Specifications

36
Figure 5.4.2. Examples of the fluid level of the stabilized platform and
vertical accelerometer sensor: (a) fluid level too low, (b) fluid level sufficient.
5.4.5 Sensor fluid and temperature
It has been written repeatedly that the accelerometer based buoy must not be stored below
temperatures of −5 ºC. This is determined by the freezing temperature of the fluid surrounding
the stabilized platform. While the buoy is deployed at sea, the temperature of the sensor fluid T
s
mainly depends on the water temperature T
w
and to a certain extent on the air temperature T
a
,
approximately as
)(1.0
waws
TTTT −+= (5.4.1)
This will limit the use of the buoy near the north and south poles.
In practice, it may be inevitable to expose the buoy to temperatures lower than −5 ºC. On
a short time scale this is acceptable as long as you consider the following. The time constant τ of
heat transfer from the sensor to the outside is about τ = 70 hrs. Starting from the initial
temperature of the sensor T
so
and the air temperature T
a
to which the buoy will be exposed, the
time it will take to cool down the sensor fluid to −5 ºC may be calculated
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−−
−
=
−
a
o
aso
C
TC
TT
t
o
5
ln
5
τ (5.4.2)
All temperatures are in degrees Celsius. As long as the exposure time remains below the
maximum of t
−5 ºC
the exposure to the low temperature is acceptable.