See below
Matt Olsen wrote:
snip.
>
> Atmospheric pressure at sea level will support a column of mercury
> approximately 29.92 inches high, as in a barometer. The actual height of
> mercury depends on weather conditions and decreases with altitude.
I think we all know this. I continue to maintain that 30" is close
enough for our purposes. When you use pi, do you use 3.141592647?
>
> A vacuum gauge reads approximately 29.92 inches of mercury (chemical symbol
> Hg) when open to atmosphere. Start to draw a vacuum, and the gauge reading
> drops toward zero. These are absolute pressure measurements.
Nope, not unless you have a very weird vacuum gauge. Every vacuum gauge
I have ever seen provides a relative measurement and when open to the
atmosphere reads 0. And I just checked the one in my basement. While
still able to inhale, and not passing out. You're describing a
barometer.
>
> Pressure can also be measured relative to atmospheric pressure. This is
> commonly expressed as "XX inches Hg vacuum." For example, 10 inches Hg
> vacuum = 19.92 inches Hg absolute pressure.
Absolute pressure anywhere in the system is irrelevant. We want to know
the difference in pressure that is squeezing the mylar or sheeting
against the core. That's why we use a vacuum gauge.
>
> The pressure on a partially evacuated volume is equal to the difference in
> internal and external pressures, which is the relative measurement. Using
> the example above and converting units, (10 inches Hg)(14.7 psi)/(29.92
> inches Hg) = applied pressure in psi.
Yes, we know. You appear to understand the physics too.
>
> Vacuum -- much ado about nothing.
Hah, hah.
>
> Matt Olson
> Glendale, AZ USA
>
--
Lincoln Ross
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