Section 6 | Frequently Asked Questions
©
2022
–
Valeport Ltd
Page | 18
6 Frequently Asked Questions
Why is the SV data from the time of flight sensor different from the CTD data?
Quite simply, the Valeport SV sensor is more accurate than the CTD data. This is because the
CTD formulae (Chen & Millero, Del Grosso etc.) all have errors in them
–
they were after all based
on observed data taken over 30 years ago using the best technology available at the time. The
Valeport SV sensor simply highlights those errors.
The purpose of this product is actually to provide you with good SV data as well as Density and
Salinity data, which were not previously available on an SVP. The ability to compare the true
sound velocity with the value calculated from CTD is an obvious consequence of this, and raises
questions that had not been previously considered.
Why is the SV data so accurate?
Several reasons. Primarily, we use an advanced digital signal processing technique that removes
virtually all noise from the data, tells us the precise moment that the sound pulse is both
transmitted and received, and allows us to measure the time of flight with a resolution of 1/100th of
a nanosecond (10-11 seconds). Secondly, we have developed a carbon composite material that
doesn’t expand or contract with temperature, so our “known distance” is a constant. Technically,
the material will expand and contract minutely, but over the operating temperatures of the probe, it
is an almost immeasurably small amount, and any change is included in our overall error budget.
Finally, our calibration method removes virtually all the error sources associated with other
techniques.
But don’t you just
calibrate it against Chen & Millero?
No we don’t –
that would defeat the purpose. While the seawater formulae (Chen & Millero, Del
Grosso etc.) have inherent errors that are accepted as being at best ±0.25m/s, we use a different
formula to calibrate the sensor. Del Grosso also published a formula for speed of sound in pure
water (with Mader, 1972), which is much more accurate. In pure water, the only variable that can
affect sound velocity is temperature (assuming that you are at atmospheric pressure in a laboratory
environment), rather than both temperature and Salinity with the seawater equations. The Del
Grosso & Mader formula therefore has an error of just ±0.015m/s. By calibrating against this rather
against the error-filled seawater equations, we can achieve significantly better performance.
Is a pure water calibration valid?
Absolutely
–
the purpose of a calibration is just to compare (and adjust) the sensor output against a
known standard
–
it doesn’t really matter what that standard is, as lon
g it is precisely defined. Our
standard happens to be pure water because it is the most accurately defined standard available.
How Robust is the Conductivity Cell? It looks like it's made of plastic.
We wouldn't go so far as to say it's indestructible, but it is pretty strong. It's actually made from
titanium, with just a thin coating of polyurethane to insulate it. The key thing is the central core,
which is a specialised ceramic material that is particularly durable. It allows the sensor to
overcome a common failing of inductive cells, which is that they rely on the dimensions of the
central core remaining absolutely constant. As pressure increases, most inductive cells compress
slightly, so they may be out of specification at high pressure. The strength of Valeport's cell keeps
the dimensions remarkably stable, so performance is maintained over the entire profile.