Why is there a difference between mean pressure and the mean AST?Follow
There have been a few questions as to why there can be a difference between the mean pressure and the mean AST. The pressure reading and the AST distance has been calibrated so that they both have the same distance, but sometimes there can be a difference of up to a meter. When the time series is plotted, it appears that there is a vertical offset between the measurements. There are probably four sources of this bias or difference between the two.
The "error" in the tide=surface elevation (as derived from pressure) will come from variations in the density profile and from fluctuations in the atmospheric pressure. If there are large atmospheric pressure variations, then this would change the mean pressure for a given burst, and this would not be seen in for the mean AST. The difference can be as much as 0.5 meters, but typically is no more 0.25 meters.
The "error" in the tide as derived from AST could show a bias in its estimate of distance to the surface due to variations in the speed of sound. Remember that the AST is really measuring the time of travel from when it sends and receives a pulse. This assumes a constant speed of sound for its travel path. If there are any variations (gradients) in the water column then there could be bias in the distance measurement. Errors would also be seen if the initial estimate for the speed of sound is incorrect. This would occur if there was something wrong with the temperature sensor (this would have to be a lot), or if the estimated for salinity was way off.
The pressure sensor can have an offset at the time of the deployment if it is not set to zero. It could simply come from the production system we have here. Because the pressure sensor cannot output negative values and because it is confusing when you see a zero output from the pressure sensor, we give the pressure sensor a positive offset of about 0.2 m during production. This can be adjusted later on by using the pressure offset function in the software.
In addition, the pressure sensor does not have linear response so in a 100-m sensor you can have absolute errors as large as 0.5 m (0.5% of full scale). The good news is that this absolute error will not affect (much) your ability to correctly estimate variations in the pressure.
A "bad" combination of the three first of these could lead to a meter or more difference between the pressure and AST. The good news is that it has little effect on the estimates of wave parameters, which is what the AST is really used for.
Please sign in to leave a comment.