NASA EML-2, Manual

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INTRODUCTION.
The EML-2 log consists of a through hull sensor and an interface box which, when
correctly installed, outputs boat speed through the water, trip distance and total
distance. The unit is factory set but has the facility for final in situ calibration.
The output, which updates twice a second, is in the NMEA-0183 format using the
sentences VHW and VLW. The trip distance is reset to zero each time the power is
turned off while the total distance is cumulative and cannot be reset. The EML-2 can
be used with the NASA Marine Clipper or Target EM log display or any other
compatible display.
The EML-2 log is designed for use in GRP hulls.
HOW IT WORKS.
The EML-2 generates an alternating magnetic field in the water below the sensor. As
the sensor passes through the water it generates an alternating electric field whose
amplitude is proportional to the boat's speed through the water. This field is detected
by small stainless electrodes on the underside of the sensor. The interface box then
amplifies this signal and converts it into speed and distance readings. An LED on the
interface box flashes each time new NMEA data is sent out.
CHOICE OF SENSOR LOCATION.
The sensor must remain in non-turbulent water flow at all times. Choose a position as
close to the centre line as possible where the water flow is unrestricted and where the
sensor is accessible for cleaning. Avoid areas around devices which could cause
electrical or magnetic interference.
Ideally the sensor should be: -
- close to the boat's centre line.
- as vertical as possible.
- clear of the keel.
- clear of sources of electromagnetic interference.
- always immersed, no matter what the heel.
- internally accessible for cleaning.
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