Why is sound velocity (SV) computed from a CTD better than sound velocity from direct measuring instruments?
Direct SV probes measure the time (flight time) required for a sound pulse to travel over a fixed length, using a high-speed clock to measure time. The clock starts when the pulse is emitted, and stops when the pulse is received. Theoretically, you only need to know the length of the path (and the frequency of the clock ‑ an easy matter) to compute SV. SV is calculated as: SV = length of acoustic path / flight time. For a typical acoustic path of 0.1 m, a flight time of 67 microseconds is expected for SV = 1492 m/s. Two problems associated with direct SV probes are: • The length is not readily determined by a ruler measurement. The true length includes some depth into the acoustic transducer at which the pulse actually arises and again some depth where it is actually detected. Consider for example the SVplus instrument made by Applied Microsystems in Canada. The claimed accuracy for this probe is 0.06 m/s. For a typical water SV of 1500 m/s and a probe acoustic path of 100 mm, achievin
