"Harry K" wrote in message
oups.com...
On Apr 11, 8:58 am, Grandpa wrote:
z wrote:
On Apr 10, 10:00 pm, "Harry K" wrote:

Poor example. The balloon expands when blown up and contracts when
able thus maintaining the pressure.

Harry K-

Which is exactly what the hose does. The volume of expansion involved,
however, is obviously much less; but that makes no difference to the
pressure. It's like a tiny capacitor vs. a big capacitor, for the
electronically inclined; both will charge up to the same voltage.

There's also a reason why oil is used instead of water in
hydraulics. Oil isn't as compressible as water. Turn off the main
water valve to the house, creating a hydraulic lock for the whole
house. Now go to the highest water tap in the house and turn it on,
water will run for a few seconds until the pressure is relieved.
Where's it coming from? The contraction of the metal pipes? Not
likely. Thermal expansion from the hot water tank? Possibly, but I
have several hundred feet of irrigation pipe that isn't connected to
a hot water source that exhibits the same phenomenon. In actuality,
water is compressible. Look it up!
--
Grandpa

I would suspect a bit of air entrained in the water as the supplier of
pressure.

Harry K

Right on Harry!!! Entrained air in water (or any fluid, for that matter) is
what allows it to be compressed. The degree of compressibility of a liquid
is defined by its "Bulk Modules". While not a big deal in most cases it is
a major consideration in hydraulic control systems and has to be accounted
for in order to keep a system from going unstable. If you have a hydraulic
lock (no expansion tank or relief valve) and allow the fluid's temperature
to increase the subsequent system pressure can increase to more than 5000
psi in a simple home hot water system.
MLD