Depth instrument giving incorrect reading

[SeaDancer]

At times when my depth reads for 350 feet (example in deep water ) , I know I am in 350 feet of water, my reading on my instruments might go to 4 feet or 7 feet, then it will go back to 350 feet what it should be. I thought my instruments might not be working correctly but it happened to another boat that was with me over the weekend. Does anyone have an explanation for this.

[sailorboy1]

something floated under, like a plastic bag or seaweed

[Tayana42]

Sometimes fish, sometimes bubbles and sometimes multiple echoes from the bottom can cause those responses.

[sailormed]

This is inherent to echo sounders. They send out a pulse and wait for the echo coming back. If the water is very deep the echo comes back just after sending out the second pulse.

It then indicate depth based on the time difference between the second pulse send and the first pulse coming back. After all a pulse is a pulse for that machine.

Of course you can get false echos also from water layers of different temperature or salinity, etc.

[geoff3nebel]

What make of depth sounder?

[dennismenace111]

Bubbles! Air! You passed a wave that swept air under your hull or bubbles from some idiopathic source. Very common in sailboats where our transducers are mounted toward the bow! Nothing to worry about!

[Frugal]

I have noted that when sailing in DEEP water my depth sounder indicates a no echo situation, it still shows the last recorded depth. Usually that reading varies up to about 10 meters and (hopefully) always from a false echo.
Flotsam, fish, turbidity, temperature lenses, going astern seem to give you a false reading.

[Redline452]

Originally Posted by sailormed

This is inherent to echo sounders. They send out a pulse and wait for the echo coming back. If the water is very deep the echo comes back just after sending out the second pulse.

It then indicate depth based on the time difference between the second pulse send and the first pulse coming back. After all a pulse is a pulse for that machine.
.

This. I used to see this regularly passing a 400-foot contour in Lake Champlain... after showing 399, the depth would steadily count down to 10' (when it was actually 410) because each echo came back just after the following pulse was sent out. Raymarine ST60. Always the same behaviour in the same place. It was weird/scary the first time, after that we'd just say that Nessie swam up to scratch his (her?) back on our keel.


A slightly more intelligent depthsounder would reduce the pulse rate (increase the interval between) as dept increased, but after 400', it wasn't much of an issue.



Radar units do something similar, changing pulse rate and duration with changes in range scale; the difference is that the range is changed manually, whereas most depthsounders do not have a "range" setting.

[ramblinrod]

Originally Posted by sailormed

This is inherent to echo sounders. They send out a pulse and wait for the echo coming back. If the water is very deep the echo comes back just after sending out the second pulse.

It then indicate depth based on the time difference between the second pulse send and the first pulse coming back. After all a pulse is a pulse for that machine.

Of course you can get false echos also from water layers of different temperature or salinity, etc.

I disagree.

If the sounder has a max software limited range of 600 ft, and at 350 it occasionally drops to 4 feet and then returns to 350 ft, the cause is most likely loss of echo. This causes the AGC (automatic gain control) to increase.

This enables the receiver to see the transducer ring down at the end of the blanking period (duration the receiver is rendered inoperative after the transmit pulse).

Which is likely the duration of an echo from 4 ft in your case.

So in normal operation when there is a strong echo from a hard close bottom (say 50 ft), the AGC is low and the transducer ringdown at the end of blanking is less significant than the true echo which is correctly processed and distance reported.

As depth increases or bottom produces a weaker echo, the AGC increases until bingo, ringdown detected and minimal distance reported.

If the false echo was due to any other cause (debris, multiple echoes, echo from previous transmit cycle) the related distance would be random, and not always 4 ft.

[Nicholson58]

False echo is most likely as noted by other posts. Is the transducer window clean?

[conchaway]

I have a gadget that NEVER gives an inaccurate reading. It's called a "lead line".

[Martkimwat]

I have seen this a few times in depths that are well within the normal range of my depth sounder (20~30metres or so) it is usually caused by a thermocline, a marked temperature difference in the water at depth. During WWII , and I expect even in this day and age, submarines used to take advantage of them to sit undetected by sonar below the sharp temperature change which the sonar bounces off.

[ramblinrod]

Originally Posted by Martkimwat

I have seen this a few times in depths that are well within the normal range of my depth sounder (20~30metres or so) it is usually caused by a thermocline, a marked temperature difference in the water at depth. During WWII , and I expect even in this day and age, submarines used to take advantage of them to sit undetected by sonar below the sharp temperature change which the sonar bounces off.

Thermoclines occur at various depths.

[Shrew]

Not limited to sailboats and not limited to deep water. Every boat I've owned has done this. 25......24......24....25.......4......24....25.

I'll zoom in on the chartplotter and look at contour lines, there is nothing at 4-5 anywhere near there. This occurs in heavily marked and travelled. If there were an obstruction someone would have hit it by now.

I assume it is a clump of seaweed or a large fish or a false echo. Usually whenI get a weird reading it also flashes as if even the gauge is confused by the reading.

[Mr B]

Originally Posted by conchaway

I have a gadget that NEVER gives an inaccurate reading. It's called a "lead line".

I would like to see that working when the bottom is 12,000 feet, Hahaha