Page 329 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

acoustic sender
receiver pair. The sender transducer transmits at a known frequency, and then the receiver measures the shift in that frequency based upon the flow through the meter. This method works well in homogeneous pure liquids but is highly attenuated by dissolved or suspended gas within the liquid due to multipath reflection of the gas
liquid bubble interface.
There are other flow sensor technologies available and more are coming onto the market as the march of technology steadily progresses.
Level sensors are quite prevalent in the subsea industry. These make use of any number of methods to sense fluid levels in various mediums. Some examples of level sensor applications are as follows:
1. Tank level indicators for measurement of potable water, fuel, or chemical levels in storage tanks
2. Hydrocarbon level measurement in sunken ships for environmental remediation (e.g., the diesel level in the tanks of the Deepwater Horizon was measured before removal after the rig sank in April 2010)
3. Fluid transfer control in various applications.
Sensors for this application fall loosely into the following categories:
a. Hydrostatic—this instrument measures differential specific gravity of a liquid in order to sense the level at various points within the sample fluid, thus deducing the level by accurately measuring the depth at the sensor (located at a known position).
b. Ultrasonic—frequency shifting under the same principles as ultrasonic flow above.
c. RF capacitance—the measurement of capacitance between the sensor and the tank/vessel wall
as the tank fluid level changes.
d. Magnetostrictive—in magnetostrictive materials, an external magnetic field can be induced that
reflects electromagnetic waves in a waveguide, which are then metered for level measurement.
e. Through-air radar—divided further into “frequency-modulated continuous wave” radar and
pulsed wave time of flight, these methods measure the air space above the liquid in a known
volume and reverse compute the liquid volume.
f. Guided wave radar—pulses of energy are transmitted down a waveguide (e.g., a rod or cable)
and the dielectric constant changes at the air
liquid interface, reflecting part of the signal back
to the transmitter and thus computing the distance to the air
liquid interface.
g. Air bubbler—a flow of air is pumped down a submerged tube to its exit point. The pressure of
the air injected into the tube at the surface is proportional to the exit point density and the pressure at depth, thus very accurately measuring the depth of the exit point due to the hydrostatic pressure on the water column evacuated from the tube. This sensor is used in the gas diving industry (termed a “pneumofathometer” or, more commonly, the abbreviated “pneumo”) for very accurate depth readings.
h. Radiological neutron backscatter—a neutron emission source transmits neutrons through a containment vessel and measures the opposite side for backscatter from either liquid or gas, thus deducing the interface point for level measurement due to differing backscatter characteristics between different compounds (e.g., diesel fuel and seawater).
i. Heated junction thermocoupler—the principle behind thermocouplers (Figure 12.18) is that dissimilar metals conduct varying degrees of current dependent upon the temperature
12.2 Sensor categories 321