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Old 14-09-2018, 09:10   #1
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Depth sounder reading really shallow depths when itís much deeper

So we used to have a NASA Clipper depth unit and in-hull transducer. Recently upgraded to a Raymarine p319 thru-hull as part of a system pack.

Both exhibited similar behaviour as follows:
Depth reading are fine until the actual depth gets to about the limit of the transducerís ping (for the NASA this was ~100m and for the new p319 itís ~140m), at which point they will read out ridiculously shallow depths (anywhere between 2-10m).

Whatís going on here? Itís like the received ping is so weak the transducer is getting confused?

Is there a way to filter this out somehow? Iíd rather it stopped displaying depth at all than give me false read outs that set the heart racing!
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Old 14-09-2018, 09:27   #2
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Re: Depth sounder reading really shallow depths when itís much deeper

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Whatís going on here? Itís like the received ping is so weak the transducer is getting confused?!
Yes, managed to get a lowrance to display depths to 130m off the coast of N Ireland but it would loose the signal beyond that. We can read the depth to 0.6m at which point not only are we aground but the transducer is in air as the water drains away.

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Old 14-09-2018, 09:46   #3
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Re: Depth sounder reading really shallow depths when itís much deeper

Noise from.bumbles
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Old 14-09-2018, 22:49   #4
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Re: Depth sounder reading really shallow depths when itís much deeper

I donít mind the depth maxing out. The issue is that when we are in roughly the ďmax depthĒ range, the readings become very shallow.

Eg motoring along yesterday in about 120m of water, the sounder is jumping around between 2-6m on the display. Itís obviously wrong so I would rather it just displayed ď- - -ď

Itís not bubbles. It happens very specifically at these depths.
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Old 14-09-2018, 23:23   #5
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Re: Depth sounder reading really shallow depths when itís much deeper

One explanation is that as the depth increases, the amplitude of the return pulse is diminished, so the instrument turns up the gain on the receiver to compensate. Eventually the signal is lost, but the gain is maxed out, and now it starts responding to some other (weaker) reflected pulses, perhaps one from a thermocline near the surface... you know, the one you dive through while snorkeling and suddenly the water gets a lot colder. Such layering can cause reflection of the sonic pulse just like the bottom can. ( I read the Hunt for Red October so I'm an expert!).

Some sounders have settings to help avoid this phenomenum, so check your manual.

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Old 14-09-2018, 23:39   #6
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Re: Depth sounder reading really shallow depths when itís much deeper

Iíve only seen the movie so Iíll take your word for it!

That sounds like it may be whatís happening. Will dig around in the manual to see if there is a fix.

Cheers
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Old 15-09-2018, 02:28   #7
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Re: Depth sounder reading really shallow depths when itís much deeper

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One explanation is that as the depth increases, the amplitude of the return pulse is diminished, so the instrument turns up the gain on the receiver to compensate. Eventually the signal is lost, but the gain is maxed out, and now it starts responding to some other (weaker) reflected pulses, perhaps one from a thermocline near the surface... you know, the one you dive through while snorkeling and suddenly the water gets a lot colder. Such layering can cause reflection of the sonic pulse just like the bottom can. ( I read the Hunt for Red October so I'm an expert!).

Some sounders have settings to help avoid this phenomenum, so check your manual.

Jim


We see exactly the same behaviour, once it goes beyond about 130m it starts reading 0.2m. I have had the same theory as you about it and it makes a lot of sense.
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Old 15-09-2018, 10:31   #8
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Re: Depth sounder reading really shallow depths when itís much deeper

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Originally Posted by benzy View Post
So we used to have a NASA Clipper depth unit and in-hull transducer. Recently upgraded to a Raymarine p319 thru-hull as part of a system pack.

Both exhibited similar behaviour as follows:
Depth reading are fine until the actual depth gets to about the limit of the transducerís ping (for the NASA this was ~100m and for the new p319 itís ~140m), at which point they will read out ridiculously shallow depths (anywhere between 2-10m).

Whatís going on here? Itís like the received ping is so weak the transducer is getting confused?

