TECHNICAL GUIDANCE NOTE
In the context of measurements using hydrophones, wetting critically affects
the coupling between the hydrophone encapsulation material (in particular,
that surrounding the active element) and the measurement medium (assumed
in this case to be water). Wetting is the readiness of a liquid to bond to the
surface of a solid. The process is governed by the relative surface energy of
the solid and the surface tension of the liquid.
Device preparation and wetting
By ensuring that the hydrophone boot material is
cleaned prior to use, surface contamination will be
minimised and the likelihood of poor wetting will be
reduced. A mild detergent will usually be sufficient for
cleaning general dirt and grease. If the device is new,
there can sometimes be a residue left by the mould
release agent used in manufacture, and this can often
cause wetting problems. Dependent upon the boot
material, gently wiping the surface with an alcohol
soaked cloth will remove the mould release agent
residue. Great care must be taken not to damage the
hydrophone through excessive use of alcohol (or any
other solvent). It is recommended that a small area away
from the active element be tested beforehand.
Application of the surfactant
The mechanism of wetting
Wetting is the readiness of a liquid to bond to the surface of a
solid. The process is governed by the relative surface energy of
the solid and the surface tension of the liquid.
Surface energy is the energy possessed by the atoms at the
surface of the solid, whereas surface tension is the result of the
attraction between molecules at the liquid surface. For a liquid
to wet a solid, the surface energy of that solid must be able to
overcome the surface tension of the liquid, thus breaking the
surface tension and forming a permanent film that will bond to
the surface.
Any change in conditions can upset the balance between a
suitable surface energy and surface tension. For example,
simply changing the temperature of the liquid will have an
effect on the ratio between surface energy and surface tension.
A more marked effect would be seen if the surface of the
hydrophone became contaminated with grease or oil, even
from simply touching the device. The grease dramatically
lowers the surface energy of the hydrophone (the solid),
potentially to the point where the surface energy is insufficient
to break the surface tension of the liquid.
A surfactant can also be used to aid the
wetting process. A surfactant (surface active
agent) when added to a liquid will reduce the
surface tension of the liquid, thus improving
its wetting abilities.
For practical purposes, simple household
washing up liquid makes for a good surfactant
to use on hydrophones. It is readily available,
cheap, non-toxic and reasonably mild in
strength.
The most suitable way to apply it is to make a
mix of water and washing up liquid of about
the same strength as when washing household
crockery and apply it to the hydrophone with a
soft to medium brush. Once applied the device
should be placed into the measurement water
straight away. This will prevent a thin film of air being trapped between the device and the water. It
can be used in its neat form to clean the
Function of a surfactant
device, but should then be washed off.
A surfactant consists of a molecule with two parts, a water-soluble
portion and an oil-soluble portion. The oil-soluble portion (the
hydrophobe) is a hydrocarbon chain usually consisting of between 8
and 18 carbon atoms. The water-soluble portion is called the
hydrophile. There are four distinct varieties of surfactant, these being
anionic, cationic, nonionic and amphoteric, the difference being in the
charge of the hydrophilic head: anionic has a negative charge;
cationic a positive charge; nonionic has no charge; and amphoteric
can exhibit both charges
When the surfactant is added to water, it will diffuse throughout its
volume. The shorter the hydrocarbon chain is, the more readily the
molecules can diffuse to the surface. Once at the surface a bond can
be made between the water surface and the solid surface, with the
surfactant acting as the interface, effectively reducing the surface
tension of the liquid.
Although a short hydrocarbon chain allows for rapid diffusion to the
surface, the bond it forms is quite weak and easily washed away. A
very long chain, although able to make a strong bond, can take too
long to diffuse for many applications.
It is not recommended that the
surfactant be added directly to the water
without first seeking expert professional
advice
and
specialist
products.
Although detergents often contain a
surfactant, they can also contain other
ingredients such as foaming agents.
When applied to the device in the
manner described above, these other
ingredients present few problems.
However, applied to a much larger
volume of water it is likely cause
significant problems due to the action
of the foaming agents, especially if the
water is agitated (for example by the
pumps in re-circulation and filtration
systems).
Temperature stabilisation
Another problem associated with wetting that can occur is gas coming out of solution and clinging to
the hydrophone surface whilst submerged. This can happen if there is a large enough temperature
difference between the water and the device under test. If the device is warmer than the medium when
submerged, the water in close proximity to the hydrophone will be heated slightly, the rise in
temperature causing dissolved gas to come out of solution and form small bubbles. These bubbles can
then stick to the hydrophone and cause measurement problems. This problem can be avoided by presoaking the device in the test tank (or in water which is at the same temperature as that in the tank).
The length of time required for pre-soaking will depend on the device under test and the temperature
difference, but at least half an hour soaking before the start of measurements is advisable (though
some devices may require much longer). It is recommended that the soaking time be reported along
with the results of a calibration.
Effect on measurements
Well Wetted
Badly Wetted
Sensitivity (dB re 1V/µPa)
.
-200
-205
-210
-215
-220
-225
30
40
50
60
Frequency (kHz)
70
80
The adjacent figure illustrates the
impact wetting can have on
calibrations. In the figure, results
are shown of two calibrations of
the same device. The blue curve
shows the results with the
hydrophone fully wetted, and the
red curve shows the results
obtained when the surface of the
hydrophone
was
severely
contaminated by surface grease.
In general, wetting effects may
vary with frequency and often the
effects are time dependent,
leading to measurements that are
unstable with time.
Also shown is a polar plot of the directional response of a hydrophone measured before and after
temperature stabilisation. The hydrophone was initially placed into the tank without being wetted with
detergent and measurements were made without allowing the temperature of the device to stabilise.
Bubbles were clearly visible on the surface of the hydrophone boot when viewed through the viewing
window of the tank, and the polar response measurement resulted in the blue curve shown. The
bubbles were brushed away and the hydrophone was allowed to soak for some time before the
measurements were repeated, resulting in the green curve shown. The red curve is the result of
measurements made on a wetted hydrophone which had been soaked in the tank to allow the
temperature to stabilise fully, and these results represent the expected directional response of the
hydrophone. It is clear that bubbles clinging to the hydrophone can significantly affect the measured
response.
For further information, contact:
Justin Ablitt
Centre for Mechanical and Acoustical Metrology
National Physical Laboratory
Queens Road
Teddington
Middlesex TW11 0LW
United Kingdom
Tel:
020 8943 6695
Fax: 020 8943 6217
Email: [email protected]
Or, why not visit the NPL Acoustics web pages at:
http://www.npl.co.uk/npl/acoustics/index.html