On Jul 10 , at 1:26 PM, Michael Payne wrote:
The benefit of Pascals is (I believe) that a vacuum with no pressure
is zero Pascals so any indication is always positive. Similar to
Kelvin starting at Zero and always being a positive value.
Any of the Physicists on this list throw any light on this? I know
there are a lot of knowledgeable people out there on this list.
OK! I'm a "physicist on this list", so here goes:
The SI unit, pascal, does NOT have a "benefit" that it is always
positive. Pascals can be used to measure ANY kind of pressure (even
that kind of "pressure" which is called "stress" in the stretching or
compressing of solid materials). Therefore, pascals can measure
negative pressures just as easily as positive ones, and pascals are
the correct (SI) unit in both cases.
What some of us are failing to realize is that one can measure any
pressure as a relative amount compared to some other pressure. That is
called "Relative Pressure". It is also called "Gauge Pressure" because
typically that is the kind of pressure which is measured by gauges.
Absolute pressure can never be zero, whether it is measured in pascals
or newtons per square metre or psi or anything else. (I know I should
never say "never" because someone will dream up an exception.)
However, gauge pressure, which is the pressure relative to another
pressure (often ambient air pressure) may be either negative or
positive, depending on how great it is relative to the reference level
(usually atmospheric pressure). If the pressure in question is higher
than the reference that is it is "relative to", then that (gauge)
pressure is positive. If it is less than the reference, then it is
negative.
Gauge pressure is almost always a more important practical measure
than the absolute pressure (with atmospheric air pressure being one of
the few exceptions). Also, gauge pressure is easier to measure (gauges
to measure it are easy to manufacture).
One of the difficulties with this subject is that, many times, the
term pressure is used without stating whether it is the absolute
pressure or the relative pressure that is meant. Usually this does not
cause any difficulty because only one or the other is reasonable to
use in any given circumstance so it is obvious which one is intended.
The situation is identical to the situation of measuring the height of
mountains and other terrain RELATIVE TO sea level. Surely it can be
positive or negative depending on whether it is above or below sea
level. An absolute measure of such heights would be the distance of a
particular mountain peak or valley floor from the center of the earth.
Surely, that could not be negative.
Bill Hooper
1810 mm tall, which means ... see below:
(When standing, my head is
1810 mm high relative to my
feet, which I consider the
reference point or the "zero point".)
Fernandina Beach, Florida, USA
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SImplification Begins With SI.
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