Depth instrument is showing double depth

[glsg91]

I have bought this year a Nexus (Silva) NX system which is delivered with a plastic rectractable thru-hull Airmar 200 Khz depth sensor with temperature sensor.
I have a sailing yacht of 10 meters. It is mounted on the same place of my previous Silva depht sensor.

Now I have a strange problem with the depth reading.
At some places the depth shown on the instrument is about twice the real depth (for example depth is really 4 meter but several times 8 meter is shown). This happens sometimes for a few seconds to a minute (sometimes longer). This not the case everywhere. It looks like it has someting to do with the depth and/or the kind of ground. I'am on the inland waterways inn the north of the Netherlands (Friesland).
I didn't cut off the cable lenght.

I tried a lot of things:
- another Nexus NX2 server (where the sensor is connected)
- use a seperate battery for the server with no other connections
- the cable through the boat without nearby other cables (interference)
- holding the sensor on a few different places inside the hull
- holding the sensor outside the boat in the water

With the last 2 it looks like it occured less but I'am not sure.

The local Nexus dealer cannot find any failure while testing. They also tell me that the sensor cannot be the problem.
The only solution is to sent the sensor and server to Nexus Sweden which will take more then a month (without a replacement). This I don't like of course.

Does anyone have a solution for my problem ?

Regards,
John Brattinga
Netherlands

 

[stevebrassett]

I sometimes get strange depths shown if the bottom is very soft mud.

 

[glsg91]

It is not very soft mud where I was.
My previous depth instrument was giving no problems at all at the same waters.

 

[JasB]

I know it is way off double, but could it be set at feet instead of metres?

 

[glsg91]

No, it is showing M from meters.
Also it is not continue showing the double depth.

 

[duncan]

welcome

your unit sensitivity is set to high for the depth

either reset to auto and reduce the sensitivity or just reduce the sensitivity manually

where you have a function of the depth it's normally related to manual sens overriding the units capability to make sense of it's data.

 

[fluffc]

Is it showing exactly twice the depth, or random large numbers?

 

[glsg91]

As far as I know I cannot adjust the sensitivity.
It is not in the manual of the NX2 multi control instrument.

 

[duncan]

you are asking me? /forums/images/graemlins/smile.gif

I took exactly double from the initial post - large random fluctuations in shallow water with that type of transducer set up would indicate high sediment confusing the unit - again redusing the gain (sensitivity) might help but there is no simple solution

 

[glsg91]

[ QUOTE ]
Is it showing exactly twice the depth, or random large numbers?

[/ QUOTE ]

Yes, about twice the depth. Of course the depth is varying continue.
If the depth is 4m is always show 8m. Never 6,7 or 10m

I recorded some data:
3.87,3.87,3.88,3.91,7.8,7.8,7.79,3.93,7.76,7.73,7.86,7.85,7.85,7.9,7.9,7.89,7.89,7.94,7.82,7.92,7.92,3.94,3.94,3.94
3.93,3.91,7.86,7.86,3.85,3.85,3.83

 

[KenMcCulloch]

When I were a lad we mostly used the rotating dial type of echosounder with a manual gain control. In shoal water you would often find that you saw multiple echoes, one for the true depth and one or more for various multiples. The conventional explanation was that on a hard bottom the pulse would bounce of the seabed, and be detected by the transducer but then also reflect back off the hull and detected on return a second time, showing double the depth. Turning the gain down made the second echo go away. I suspect that's what's happening to yours. These modern digital things just tell you what their best approximation is I think. When I turn my modern digital echosounder on I often get all kinds of supurious readings for 30 seconds or so before it settles down.

 

[gandy]

Does it only happen over a hard flat bottom? It could be hearing a second echo, boat to bottom then bouncing of the boat and off the bottom again. On the old paper trace sounders you used to see multiple bottom lines over a hard seabed.

 

[wooslehunter]

Sounds like something's wrong with the processing. Some units will gave a gain control. Sounds as if your unit's gain is too high but there's no user control. If it reads exactly double then it's measuring a second echo. The sound wave is bouncing from the surface to the bottom and then from the surface to the bottom again.

On fish finder type displays, you can often see this if you set the range high enough. The composition of the bottom will affect the strength of the second echo as will the sea state.

The unit should not spot the second echo since the first will be far stronger and arrive first.

Clearly the unit is not working correctly & it is up to the supplier to sort it out.

 

[duncan]

one minute you are suggesting the gains to high the next [ QUOTE ]
Clearly the unit is not working correctly & it is up to the supplier to sort it out.

[/ QUOTE ]

oh for such clarity........

 

[Dipper]

I agree with Duncan (and some others). The sensitivity is too high and it is picking up echoes from the hull, down to the bottom and back again. I had this on my NASA sounder but only in shallow waters below about 3.5 metres (when I needed it most!) and the readings were exactly double. Lowering the sensitivity sorted it out. On the NASA, you do this by pressing a button as you power up - I can't remember which one.

I’ve had a look at the Silva manual on-line and as you say, there does not appear to be any facility to change the depth sensitivity. You can alter both the wind and speed so this seems slightly odd. It might be worth entering the calibration menu on your instrument to see if the instructions have been left out of the manual but are actually in the system.

 

[glsg91]

[ QUOTE ]
Does it only happen over a hard flat bottom? It could be hearing a second echo, boat to bottom then bouncing of the boat and off the bottom again. On the old paper trace sounders you used to see multiple bottom lines over a hard seabed.

[/ QUOTE ]

I think the bottom is mud. Not very soft but also not hard.
It is on the inland waterway's.

 

[glsg91]

[ QUOTE ]

I’ve had a look at the Silva manual on-line and as you say, there does not appear to be any facility to change the depth sensitivity. You can alter both the wind and speed so this seems slightly odd. It might be worth entering the calibration menu on your instrument to see if the instructions have been left out of the manual but are actually in the system.

[/ QUOTE ]

As far as I can remember this option is not in the instrument. I also will ask the dealer about it.
Is there another way to lower the sensitivity ?
(extend the cable, adding resistors to the cable connector, put something before the sensor)

 

[wooslehunter]

"The gains set too high" is a possible cause of the problem. Given that there is no way for a user to adjust the gains, or for that matter anything else, if the device is not functioning as it should, then it's up to the supplier to sort it.

If a replacement unit still doesn't work then there's a design problem.

Of course sarcastic answers are likely to result in the unit magically fixing itself so you're right to throw that one in.

 

[tome]

Sorry to say that this sounds like a design fault. Echo sounders generally have an auto-gain function so that the power transmitted is reduced in shallow water, conversely increased in deeper water

It wouldn't be the first design fault from Silva and one wonders how thoroughly they test their products

 

[duncan]

my apologies for both tone and also failing to spot your observation that no user control over this exists on this unit.

my unit certainly gives double depths from time to time where it increases the gain automatically on soft bottoms hunting a solid surface - showing twin bottoms until it worked out what was happening.

In fact it's often recomended to tune manual gain by increasing until the double shows then backing off a notch. Electronics are almost getting to clever for themselves!

sorry again.