typical mass flow at a pressure ratio of 50 is 6 x 10(-3) T L/s for nitrogen with a nominal conductance of 4 x 10(-4) L/s. The vacuum system design program, VSD-II, was used extensively to determine the values necessary for mass flow, pressure, and conductance of the leak. This calibrated variable leak can be used in many applications other than in accelerators.