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Please indulge a brief rant about the common
terminology for Atmospheric Pressure, Absolute Pressure and Gauge
Pressure. We specify that atmospheric pressure added to gauge
pressure equals absolute pressure: PSIG + PSIA = PSIA.
???????
Atmospheric pressure is barometric pressure and
might be designated PSIB then the equation for absolute pressure
would make sense. Gauge pressure plus barometric pressure equals
absolute pressure, PSIG + PSIB = PSIA.
Many of you know that when we use the standard
atmospheric (barometric) pressure at sea level for calculations
about compressed air it is an approximation from the average of the
fluctuating atmospheric pressure. Also this close approximation
based on the most common barometric pressure is incorrect for every
application except right at sea level.
If your application is not floating in a raft
in the Ocean the local atmospheric (barometric) pressure is most
often lower than the standard atmospheric (barometric) pressure
which is given as 29.92 inches of Mercury (Hg) and 14.7 (14.696)
PSIA (Should be PSIB) and 1.013 Bar. (A bar is .986 atmospheres).
One bar is 14.5 PSI.
It is uncommon for a gauge or pressure switch
to show absolute pressure. When absolute pressure is required we
read the gauge and add atmospheric pressure.
Most of us have been convinced that 14.7 is the
atmospheric pressure. When you change the word from atmospheric to
barometric it changes he perspective.
By the way what is atmospheric pressure? If we
had a cubic inch of air so that any one side was one square inch and
laid that on a scale in a prefect vacuum it would weigh .0000434 lbs
(.075 lb/cu ft / 1728 cu in/cu ft). How the heck can it have almost
15 pounds of pressure per square inch? The air around us is stacked
up approximately 200,000 feet. Let us imagine that you got on the
scale and weighed 200 Lbs. If a person who weighed 150 pounds sat on
your shoulders and a 100 pounder on theirs and a 50 pounder on top.
You would be the only one touching the scale but the scale would
show 500 pounds. |
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The air weighs less and
has less pressure as the elevation increases. An uncorrected
barometer that shows the local barometric pressure is also reporting
the altitude. If a pilot zeros the altimeter on the ground at the
airport before take off he essentially makes the airport ground
zero. As he flies above the airport the altimeter will show the
altitude based on the reduced atmospheric, barometric pressure.
This formulas, adapted
from the Nosler reloading manual is for those who dislike charts
because the number they need lies between listings.
B = 14.7 psib or 29.92
In. Hg. h = the altitude in feet
f = conversion factor, 3.69^-5 (.0000369)
Atmospheric Pressure = 14.7 x (e^-fh).
Barometric Pressure = 29.92 x (e^-fh)
|
ALT.(ft) |
ATMOSPHERIC
PRESSURE
(psi) |
BAROMETER
(hg) |
|
0 |
14.70 |
29.92 |
|
100 |
14.64 |
29.81 |
|
500 |
14.43 |
29.38 |
|
1000 |
14.17 |
28.85 |
|
2000 |
13.66 |
27.82 |
|
3000 |
13.17 |
26.81 |
|
4000 |
12.69 |
25.84 |
|
5000 |
12.23 |
24.89 |
|
6000 |
11.78 |
23.98 |
|
7000 |
11.34 |
23.08 |
|
8000 |
10.91 |
22.22 |
|
9000 |
10.50 |
21.38 |
|
10000 |
10.10 |
20.57 |
|
15000 |
8.29 |
16.88 |
|
20000 |
6.75 |
13.74 |
|
25000 |
5.45 |
11.10 |
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