Page 328 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition
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  320 CHAPTER 12 Sensor Theory
Thermal anemometers measure the transfer/removal of heat from a heated sensor (also referred to as “hot wire”) to the flowing fluid to compute flow based upon the heat transfer rate. These sen- sors are typically used in air flow measurements.
Differential pressure measurement (DPM) sensors are the most common type of liquid flow sen- sors currently used. The DPM sensor isolates the pressure drop through the meter. The pressure drop across the meter is proportional to the square of the flow rate (i.e., the square root of the pres- sure differential). Most of these types of measurement devices have multiple sensor stations for sensing differentials over a range of locations in order to mitigate the effects of local measurement variations for the single pick-up (i.e., measurement) point. DPM sensors can either measure veloc- ity pressure through tube insertion into flow (e.g., Pitot tube) or pressure drop across a restriction (e.g., Venturi tube).
Examples of these types of sensors are as follows:
1. The Pitot tube (measuring kinetic versus static pressures)—this device offers the least pressure drop from the incoming fluid stream. The most common use of this type of sensor is for airspeed on aircraft, but it is somewhat inaccurate compared to other devices. The instrument’s basic function measures both dynamic fluid flow pressure (from the oncoming fluid stream), then compares this to static fluid pressure (from the ambient/static fluid reservoir), and outputs the pressure differential. The sensing tube is subject to obstruction (as with the 2009 Air France 447 accident involving Pitot tube blockage due to ice), which blocks the sensing unit. Aircraft manufacturers typically use this type of sensor because of its simplicity, durability, and cost. Subsea vehicle engineers use this sensor for travel speed through the water column on high- speed vehicles and pipeline engineers for fluid flow speed through a pipe.
2. The Venturi tube (measuring pressure drop through a restriction)
3. The concentric orifice
4. The flow nozzle
Vortex-shedding sensors make use of the Von Karman principal by measuring the vortices shed
downstream of an object due to the eddying effect of turbulent flow. Vortex-shedding frequency is measured as it is proportional to the velocity of the flowing fluid, thus deducing the fluid volume by measurement of the velocity through a fixed volume.
Positive displacement flow sensors separate the fluid into measured volumes and then send the fluid on after precise volumetric measurement. This type of sensor is the most energy inefficient (as it requires mechanical sample separation), but it is the most accurate since it precisely measures the volume during each sample.
Turbine-based flow sensors force the fluid flow through a fixed volume that turns a turbine or propeller, which in turn spins at a rate proportional to the fluid flow rate. The turn rate is then mea- sured to derive the fluid flow.
Mass flowmeter sensors make use of the Coriolis effect to directly measure the mass of the fluid flowing through the meter. The fluid volume is vibrated at a known frequency as it passes through the sensor. The vibrating fluid induces a twisting moment into the volume of water that is then measured as torque in the piping. The torque is directly proportional to the mass flowing through the pipe and is a direct measurement of the fluid’s mass.
Ultrasonic flow sensors are either Doppler or time-of-travel/flight meters. With Doppler sensors, the meter measures the frequency shift caused by the fluid’s flow through the meter across an





















































































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