REFLECTOR:RE:weak gas struts

[reflector@tvbf.org]

At 07:24 PM 1/1/04 -0500, you wrote:
>Well, the syringe example was not exactly what I was inquiring about, though
>its certainly illustrative of how a vacuum is formed.  My question was given
>the very high (or is it low, I always confuse the two) vapor pressure of
>oil, it's going to take a very high vacuum to form a bubble.  Is the "pull"
>on the plunger sufficient to cause the hydraulic oil to cavitate?

Think of it this way.  If the fluid were ether, right on the edge of 
boiling at room temperature and pressure, just a slight drop in pressure 
would cause a bubble full of vapor to form.

If the fluid were something like concrete, that isn't anywhere close to 
boiling, and you pull HARD, you'll get a bubble that's full of nothing.  No 
vapor at all, just vacuum.

Hydraulic fluid is much closer to concrete than it is to ether.  But it 
doesn't matter.  Pull hard enough, and an empty space, a void, filled with 
nothing, will form.


>Isn't this the reason we use hydraulic oils and brake fluids...for their
>very high vapor pressure.  When brake fluid gets contaminated with water,
>the hot brakes will heat the water/brake fluid to +212F which will result in
>water boiling off which is just another way of saying its vapor pressure was
>exceeded?  Richard, there may well be a bubble form but it just seems it
>would take a very robust vacuum to vaporize the oil.

Absolutely.  But if the thing we're concerned with is the "vacuum" side of 
a hydraulic system keeping things locked, it's probably not going to happen.

>Of course, there is an unrelated but coincident reaction that often occurs
>when needing to brake sufficiently hard so as to vaporize the water in the
>contaminated brake fluid; namely, sweat formation on the brow.

Absolutely true.  There's an absolute correlation - the sweat will drip 
into your eyes at exactly the same moment the brakes fade.