Tigers,
Just a short note. Why is it that there are many who believe that fluids
are compressible?
Well, in a gas, there is a lot of room between the molecules for them to
get closer together with increasing pressure. With enough pressure, and
at the right temperature (minus 400 deg, F.), gases can get compressed
so much that oxygen, and even hydrogen, can be made into liquids.
They are very dense, very cold, and usable as rocket fuel as they take
up less volume and can be pumped at very high rates.
In a fluid, such as water, the pressure can be increased a great deal at
normal temperatures, and the volume, in any meaningful measure of such a
property except to a molecular physicist, does not change. If the
pressure gets high enough, the liquid actually turns into a solid, and
the solid (generally) takes up less volume. Water is a peculiar
substance in this regard, it actually gets larger when a solid, taking
up more volume for the same mass. That is why ice cubes float, in case
you wondered.
Now water is the medium used to pressure proof test the solid rocket
engine cases for the Shuttle and the Titan solid rocket boosters. These
extremely large cylinders (over 10 feet in diameter) are sealed at both
ends, and filled with water. Then the pressure is increased with special
pumps to be over the expected use pressure, to verify the margin of safety.
When this is done, many lay observers see the additional water being
pumped into these cases and remark on the compression of the water under
pressure. More knowledgeable sources point out that indeed, more water
is entering the pressure vessel. But this is NOT because the water is
compressing (at 1000 psi), but the rugged pressure vessel being tested
is expanding, and the water is taking up the increased volume of the cylinder.
Similarly, in a brake system, some may think the hydraulic fluid is
being compressed, as the pedal moves further. Other factors are causing
this, not fluid compressibility. Some examples might be entrapped air
bubbles (even microscopic) are in the fluid and are compressing. This is
no more likely in different fluids. Another cause can be swelling of the
flexible lines. Again, not fluid related. Still another cause could be
minute leakage of seals, or deformation of brake components. This can be
particularly true in drum brakes, due to the number of mechanical
components in the load train that can bend.
I hope this has brought some clarity to the incorrect conclusion that
one, or another brake fluid "compresses under pressure" more than
another. The observations of a moving pedal may be correct, but the
cause is not.
Steve
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Steve Laifman
Editor
<http://www.TigersUnited.com>
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