Directly digital flow controller.

An improved directly digital flow controller is evaluated for its ability to modulate gas flow rates. As in the older device, the "GasDAC" (named for its similarity to a weighted-resistor digital-to-analog converter) is capable of controlling gas flow in a linear and reproducible manner with the advantage of having an adjustable range of flow rates. The new design incorporates venting to prevent "puffing" when the individual flow channels are opened. The temporal characteristics of the GasDAC are also examined; modulation frequencies of 10 Hz with various types of waveforms are possible with the new device.