Effect of mixing in continuous ocean sampling systems

Abstract

Small-scale variability of many properties in the ocean can best be studied by pumping water through a hose to analytical systems on board a ship. Sampling, however, tends to obliterate small-scale features of the resulting data. The effect of this process can be assessed using the theory of systems analysis. This treats the sampling system as a linear filter acting on the inlet concentration stream to produce a filtered output. The sampling system consists of a centrifugal pump coupled to a long hose. Impulse response functions are derived for the pump and for the hose. Frequency response functions are then derived for the pump and for the hose. The frequency response of the sampling system is the product of the frequency response functions of the individual components. The system gain function is transposed to a environmental scale by dividing frequency into the velocity of the pump inlet through the water. For sampling from a ship while underway, the spatial resolution at the analyzer is decreased in direct proportion to the ship's speed and the length of the hose. Resolution is increased in proportion to the square root of the pressure generated by the pump. Pump internal volumes substantially reduce resolution when they approach 2% of the volume of the hose.