Is your depth sounder showing 'old' data?

[ChasB]

Is your depth sounder showing \'old\' data?

I came across this: [ QUOTE ]
**Simple digital sounders show historical data, they may show a depth which is 10 to 15 seconds out of date and represents the seabed your boat passed over a hundred metres ago.

[/ QUOTE ] on the blurb for this Echopilot FLS Bronze.

How correct is this claim? If true, is it not then better to purchase a simple sounder displaying a number rather than one with a fancy display?

(Can't wait till the price of these FLS units goes down!) /forums/images/graemlins/cool.gif

 

[reeac]

Re: Is your depth sounder showing \'old\' data?

[ QUOTE ]
I came across this: [ QUOTE ]
**Simple digital sounders show historical data, they may show a depth which is 10 to 15 seconds out of date and represents the seabed your boat passed over a hundred metres ago.

[/ QUOTE ] on the blurb for this Echopilot FLS Bronze.

How correct is this claim? If true, is it not then better to purchase a simple sounder displaying a number rather than one with a fancy display?

(Can't wait till the price of these FLS units goes down!) /forums/images/graemlins/cool.gif

[/ QUOTE ]

Those distances and times equate to a speed of 16 knots if my arithmetic is correct so it's motorboat territory but even my cruiser would travel 35 metres in that time so it's a significant delay.

 

[Lakesailor]

Re: Is your depth sounder showing \'old\' data?

They would say that to sell a £430 depth sounder.
My £90 NASA Clipper had adjustments for sampling rate and averaging rate down to something like 3 times a second.

 

[Danny]

Re: Is your depth sounder showing \'old\' data?

Would you be doing 20kn in shoal waters? I think that's what you'd need to be doing to cover 100m in 10s.

 

[pau1gray]

Re: Is your depth sounder showing \'old\' data?

Alot of this depends entirely on how the depth is measured there are two options:-

1) For a SIMPLE system the sytem will only transmit once a reply has been received and depth can only update at a frequency related to the depth - if for instance the sound speed is 1500m/s (typical value) then in 1500 metres of water it takes 1 second for the signal to descend and 1 second for it to ascend. So the question is how far can you move in 2 seconds? At 20 Kts (approx 40m/s) you will have moved 80m

2) You could transmit a train of pulses at a known time spacing apart and depth will be measured by the first reply then the time betweeen the replies will correlate with the change in depth since you started. Then the question of how far you have travelled is dependant on the pulse repetition frequency.

There is also the question over how long the system takes to process these answers which will vary based on the complexity of the system.

As you start to add more complexity into the system, for instance averaging) then the time taken for the updates may increase again.

So the time since the depth fix was correct will depend upon depth, sound speed (depth, temperature and conductivity) and also what is known as processing time.

The claim did state on a simple system - I think they might have meant lead line

Right another Sonar Class over - I spend the week teaching this to surveyors so can I go and get some beer now? /forums/images/graemlins/smirk.gif /forums/images/graemlins/smirk.gif /forums/images/graemlins/laugh.gif /forums/images/graemlins/laugh.gif

 

[ChasB]

Re: Is your depth sounder showing \'old\' data?

Thanks Paul.

So it really is a case of the more you pay the less you actually get (unless you pay a lot). I think the simple NASA sounder above is probably my best bet then. In shallow waters (where response time really matters) this would theoretically give the fastest updates.

 

[Lakesailor]

Re: Is your depth sounder showing \'old\' data?

I don't have it now as I sold the boat. But on the mooring if the wash from the ferry passed the boat the readout was changing several times a second, almost a constant scrolling of the display. I think that's why they offer slower averaging rates so as to make the display more readable and to discount chop and swell.

 

[ChasB]

Re: Is your depth sounder showing \'old\' data?

That also answers my question about how you would check these things.

 

[Danny]

Re: Is your depth sounder showing \'old\' data?

Paul, I hope your students don't read this!

[ QUOTE ]
At 20 Kts (approx 40m/s) you will have moved 80m

[/ QUOTE ] Surely 20kn is closer to 10m/s than 40m/s! So you'll only have moved about 20m in 2s.

1 nautical mile = 1852m
20 kn = 1852*20 m/h = 1852*20/3600 m/s = 10.29 m/s

 

[pau1gray]

Re: Is your depth sounder showing \'old\' data?

Oh Bugger - was doing this without the aid of a safety net and couldn't be a**d to work it out - I knew it was either 1 Kt = 2m/s or 2Kts = 1m/s.... so I erred on the side of caution





and obviously got caught out /forums/images/graemlins/laugh.gif (well I was getting ready to go out for a beer.) /forums/images/graemlins/smirk.gif /forums/images/graemlins/smirk.gif /forums/images/graemlins/smirk.gif

 

[brianrunyard]

Re: Is your depth sounder showing \'old\' data?

They're trying to sell you something, so it's not impartial.
It's a salesman's statement:
1. "Simple digital sounders", doesn't state which ones.
2. "...may show a depth which is 10 to 15 seconds out of date ...", then and again it might not be 10 to 15 seconds out of date. The getout clause is "MAY SHOW".

Normal advertising, make your product sound better than the rest.

 

[Andrew_Fanner]

Re: Is your depth sounder showing \'old\' data?

So a cheapy fishfinder using pretty much the same technology only shows fish behind you?

Don't think they would sell well

 

[stevebrassett]

Re: Is your depth sounder showing \'old\' data?

[ QUOTE ]
So a cheapy fishfinder using pretty much the same technology only shows fish behind you?

Don't think they would sell well

[/ QUOTE ]
You mean a "look what you could have caught" finder?

Marketed by Jim Bowen, perhaps?

 

[Guest]

Re: Is your depth sounder showing \'old\' data?

Go fast enough and even a FLS will show "historical data !"

The sort of numbers being quoted though are really at extremes .... especially in terms of sail-cruisers ......

The whole subject is a bit like Navigation - The Art of Knowing where you were ...... and trick of extrapolating to where you might be later !

Sales jargon quoting "other is old data" has to be a good one !!

 

[rudolph_hart]

Re: Is your depth sounder showing \'old\' data?

FWIW, I am able to give a measure of the delay, having seen it when we exited over the Ore Bar at 0700 on Tues, 24th. There was a regular 1.0-1.2 metre onshore swell left over from the previous night's Easterly blow, but the wind had moved to NW (i.e. offshore) so there was no sea running. This made it easy to observe the sounder's reaction time, once we turned offshore into the swells.

The delay was about 5 seconds, which was a bit scary when we approached the outer knolls as the swells became steeper (not higher), and yet the sounder was actually telling us the depth in the last trough when we were on the next crest.

Sounder is a Simrad/Robertson/Stowe Dataline, which processes the sensors' data then 'publishes' it on an NMEA cable bus.

 

[Guest]

Re: Is your depth sounder showing \'old\' data?

Funny thing is ... my boat sits in my canal .... and I could go there now ... switch on E/S and watch it flick about as I move side to side on the boat ... my t/ducer is to port of C/L and heel affects it.
I can honestly say that I cannot recall a delay anywhere near a 5 sec magnitude ! OK my E/S is a stand alone Echopilot Duo with standard length t/ducer lead etc. - with no bus or repeater connections. But I would not expect electronic data trsfr to increase display data delay so significantly ?