Please note this simple fact. Every syringe is a cylinder containing a piston of some sort that decreases the volume of the cylinder by moving toward its open end.

How much fluid escapes the open end is determined by the decrease in volume ahead of the piston.

In engineering terms this is known as displacement.

The way to calculate this displacement is simply to calculate the volume of a cylinder.

In other words Pi times radius squared times travel of the piston.

You will notice that if we were to double the radius (or diameter if you wish) of the cylinder, this would quadruple the volume.

Conversely, if we halve the diameter then we decrease the volume to one-fourth.

The operating principle for syringe design is that with the same stroke, the displacement varies as the square of the diameter (or radius).

You can quickly see that the smaller the diameter the more controllable is the syringe.

The typical hypodermic syringe moves the piston(plunger) by pushing directly on it.

As discussed previously, with thick fluids and small diameter needles, this results in a mechanical DIS-advantage i.e. it gets pretty hard to push.

This is solvable in one of two ways.

- You use a mechanical lever system to magnify the pressure you can apply manually.
(Remember levers and pulleys in high school physics?)

- Or you can use a screw mechanism to magnify your manual effort.

- hypodermic type and
- micrometer screw types.

How much you magnify your manual effort with a micrometer screw type is determined by the pitch of the screw(threads per inch).

In other words, now far does the piston (plunger) move per full turn of the screw.

The finer the pitch (allowing for friction) generally means more pressure and easier turning.

It certainly results in better control because for a given amount of turn, a smaller amount of fluid is ejected.

Therefore controllability of a micrometer screw syringe is directly proportional to the pitch of it's screw.

So in general, we want to make a barrel with as small an internal diameter as possible and a screw with as small a pitch as possible, i.e. a greater number of threads per inch.

The only other factor is the diameter of the handle which determines the leverage that you have in turning the screw, i.e. the larger the handle (knob) the easier it is to turn, i.e. the lever arm is longer.