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Why does the depth on my instrument jump to 28 cm from 0?

Full Question: When I lower the sensor from a ship, then it is not easy to see how deep the instrument is below the surface. With waves and a moving ship, the sensor is kept at a depth so that it stays in the water. And subsequently, the profile goes down to >10 or 20 m. However, recently profiles were obtained in mesocosms, which are only 1.5 m deep and have no waves. When I look at the raw data of the profiles it seems that the moment the PUV goes into the water and the sensor is kept for temperature equilibration just under the surface, the depth is about 28 cm. Is this correct? Is this because the depth sensor is at the lower part of the PUV and the light sensor is at the top? I assume that such an offset would be corrected by the offset obtained during the calibration of the PUV. This 28 cm offset is insignificant for our deep profiles, however, for our shallow data, this becomes significant.

Answer: It's important to remember that the depth channel is actually a pressure transducer. You should always start your recordings with the instrument above the surface so that any "zero" changes caused by changes in atmospheric pressure will be recorded. The calibration in our darkroom does NOT account for baseline changes caused by atmospheric pressure or altitude. Yes, when working in shallow water you must take into account the difference in position of the pressure transducer and the top of the cosine collector. Fortunately, this is a constant.