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Old 15-09-2018, 16:22   #16
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Re: Depth sounder reading really shallow depths when itís much deeper

That sounds to me as though there is a source of reflections of sound somewhere on your vessel which gives a very weak signal, but is close to the transducer. It could be a propeller, skeg, rudder tab, self-steering autovane, even a sacrificial anode close to the beam. As soon as the main signal is sufficiently weak that this extraneous reflection becomes comparable or is larger than the reflected signal from the sea bed, the sounder switches to measure the distance to the source of reflection, bearing in mind the angle of acceptance of the transducer.

Because many transducers are not set exactly vertically in the hull but are set normal to the curvature of the hull, this sort of thing might occur. My advice is to get into a calm anchorage and set up a point directly above the transducer using a plum-line. Mark this point. Check the vertical placement of the transducer housing against this. If it is not vertical, you should make it so.

Now, when out of the water, (such as when tide has ebbed if you can find a careening grid,. (the feel-gooders in Oz had them all dismantled) or the vessel out of the water one way or another, set the transducer to point DIRECTLY at the bottom and make sure that there are NO slight obstructions that could deflect the signal anywhere else, or deflect the returning echo to the transducer via some other part of ship's equipment..

Mounting a transducer alongside a propellor might produce some intermittent reflections, particularly if it is spinning.

I prefer to mount a transducer as far forward as is practical and clear of anything else. In my case I do not use through-hulls, I mount the transducer on top of a polycarbonate panel set into the hull. Polycarbonate has the same characteristic as sea water and is invisible to the transducer. It has a circle of PVC tube set vertically to the seabed and silicon glued to it giving me a perfect vertical. In this I mount the transducer in either clean salted water or a little light oil. The oil evaporates very slowly. In readings the difference is negligible.

There is another very faint possibility. The sounder is reading itself--not all of the sounds emitted by the crystal goes down into the water. Some comes out of the side of the transducer housing, and can bounce off anything at all and be picked up in the same way, through the housing.

The only way one could minimise this is to absorb the sound before it hits anything else--a plastic bucket with a heavily padded interior placed over the transducer would be interesting, to see if it made any difference. How it might affect the heating up of the transducer is hard to say. It would just be a way of finding out the source of a problem.


Reading the thermocline might make a difference too--but it is usually located at a depth down further than your false depth reading is indicating.
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Old 15-09-2018, 21:57   #17
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Re: Depth sounder reading really shallow depths when itís much deeper

Quote:
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!
What's happening is that there is very good signal transmission thru the water & reflection from the bottom and the path time to the bottom and back is just a bit longer than period between pulses.

Depthsounders send out a sound pulse and then listen for it to return. The depth of water is related to the time it take for sound to get to the bottom and return back to the boat. Equipment is usually designed to work up to a specific depth. In this case let's say 100m. Most recreational equipment maxes out at 100-200m. Let's say the quality of the electronics and sounder are such that a good signal is not going to come back at more than 75m most of the time. Speed of sound in saltwater is 1500m/sec. That means the path time to 75m and back is 0.1sec. The designers don't want confusing and spurious results like what you are getting so the use a pulse spacing of 0.133 sec which is equivalent to about 100m depth.

The problem is that most of the time is not all of the time. Sometimes water conditions and bottom conditions are good enough that a good signal will return from 100m or more down. That means that the depth sounder sends out pulse #1 but it doesn't come back until pulse #2 has been sent which resets the electronic timer that correlates with depth. The depthsounder hears the pulse #1 just after it sent pulse #2 and thinks it is hearing pulse #2. Because of the very short time between sending pulse #2 and hearing pulse #1 it thinks it's in shallow water.

When I learned to sail, I hung out on a friend's boat the used an old rotating light depthsounder. In good conditions we could use the depthsounder deeper than it's greatest rated scale by knowing that the pulse we were seeing was on the second spin of the light, we just added the max depth rating to whatever number we were seeing. So if it read 20' and it was set for 200' depth reading the depth was really 220'.
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Old 15-09-2018, 22:36   #18
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Re: Depth sounder reading really shallow depths when itís much deeper

The sounder is mounted just forward of the keel, and on an angle that faces slightly to port and slightly forward - itís not mounted straight down. The keel angles back fairly quickly, so itís also mounted ďawayĒ from the keel. Sometimes the false readings are 6 or 7 metres, so not sure this could come from the keel which is only a meter below the sounder?

Adelie, your pulse timing explanation makes sense. No idea if this can be helped or not?

It is a p319 Airmar transducer going into a Raymarine ITC-5 unit and displayed on an i70s screen. The only option in the settings I can find is about the depth offset. Will keep investigating.
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Old 15-09-2018, 23:31   #19
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Re: Depth sounder reading really shallow depths when itís much deeper

Quote:
Originally Posted by Adelie View Post
What's happening is that there is very good signal transmission thru the water & reflection from the bottom and the path time to the bottom and back is just a bit longer than period between pulses.

