Inaccurate depth readings on Lowrance LCX-25C during red tide

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Author Topic:   Inaccurate depth readings on Lowrance LCX-25C during red tide

Frank O posted 06-11-2007 04:49 PM ET (US)
I have a Lowrance LCX-25C fishfinder/GPS on my 1995 Outrage 21, with the dual-frequency transducer mounted on the transom. This appeared to operate normally for the better part of a year after I installed it.
Recently, however, I've noticed inaccurate depth readings at depths greater than 120 feet or so. Sometimes at these depths the reading will all of a sudden become smaller by a factor of 100 -- the depth reports a depth, for example, of 1.9 feet.
One possible factor that I've been wondering about is that we've been experiencing a heavy red tide lately -- the water looks like root beer in many spots.
I emailed Lowrance, and they told me that (a) yes, the red tide might be a contributing factor, and (b) this especially happens with the sonar set at the 200 kHz frequency, which is the normal one used for depths shallower than 300 feet or so.
I guess I'll wait out the red tide, but I'm curious -- has anyone else ever encountered this phenomenon?

jimh posted 06-11-2007 08:24 PM ET (US)
On our old sailboat we used to often see a situation where the depth sounder would give false readings when the depth of water was over 200-feet. My theory was that as the water depth increased the sounder was increasing the gain in order to pick up the bottom echo. At some point, the gain increased enough so that it began to hear an echo off the edge of our own keel. We would be sailing along in 200-feet of water, then out of the blue (literally) the alarm would sound and the sounder would show us to be in about 4-feet of water (or the depth of our own keel). You may be experiencing something similar.

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Author	Topic: Inaccurate depth readings on Lowrance LCX-25C during red tide
Frank O	posted 06-11-2007 04:49 PM ET (US) I have a Lowrance LCX-25C fishfinder/GPS on my 1995 Outrage 21, with the dual-frequency transducer mounted on the transom. This appeared to operate normally for the better part of a year after I installed it. Recently, however, I've noticed inaccurate depth readings at depths greater than 120 feet or so. Sometimes at these depths the reading will all of a sudden become smaller by a factor of 100 -- the depth reports a depth, for example, of 1.9 feet. One possible factor that I've been wondering about is that we've been experiencing a heavy red tide lately -- the water looks like root beer in many spots. I emailed Lowrance, and they told me that (a) yes, the red tide might be a contributing factor, and (b) this especially happens with the sonar set at the 200 kHz frequency, which is the normal one used for depths shallower than 300 feet or so. I guess I'll wait out the red tide, but I'm curious -- has anyone else ever encountered this phenomenon?
jimh	posted 06-11-2007 08:24 PM ET (US) On our old sailboat we used to often see a situation where the depth sounder would give false readings when the depth of water was over 200-feet. My theory was that as the water depth increased the sounder was increasing the gain in order to pick up the bottom echo. At some point, the gain increased enough so that it began to hear an echo off the edge of our own keel. We would be sailing along in 200-feet of water, then out of the blue (literally) the alarm would sound and the sounder would show us to be in about 4-feet of water (or the depth of our own keel). You may be experiencing something similar.