Measurement of sound velocity and what sensor to use
There are currently two methods of obtaining sound speed in the oceans: calculating sound velocity from other physical parameters and measuring sound velocity directly.Temperature has the strongest influence of sound speed and salinity the strongest influence on attenuation. At greater water depths the increased pressure will result in increased sound speed which is calculated through various known equations.
The two most common in use are by Chen & Millero, and Del Grosso. They give slightly different answers, and their use is often confined to specific depths. Regardless of which formula is used, it can be said that the resulting sound velocity value is subject to a variety of errors.
Nowadays, modern CTDs provide accurate measurement, so it is not uncommon for the combined effect of these errors to be less than 0.05m/s. However, the inherent errors and inaccuracies in the various sound speed formulae cannot be overcome. It is accepted that the best accuracy which can be attributed is ±0.25m/s.
In recent years, a method called time of flight which provides a timed acoustic pulse over a known distance has created a breakthrough in new products.
During calibration, the sensor output is measured in fresh water at different temperatures. The sound speed in fresh water is only affected by temperature (pressure is negligible), and so the relationship is much more accurately defined, and is expressed in an equation by Del Grosso and Mader . It is accurate to ± 0.015m/s, more than ten times more accurate than the CTD equations.
Valeport Limited is based in Totnes, Devon, UK. www.valeport.co.uk