Depthsounders send out a sound pulse and then listen for it to return. The depth of water is related to the time it take for sound to get to the bottom and return back to the boat. Equipment is usually designed to work up to a specific depth. In this case let's say 100m. Most recreational equipment maxes out at 100-200m. Let's say the quality of the electronics and sounder are such that a good signal is not going to come back at more than 75m most of the time. Speed of sound in saltwater is 1500m/sec. That means the path time to 75m and back is 0.1sec. The designers don't want confusing and spurious results like what you are getting so the use a pulse spacing of 0.133 sec which is equivalent to about 100m depth.

The problem is that most of the time is not all of the time. Sometimes water conditions and bottom conditions are good enough that a good signal will return from 100m or more down. That means that the depth sounder sends out pulse #1 but it doesn't come back until pulse #2 has been sent which resets the electronic timer that correlates with depth. The depthsounder hears the pulse #1 just after it sent pulse #2 and thinks it is hearing pulse #2. Because of the very short time between sending pulse #2 and hearing pulse #1 it thinks it's in shallow water.

When I learned to sail, I hung out on a friend's boat the used an old rotating light depthsounder. In good conditions we could use the depthsounder deeper than it's greatest rated scale by knowing that the pulse we were seeing was on the second spin of the light, we just added the max depth rating to whatever number we were seeing. So if it read 20' and it was set for 200' depth reading the depth was really 220'.
A very good description of a second trace return. Pulse radars have the same issue in some conditions. Ranging out is the normal cure, or increasing the PRF (pulse repetition frequency) by going to long pulse.
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Old 16-09-2018, 00:17   #20
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Re: Depth sounder reading really shallow depths when itís much deeper

Quote:
Originally Posted by benzy View Post
The sounder is mounted just forward of the keel, and on an angle that faces slightly to port and slightly forward - itís not mounted straight down. The keel angles back fairly quickly, so itís also mounted ďawayĒ from the keel. Sometimes the false readings are 6 or 7 metres, so not sure this could come from the keel which is only a meter below the sounder?

Adelie, your pulse timing explanation makes sense. No idea if this can be helped or not?

It is a p319 Airmar transducer going into a Raymarine ITC-5 unit and displayed on an i70s screen. The only option in the settings I can find is about the depth offset. Will keep investigating.
Transducers also have a Side lobe where a bit of excess signal goes out the side not in the main beam. If the gain is set to high the side lobe can cause it to register the keel, or other object near the unit.
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Old 16-09-2018, 01:37   #21
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Re: Depth sounder reading really shallow depths when itís much deeper

I've seen this phenomenon as well with my Airmar P19. The second trace return makes a lot of sense to me.

Quote:
Originally Posted by Alita49DS View Post
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.
Yes, it doesn't matter what the chart reads. The prudent pilot trusts his instruments. Personally, if I'm offshore in any kind of swell, an error is immediate cause for concern.
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Old 17-09-2018, 06:41   #22
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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
Depth reading are fine until the actual depth gets to about the limit of the transducer’s ping (for the NASA this was ~100m and for the new p319 it’s ~140m), at which point they will read out ridiculously shallow depths (anywhere between 2-10m).

I've had that happen on a couple of brands (Standard Horizon and now Raymarine ST60) - what happens is that the round-trip ping time becomes greater than the sounder's pinging interval.

ping1... echo1... ping2... echo2


as depth increases


ping1....... echo1.ping2...... echo2


then with still greater depth


ping1.................ping2.echo1............ping3 .echo2......


But all pings "sound alike", so the sounder just sees


ping.echo......ping.echo.....



which looks like really shallow water. In reality the sounder sees the depth go from 400' (in my case, one spot in the middle of Lake Champlain) to 2' instantly, but the display averages it down more slowly.



The "cure" is to have the ping rate slow down at greater ranges, (akin to what radars do), but most (any?) don't have a "range" setting so it would have to be adaptive, something designed in by the manufacturer. Power-cycling the instrument *might* cause some to auto-calibrate their ping rate, but beyond that not much a user can do.


(posting before seeing Adelie's update, good explanation)
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Old 17-09-2018, 09:00   #23
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Depth sounder reading really shallow depths when itís much deeper

Iím betting side lobe, cause the second ping theory stabilizing at less than a foot would be very unlikely.
If you can adjust frequency, do so from 200 Hz to 50 Hz, the lower frequency gives a stronger but less detailed signal.

