From: Quarterdeck Volume 5, Number 1, Spring 1997

a-c meter

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1. As the a-c meter descends, water is pulled into two tubes. One measures light absorption (a) and one measures attenuation (c). In each tube, the meter projects a beam of light with a wavelength that rotates among three values.

2. The attenuation tube determines how much light is absorbed and scattered by particles. To do this, a detector at the end of the tube measures how much of the original light beam remains after it passes through the seawater inside. To make sure the detector does not measure any light that has hit particles and bounced off, the tube walls are black and therefore absorb scattered light.

3. The absorption tube only determines how much light is absorbed by particles. Its detector measures how much light is left of the original beam, including any light that has bounced off particles. To ensure the detector receives all scattered light, the tube walls are lined with a quartz mirror that reflects scattered light toward the detector.

4. A small pump pulls seawater through the tubes then ejects it back into the water column.

5. The pressure case houses electronics and a microprocessor to collect data and transmit them to the surface.

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