The RCM 9 - a new instrument for measuring Current Speed and other ocean parameters
by Ivar Aanderaa, Aanderaa Instruments Norway, USA and Japan
Ocean scientists have been studying ocean currents and other important ocean parame-ters for more than 100 years and are likely to continue to do so for at least another 100 years. This activity is therefore a task with a very long time horizon. For this reason equip- ment that is well suited for the task and that can find long lasting application is desirable. Aanderaa Instruments of Nesttun, Norway has manufactured over 15000 such instruments during the last 30 years and is working continuously toward design of new and better instruments for this task.
The RCM 9
The RCM 9 is a new instrument that opens up for many new uses in this field. These will shortly be described in this paper. The designa - tion RCM 9 is an acronym for Recording Current Meter Model 9. Although this instru- ment measures several parameters other than current speed and direction, the same letter combination used for the earlier RCM 4 and RCM 7 has been maintained since these instru- ments have been the most commonly used instruments for recording ocean currents for over three decades.
A basic feature of the RCM 9 is that it is a self- contained unit that can be left in the sea, at a depth to 2000 meters, for a period of up to two years. It monitors a series of parameters in sequence at regular intervals, stores the data obtained and at the same time transmits the data obtained by acoustic pulses to a receiving point up to about 1000 meters away.
The parameters measured are current speed and direction, water temperature, con-
ductivity, turbidity of water and the depth at which the instrument operates. Besides it has an extra channel for an optional sensor, for example for the amount of dissolved oxygen in the water or for another parameter of particular interest.
The current speed is measured by use of an acoustic Doppler shift prin- ciple. The use of this princi- ple measures the current speed at a distance of about 1 meter from the sensor itself, a feature that removes the disturbing effect of the instrument from the movement of the water. It further has the advantage that the sensor performance is not likely to be influenced by marine fouling and drifting seaweed.