I have two depth finders, the Raymarine that just reads depth of course and you canít adjust frequency has an installed thru the hull transducer.
It will work to about 300í. When depth exceeds its capability it just displays last depth, with the word last on the screen to inform you itís not longer working.
Then I have the B&G plotter with a built in sounder that I installed a P79 shoot thru the hull transducer, now I would think logically that having to shoot through the fiberglass hull would have to reduce the range, but it works to about 1,500í or five times deeper than the Raymarine one.

I wanted the second as a back up, partially for redundancy, but also when weather gets real bad and we are taking water over the bow etc. enough air will get under the hull to stop the Raymarine one from working, and all I get is a depth that says ďlastĒ. When weather is this bad, it also churns the water up so you canít see the bottom, and I used to do a lot of sailing in shallow waters of the Gulf of Mexico.
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Old 17-09-2018, 09:30   #24
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Re: Depth sounder reading really shallow depths when itís much deeper

All of the above descriptions of the pinging behavior of sonar units describe the older technology single frequency or two frequency units. I advise all of you to read up on CHIRP sonar technology which broadcasts a broad signal of multiple frequencies at one time and then uses a sophisticated series of algorithms to analyze the multiple returns. The resolution is much better, the depth penetration is much deeper, and interference is dramatically reduced. This is the technology that naval vessels have been using for many years and that most commercial vessels now use. Dramatic decreases in price now make these improvements available to anyone. Here is a short explanation of what CHIRP technology is all about.

https://www.westmarine.com/WestAdvis...Scanning-Sonar
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Old 17-09-2018, 10:02   #25
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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
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.

I have the exact same problem, raymarine too. Very annoying indeed !


I seem to remember things were not so bad before I installed the itc-5 and connected the sounder to it. It used to blink when above ~100m and now it's moving up & down, sometimes does it in less than 10m as you describe. I was blaming the antifouling but I wonder if it could be the itc5...


Please let us know if you find something from raymarine forum, I do not have access to it.
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Old 17-09-2018, 14:20   #26
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Re: Depth sounder reading really shallow depths when itís much deeper

Benzy, the second ping confusion is a reasonable explanation when in depths that are slightly over the proper operating depths. But I don't think it would apply in significantly deeper water, so query: do you see the false returns in really deep water... several times the operational depth? If so, the second ping theory fails, I think. If not, then Adelie's explanation is likely correct.

I think the side lobe explanation is unlikely...

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Old 17-09-2018, 21:22   #27
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Re: Depth sounder reading really shallow depths when itís much deeper

Rom I also canít seem to access the Raymarine forums at the moment. Will try and contact them some other way.

Jim, it only occurs around the depth limit. At deeper depths the display changes to ď- - -ď

We have just been sailing around Skopea Limani in Turkey where the bottom in many places is roughly the max depth, so I have watched this behaviour a lot over the last few days.

The p319 is touted as a dual frequency transducer (50Hz and 200Hz) but there doesnít seem to be any way of controlling this or the intervals.
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Old 22-09-2018, 03:11   #28
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Re: Depth sounder reading really shallow depths when itís much deeper

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Rom I also canít seem to access the Raymarine forums at the moment. Will try and contact them some other way.

Jim, it only occurs around the depth limit. At deeper depths the display changes to ď- - -ď

We have just been sailing around Skopea Limani in Turkey where the bottom in many places is roughly the max depth, so I have watched this behaviour a lot over the last few days.

The p319 is touted as a dual frequency transducer (50Hz and 200Hz) but there doesnít seem to be any way of controlling this or the intervals.

Following your post I have been playing with the I70 and found that depth transducer was in "Auto" mode. So I selected "iTC5 Converter" instead. Problem solved !!
(setup->system setup->data sources->depth)
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Old 22-09-2018, 23:42   #29
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Re: Depth sounder reading really shallow depths when itís much deeper

Hi Rom,

That is interesting. As I understood the manuals, the setting you describe is simply telling the i70s unit which data source to use on the network. Eg if you have multiple sources for the same data (like depth, such as having two transducers) then this setting allows you to designate which source from the network to use. The ďautoĒ setting means let the i70s decide automatically.

That was my understanding but I may have that wrong.

But you are saying that you no longer get dodgy depth readings?

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Old 24-09-2018, 03:59   #30
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Re: Depth sounder reading really shallow depths when itís much deeper

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Hi Rom,

That is interesting. As I understood the manuals, the setting you describe is simply telling the i70s unit which data source to use on the network. Eg if you have multiple sources for the same data (like depth, such as having two transducers) then this setting allows you to designate which source from the network to use. The ďautoĒ setting means let the i70s decide automatically.

That was my understanding but I may have that wrong.

But you are saying that you no longer get dodgy depth readings?

Cheers

It has been working fine for 2 days, but I am finally seeing the same dodgy readings ... sorry... It's been particularly scary yesterday with readings falling suddenly down to 1 meter in a place where I know there are rocks here & there ! Maybe the transducer is just too old (10 years) and I should buy a new one ?
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