Is there a way to filter this out somehow? Iíd rather it stopped displaying depth at all than give me false read outs that set the heart racing!
Put your Fishing Line overboard and start jigging,you might have a Fish Ball under the boat that reflexes the sonar of your Saunder
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Old 15-09-2018, 11:05   #9
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Re: Depth sounder reading really shallow depths when itís much deeper

All depth sounders max out.

Why on earth do you find this a problem?

If the chart shows that you are in 500m of water you do not need to know if it is 499 or 501. Unless you are a complete pedant. Or unable to read a chart.
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Old 15-09-2018, 11:58   #10
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Re: Depth sounder reading really shallow depths when itís much deeper

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All depth sounders max out.

Why on earth do you find this a problem?

If the chart shows that you are in 500m of water you do not need to know if it is 499 or 501. Unless you are a complete pedant. Or unable to read a chart.


Itís not the maxing out thatís an issue, itís the reporting of very shallow depths when it has maxed out, and in our case setting off alarms in the cockpit and below.
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Old 15-09-2018, 12:28   #11
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Re: Depth sounder reading really shallow depths when itís much deeper

I recently upgraded my sonar and transducer to CHIRP technology (in my case, Garmin ClearVue). The increase in resolution and capability is astounding. Penetrates much deeper, provides much great detail, rejects false returns and harmonics. This is an upgrade I would recommend to anyone.
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Old 15-09-2018, 12:47   #12
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Re: Depth sounder reading really shallow depths when itís much deeper

A couple of years ago we were sailing well west of the 1.5 km exclusion zone around Kick-em-Jenny Volcano in the Windwards. Our depth gauge showed ---, as expected, since the charts indicated >1000 foot depths. Suddenly, we saw a reading of 100 ft, which quickly rose to 50 and then 20 feet. But we were supposedly in really deep water, and I'm quite confidant of our position fix. WTF? We noticed that there were breaking waves all around us. Needless to say, we headed out into the Caribbean sea. Was it volcanic activity? A new island being built? No other cruisers reported anything unusual. Very strange.
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Old 15-09-2018, 13:56   #13
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Re: Depth sounder reading really shallow depths when itís much deeper

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All depth sounders max out.

Why on earth do you find this a problem?

If the chart shows that you are in 500m of water you do not need to know if it is 499 or 501. Unless you are a complete pedant. Or unable to read a chart.


Well I hope your ability to read a chart is better than your ability to read a post. Itís the false shallow depth readouts which are the issue.

Iíll post to the Raymarine forums to see if there is anything they can suggest.
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Old 15-09-2018, 15:42   #14
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pirate Re: Depth sounder reading really shallow depths when itís much deeper

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A couple of years ago we were sailing well west of the 1.5 km exclusion zone around Kick-em-Jenny Volcano in the Windwards. Our depth gauge showed ---, as expected, since the charts indicated >1000 foot depths. Suddenly, we saw a reading of 100 ft, which quickly rose to 50 and then 20 feet. But we were supposedly in really deep water, and I'm quite confidant of our position fix. WTF? We noticed that there were breaking waves all around us. Needless to say, we headed out into the Caribbean sea. Was it volcanic activity? A new island being built? No other cruisers reported anything unusual. Very strange.
I saw the same thing when I went close to kick em Jenny
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Old 15-09-2018, 16:22   #15
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Re: Depth sounder reading really shallow depths when itís much deeper

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Originally Posted by Jim Cate View Post
One explanation is that as the depth increases, the amplitude of the return pulse is diminished, so the instrument turns up the gain on the receiver to compensate. Eventually the signal is lost, but the gain is maxed out, and now it starts responding to some other (weaker) reflected pulses, perhaps one from a thermocline near the surface... you know, the one you dive through while snorkeling and suddenly the water gets a lot colder. Such layering can cause reflection of the sonic pulse just like the bottom can. ( I read the Hunt for Red October so I'm an expert!).

Some sounders have settings to help avoid this phenomenum, so check your manual.

Jim
I like your theory Jim. Another thing to check out is second trace returns.
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