How Provbio-B Works - Kannad PROVBIO-B Manuel Utilisateur

USER MANUAL / MANUEL UTILISATEUR

8. HOW PROVBIO-B WORKS

Movement of the float through its profile is accomplished by a pump and valve system. The pump transfers
oil from the inner reservoir to the outer bladder. Oil moves back to the reservoir when the valve is opened-
-driven by the difference between the float's internal and external pressures.
As seen in figure below, the float's speed of ascent oscillates. This oscillation is due to the way in which the
float's controller regulates its speed. The controller, using depth measurements from the float's pressure
sensor, calculates the change in depth over a set period of time. With this information, the controller
determines the float's speed.
Figure 6: Display of speed of a PROVBIO-B float as it ascends in a deep-sea test
When ascending, if the calculated speed is lower than desired, the pump is activated for about 10 seconds,
pumping oil into the outer bladder. This produces an increase in buoyancy, which increases the speed of
ascent.
As the float rises to shallower depths, its buoyancy decreases, causing the ascent speed to also decrease.
When the calculated speed is too low, the pump is activated again.
This cycle repeats until the float reaches the surface.
The same regulating method is used to control the float's descent speed, by opening the valve and allowing
oil to flow from the external bladder to the internal reservoir.
Why does PROVBIO-B's speed decrease as it ascends?
The buoyancy of a float is determined principally by its mass and its volume, but another factor, hull
compressibility, also plays an important role. As PROVBIO-B ascends, the decrease in water density
reduces the float's buoyancy. At the same time, the decrease in water pressure causes PROVBIO-B's hull
to expand, which increases the float's buoyancy. The two effects tend to counteract each other.
Because PROVBIO-B's compressibility is actually less than that of sea water, the decrease in buoyancy due
to decreasing water density is greater than the increase in buoyancy due to hull expansion. This causes
PROVBIO-B's speed of ascent to decrease as it rises in the water column.
Conversely, as the float descends, the increasing water density increases the buoyancy more than the
decreasing buoyancy from hull compression. This causes PROVBIO-B's speed of descent to slow as it goes
deeper (see Figure 7: Example of graph of PROVBIO-B's descent speed versus depth in an actual
deployment).
PROVBIO-B
PAGE : 29 / 72
DATE : 26/03/2008
INDEX : A
REF. : DOC08018