Thanks Bill, very well explained.
----- Original Message -----
From: Bill Hooper
To: U.S. Metric Association
Sent: Sunday, 12 July 2009 01:35
Subject: [USMA:45344] Re: Vacuum display
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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