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  SONAR Displays Inaccurate Depth

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Author Topic:   SONAR Displays Inaccurate Depth
nekoma1948 posted 06-09-2008 11:40 AM ET (US)   Profile for nekoma1948   Send Email to nekoma1948  
My unit displays erratic and inaccurate depths. This is quite a concern for safety purposes. What is the problem and how can it be corrected?
jimh posted 06-09-2008 08:14 PM ET (US)     Profile for jimh  Send Email to jimh     
SONAR works on the principle of measuring the time between transmission of a sound pulse and return of an echo off the bottom. If your unit is inaccurate, the problem is probably one of improper selection of the bottom echo to use in the measurement. Modern echo sounders make automatic judgments about which echo is the bottom echo. If they pick the wrong echo the measured depth will be inaccurate.
jimh posted 06-09-2008 08:46 PM ET (US)     Profile for jimh  Send Email to jimh     
As for advice on how to correct the erratic and inaccurate readings on your SONAR echo sounder display, you will most likely have to provide the unit with a stronger and more clearly defined bottom echo. One way to affect the bottom echo reception is by the orientation of the transducer. Is there any way for you to observe the bottom echo signal on this device of yours? You have given us very little information or detail about it, which is why the reply has been in such a vague and general manner.
davej14 posted 06-09-2008 09:25 PM ET (US)     Profile for davej14  Send Email to davej14     
It would also be useful to know what depths you are trying to measure and your expectation of the accuracy.
jimh posted 06-09-2008 09:42 PM ET (US)     Profile for jimh  Send Email to jimh     
Regarding the accuracy of depths measured by echo sounding, there are two additional factors which influence:

--the accuracy of the time measurement
--the speed of propagation of sound in water

We allow that the speed of propagation of sound in water is generally in the range of 1,500 M/second

Cf.; http://en.wikipedia.org/wiki/Speed_of_sound#Seawater

In other word, the time it takes sound to travel one tenth of a foot in water is only 0.000020512821 seconds. These are rather short slices of time, so short, for example, that light only travels about 6,300-feet in that same interval. So when one gets depth soundings with a resolution of 0.1-feet, they are getting rather precise measurements of time.

Regarding the speed of propagation of sound in sea water, this is a variable, but in most recreational echo sounders a fixed value is used. This affects the calibration of the unit. Actual speed of sound varies with salinity, temperature, and depth. Most recreational echo sounders do not offer any calibration or adjustment for these variables. This will also influence accuracy of echo sounding measurements.

Frank O posted 06-09-2008 11:04 PM ET (US)     Profile for Frank O  Send Email to Frank O     
Which unit do you have, and what kind of inaccuracies are you observing?

I have a Lowrance LCX-25C that seemed to work fine at first. After a few months, though, I noticed that when I entered water deeper than 150 feet or so, all of a sudden the depth would read 1.3 feet (off by more than 100x). Lowrance sent me a few transducer to try, but I noticed the same issue with it. Perhaps there is an adjustment I can make to resolve this, but since I rarely run in deep water (deeper than I can dive in, anyway), it's not really a pressing problem for me.


towboater posted 06-09-2008 11:58 PM ET (US)     Profile for towboater  Send Email to towboater     
Id bet the problem is either:
1. turbulance. make sure transducer is secure to the hull and touching the water where it is supposed to. sometimes the transducers pop out of the mounting brackets.

2. a wire is pinched or frayed. check the wiring and clean the units connections...especialy the ground loop.

3. the transducer is malfunctioning. First, get a price quote from a repairman who can diagnoss. I expect his quote to exceed $100...whereas a new transducer will probably only cost $60.

IF the new transducer does not fix the problem, your monitor has issues. Sell your used transducer for $25 and buy a new monitor with same pin connectors (usually 5) and frequency.

Double check 1 & 2.

mk

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