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smpsmp45
01-10-2008, 03:33 PM
Can someone explain the definition of sea level say for Pshycrometric charts. How it is defined??

The Viking
01-10-2008, 05:08 PM
???
Not 100% sure that I understood the question...

Sea level is the "height" of the sea's surface at mean tidal conditions. Generally accepted as being 0 when measuring altitude (Note: Don't count on this if you are a pilot!).

As everything (including air) has a weight, pressures will decrease the higher up you get (or increase the further down you get).
1 Bar = 10m of water-column = 10000m of air-column
If you have 1 bar at sea level you would have 0.9999 bar at 1000m altitude and 2 Bar at 10m under the seas surface.
As we measure pressures we need to know that we are talking the same values, hence most pressure scales relates to sea level (at mean tidal condition). Other pressure scales refers to absolute vacuum.

US Iceman
01-10-2008, 06:20 PM
Sea level is used as a reference point for the atmospheric pressure as a basis for the density/specific volume of air.

smpsmp45
02-10-2008, 06:09 AM
Yes this is fine. But can there be a specific point per country basis for this level????

Peter_1
02-10-2008, 07:05 AM
Pilots use MSL, Mean Sea Level and all altitudes are
expressed with MSL as reference.

http://en.wikipedia.org/wiki/Sea_level.

MSL in aviation is used to give for example the height of an airfield above seal level at standard conditions (1013.2 mbar, 15C, 1.221 kg/m), terrain height above sea level,
When approaching an airfield, pilot must ask the local pressure and dial this in his altitude meter which then 'converts' to actual height for a pressure occurring in the region he/she flies.
He then can look on his map with altitudes and he then see the real actual height of the terrain on his altitude meter.

This is one of the parameters a pilot must always repeat when tower or Air Traffic Controls gives you the actual pressure to avoid any misunderstandings.
ATC always gives you pressure when first contacting them.

Once flying at higher altitudes, then you change to Flight levels (expressed FL), referred to a height above standard pressure 1013.2 mbar.
In Belgium this is 4.500 ft, the US mostly 18.000 ft.
So all traffic pilots are flying with the same reference to prevent collisions.

The Viking
02-10-2008, 06:56 PM
Yes this is fine. But can there be a specific point per country basis for this level????

Only if we got problems with the gravity...:D

Seriously, yes there are always variations. Even from day to day at the same place.

This is something meteorologists use to try and predict weather changes.
In real life, for us in our trade, no you don't have to consider it.

However, your posts makes me wonder if we have misunderstood you, or why you think this would be important.

Peter_1
02-10-2008, 07:02 PM
Yes this is fine. But can there be a specific point per country basis for this level????

The MSL is the same for the whole world.

nike123
03-10-2008, 12:22 AM
1 Bar = 10m of water-column = 10000m of air-column
If you have 1 bar at sea level you would have 0.9999 bar at 1000m altitude and 2 Bar at 10m under the seas surface.

I taught that 1 bar is 0,986923 atmosphere.
If at sea level pressure is 1013.25 mbars and 1 atmosphere is pressure at 0 m than what about atmosphere weight above 10000 m. ;)

At 11000 m static pressure is 0,226320 bar. So, that equation of 1 bar is 10000 m of air is not correct. 1 bar is approximately 10000 km of air column above sea level (there is where exosphere officially ends).
Correct pressures according to height are given by formula and tables here:
http://en.wikipedia.org/wiki/Atmospheric_pressure

Chef
03-10-2008, 03:43 AM
Yes this is fine. But can there be a specific point per country basis for this level????

Your quite right, every country and even parts of a country have differing referance levels for sea level.

Heights of items on land are quoted as being above sea level which is always referred to MHWS or Mean High Water Springs. Each area in the world has its MHWS defined relative the land around the sea in that area. Some use a stone placed in a recorded position with a brass plaque and some older references use dead tree stumps with a plaque showing the data. (Luckily no-one was bright enough to pin the plaque onto a growing tree!!!!!)

It can be important because if MSL is used or mean sea level the average height of the tide is used. In some areas of the world this is almost 30 feet below MHWS. Bridges, towers and communcations pylons etc are therefore all quoted as height above MHWS. There are some areas in the world that have no tidal variation or very little like the Med.

Chef

smpsmp45
03-10-2008, 06:06 AM
Well it turned out to be very special issue. Many thanks for all

Vishwaprasanna
03-10-2008, 05:12 PM
In our country nepal we don't have specific value. We always taking value from sea level then correcting by its altitude.

nh3simman
18-10-2008, 12:24 PM
Sea level conditions are based
of a standard pressure
of Patm = 101.325 kPa