Calibrating the MOCNESS Flow Meter

The Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) is a mainstay in the plankton sampling on this Ocean Acidification cruise.  The system carries nine nets that are triggered via a shipboard computer, allowing us to sample different depth layers in the ocean. The system also carries a suite of environmental sensors that measure pressure (from which we calculate depth), temperature, conductivity (from which we calculate salinity), frame angle, and water flow past the net.  The frame angle and water flow are used to calculate the volume of water filtered by each sequentially opened net. The flow meter is mechanical and has a propeller driving a gear shaft that rotates a cylinder with an embedded magnet that moves past an underwater reed switch. With each 360 degrees the magnet passes by the switch causing it to close for a moment, which is recorded as a flow count.  Each flow count thus represents a distance traveled by the flow meter. Quantifying this distance requires calibrating the flow meter.

Leo, Gareth, and Alex on the fantail preparing the MOCNESS for the flow meter calibration (Photo: P. Wiebe)

On each cruise, calibrating the flow meter is done by deploying MOCNESS to a shallow depth and then towing it horizontally back and forth on a trackline over a prescribed distance. We did this on 11 August at our test station.  To calibrate the flow meter, we towed the MOCNESS at a depth of approximately 50 m for a distance of about one half a nautical mile from southeast to northwest (a course of 340 degrees) and then in the reverse direction (160 degrees) the same distance in order to eliminate any effect of differential current flow on the calibration.

Map showing the MOCNESS flow meter calibration runs

During the tow, the MOCNESS software logged the data coming from the underwater unit and the GPS latitude and longitude positions. The bridge called down start and end marks for each half-mile segment and the MOCNESS operator wrote down the flow count number at each of the marks.  We then used a custom Matlab program to compute the distance for each run and to make a plot of the tow segments superimposed on the total tow.  The calibration coefficient determined on this cruise ended up indicating that the flowmeter moves a distance of 6.3425 m during each count, a number that closely matches the coefficients determined on two earlier cruises. We will use the average of the three calibrations as the calibration coefficient for the MOCNESS tows taken on this cruise.

Peter Wiebe