Some General Questions on Pressure

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Some General Questions on Pressure

Posted by Jose Borrero at April 02. 2009

Some Background:

Instrument: Aquadopp current profiler

From the .hdr file:

=============================================

Software version 1.27

Pressure Sensor Calibration 0 0 26579 8425

=============================================

according to the file, the pressures listed in the .dia and .dat files are in 'dbar'

Q1) the pressure that is spit out from the instrument, does that include any shift or compensation for standard atmospheric pressure?

It may have been calibrated in air previously (i am not sure), maybe the line above will tell you...

Due to limitations in the instrument deployment settings (we were more interested in sea levels and currents over a month), i could only get 150 pressure samples in each burst. From the .dia file, each pressure reading is 1 second apart, so 1Hz. Because of the short time series, i cannot use Quickwave (or can I?)

So I took each 150 sample long time series of pressure and interpolated it to be 1024 points long so I could do FFT type analysis on it. Based on our own in-house software for waves we are seeing wave heights on the order of 0.2 m at the 6 m deep location, with periods in the 3-5 second range.

Q2) Do I have to worry about any attenuation in the pressure signal with depth? The 0.2 m wave heights make intuitive sense (wind generated, short fetch) but I was just wondering if I might be missing something by not compensating for depth. The pressure sensor was mounted on a frame resting on a sea bed with the bottom of the instrument just touching the sea bed, so ethe pressure sensor was approx 0.65 m above the sea bed.

I have looked around the web for general inforamtion on pressure corrections with depth, and I cam across this document from NIWA with regards to 'DOBIE' type pressure sensors. They suggest the use of a function to calculate the 'penetration'

see:

http://www.niwa.cri.nz/rc/instrumentsystems/dobie/dobie_notes/dobie_tnote1

Penetration = cosh [ k (z+h) ] / cosh (kh)

where k = wave number based on mean spectral period and mean depth, z = depth of sensor below water level and h = mean water depth

if we assume a 5 sec wave in 6 m of water, L =~20 - 30 m (lets say 25 m)

then k = 2 pi/L = 0.25

h = 6.61 m

z = 5.96 m

putting all this together penetration = 4.32, which doesn't make sense, because it implies that the pressure signal INCREASES with depth. !?!?

In fact, just looking at the formula, it can never be less than zero if you add z to h then divide by h, i.e. cosh [ k (z+h) ] will always be greater than cosh(kh) !

If i then forget the average depth in the numerator and just do cosh (kz) / cosh (kh) then i get 0.86, which (I THINK) means a 14% atenuation of the pressure signal due to depth.

Has anyone seen/dealt with this before?

What is the simple answer ?

Thanks for any help!

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Some General Questions on Pressure

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