Some additional input on "enema engineering". Admittedly, very dry material on a decidedly wet subject, but it may be interesting to a few members:
The PSI (pounds per square inch) equivalent of a "head" of one foot of water is indeed 0.433, as stated by Master Do. That gives some idea of what gravity provides to propel the solution from the bag into the rectum, and why flow slows somewhat as the bag empties without being raised. With a bulb syringe, gravity is not a factor -- injection pressure is produced by squeezing the bulb. That's why the rate of injection is less controlled with the bulb than with the bag -- gravity is constant; force of hand tends not to be.
Another factor that affects flow rate -- as a function of flow resistance -- is pressure drop through the hose. This became significant for my wife and I when I replaced the existing hose, which had an inside diameter ("ID") of something between 5/16" and 3/8", with small-diameter amber latex "surgical tubing". To reach from the hook on closet door to the center of the bed with a convenient amount of slack still in the hose, I made the replacement something like 10 feet long. It had an ID of 3/16" to 1/4" -- never measured it, that's from eyeballing. The combination of smaller ID and longer length slowed the flow perceptibly, as was evident when the clamp was opened to "bleed" air until only solution shot back into the preparation pitcher. The stream emitting from the nozzle was noticeably less energetic than with the original set-up. Note that the hole through the nozzle was only 1/8" to 5/32" ID, less than that of the hose. (At these smaller IDs, a change as small as 1/32" is significant in terms of through-flow at a given pressure.) But the effect of the smaller, longer hose was still significant, because the velocity of the water in the new hose compared to the original would have to have been greater for the same flow rate. And the requirement for increased velocity for equal nozzle flow means that an increase in entering end hose pressure would have to have been available to compensate for the greater flow resistance through the length of hose. Solution to the problem: raise the bag enough for increased "head" pressure to overcome the increased flow resistance and produce the needed extra velocity.
About the revolving-ball type of flow indicator (not meter -- it doesn't measure): It's not affected by bag height, hose or nozzle ID, hose length, a partially closed clamp or peristaltic backpressure in the rectum. It is a "positive displacement", "pressure independent" device -- in practical terms, it responds only to flow, and indicates the rate proportionally by the speed of the ball. I have a Zity friend who uses one to detect the onset of a cramp in the person he's giving to. From what he told me, if he sees the ball slow down, it signals backpressure, which in turn "anticipates" an imminent cramp. He stops the flow while spasmodic pressure is still relatively low, waits a few minutes to allow peristalsis to subside, and then continues giving the enema. Very clever, very caring!
For what it's all worth. . .