Echo Sounder Checkout

[Danny]

I recently had intermittent problems with my Nasa echo sounder and couldn't decide if it was the transducer or the head causing it. I took both into Nasa who sorted the problem out for me. Their premises seem a bit on the scruffy side but their service was excellent and they're very friendly. Anyway, I picked up some tips from the resident guru which might be of help to someone sometime...

1/ If the head's transmitting you should be able to hear the transducer clicking.

2/ To see if the receiver's working rub the palm of your hand over the face of the transducer. You should get random readings on the display.

3/ To check the accuracy you can use a 1 metre length of plastic drainpipe of around the diameter of the transducer, or more. Poke the transducer in one end and hold the palm of your hand over the other end. You should get a reading of 4.5 metres.

All this works for Nasa Cipper E/S and I imagine will work for other E/S's as well.

 

[Lakesailor]

Thanks for that. I just installed a Clipper and noticed the readout going nutty when I wiped the display. I thought I'd overtightened the mounting screws (to nip the "O" ring) and distorted the screen.

 

[cpedw]

Lakesailor,
I think you misinterpreted
"2/ To see if the receiver's working rub the palm of your hand over the face of the transducer. You should get random readings on the display."

The transducer is the very wet end. Wiping the display might well show funny signs due to static, but it doesn't necessarily mean the display is working.

Regards,
Derek.

 

[Lakesailor]

You could have a point /forums/images/graemlins/blush.gif

 

[Danny]

[ QUOTE ]
You could have a point

[/ QUOTE ]Yes, Derek's right - I meant the transducer, not the display. If you just installed it and the transducer is out of the water you may well get a few random readings and then the display will probably say "Out" to show it's receiving no echos.

 

[Guest]

Quote "3/ To check the accuracy you can use a 1 metre length of plastic drainpipe of around the diameter of the transducer, or more. Poke the transducer in one end and hold the palm of your hand over the other end. You should get a reading of 4.5 metres." unquote ...

That intrigues me .... I'm not saying it's wrong - no way - they know far more about it.
I just am trying to imagine a sound pulse passing through air faster than it would in water ... over a distance of 1m and indicating 4.5m ?? Seems to defy logic ... and also I was always under impression that an echo-sounder needs the greater density medium of water to create that pulse ... that air was too light .......
I've heard of a pipe with water in to create a water column to check an E/S .... even about using a rain-butt as long as over approx. 1/2 m depth.

Interesting !!

 

[pvb]

Speed of sound...

Sound travels about 4.5 times faster in water than in air (say 1500 metres/sec vs 340 metres/sec). So a 1 metre column of air should read about 4.5 metres on an echo sounder designed to measure depth in seawater.

 

[Danny]

Yes - PVB has it right. Although it seems counter intuitive to me, sound really does travel almost exactly 4.5 times faster in salt water than air.

You are right about the fact that the transducers don't perform well in air. This is the reason for the drain pipe. Without the pipe there's too much dispersion of energy and you probably won't get a good echo.

Incidently, the speed of sound in fresh water is somewhat less than sea water so that must mean that echo sounders will over-read in rivers and lakes!

 

[tome]

There's a little company in Petersfield called Ocean Scientific who sell 'standard' sea water for calibration purposes. Nice little earner!

When we were surveying, I had to take the ships boat out weekly to do a temperature and salinity dip throughout the entire water column for calibration of our acoustic sensors

 

[Danny]

[ QUOTE ]
...speed of sound in fresh water is somewhat less than sea water so that must mean that echo sounders will over-read in rivers and lakes!

[/ QUOTE ]As well as the e/s giving the wrong depth the boat will also be less buoyant and have a greater draught in fresh water - so that's why I went aground up the Fal last year. /forums/images/graemlins/smile.gif