On Tue, 31 May 2005, Mike Hord wrote:

> I'm trying to get a feel for the "normal" range of atmospheric
> pressure.
>
> Wikipedia tells me that the highest and lowest ever recorded
> barometric pressures are 108.57 kPa and 86.996 kPa,
> respectively.  It also tells me that "standard" atmospheric
> pressure is 101.325 kPa.  What it does NOT tell me (nor
> does anywhere else) is what the "normal" range is; that is,
> if a good, strong thunderstorm passes through, what can
> I expect the pressure to be?  Likewise, in a typical high
> pressure area, what can I expect the pressure to be?

It depends at what altitude you are.

> I'm trying to develop a water depth gauge, and if possible,
> I'd like to get some feel for the variation of pressure based
> on atmospheric variations.  The highest to lowest cited
> above would represent a 2.2 meter(!) fluctuation in the
> depth of a body of water, if one were basing the depth on
> the pressure at the bottom.  Needless to say, that
> kind of variation would render the information useless.

Pressure based depth gauges use a differential transducer. One end is 
exposed to the air. This removes the problem you describe and adds the 
problem of having two tube openings to keep clean.

> Normal diurnal cycles are <.5 kPa, which corresponds
> to approximately 5 cm of water depth.  No info on
> typical changes outside those variations, however.
>
> Differential measurement is NOT an option for this
> application.

You can measure depth pretty accurately using ultrasound from below. The 
water/air interface bounces it back strongly. But if you have waves it 
will be hard anyway.

Peter
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