Drop-counting flow computer.
An inexpensive microcomputer (VIC-20) was adapted to count drops of fluid and calculate flow. To minimize both the expense and the bench space occupied by the flow computer, we eliminated the need for a video monitor by employing a liquid crystal alphanumeric display. Neither tape recorders nor disk drives are needed because the flow-computing program resides in a "game cartridge." Furthermore, the power supply of the computer powers the interface and display. The computer's real-time clock is utilized to time intervals between drops falling through an infrared beam. The computed flow values are then shown on the liquid crystal display, and they are sent to an external recorder via a digital-to-analog converter. A second digital-to-analog converter can be used to trigger a fraction collector. When compared with timed manual collections, the flow computer was shown to yield highly accurate, linear measurements of water and blood flow. Although the drops-per-milliliter constant varied with orifice size and hematocrit, the hematocrit fluctuations observed in typical isolated organ experiments would not appreciably affect the blood flow determinations.