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Author Topic:   Speed of Sound Travel
jimh posted 09-17-2013 01:43 AM ET (US)   Profile for jimh   Send Email to jimh  
The speed at which sound travels through water varies with the salinity. In pure fresh water the speed of sound travel is about 1497 m/s at 25 °C. However, in saltwater sound is said to travel at about 1560 m/s.

Cf.: http://en.wikipedia.org/wiki/Speed_of_sound#Water

How do recreational marine SONAR units adjust for this difference in speed of sound travel when the devices are used in saltwater or fresh water? It would seem like some compensation would be needed to get the proper calculation of depth. Is any such compensation done in recreational SONAR units?

Tom Hemphill posted 09-17-2013 07:07 AM ET (US)     Profile for Tom Hemphill  Send Email to Tom Hemphill     
My Humminbird 947c combination chartplotter/sonar unit has a setting for fresh or salt water. The manual says "...this affects the accuracy of deep water depth readings."
Peter posted 09-17-2013 08:19 AM ET (US)     Profile for Peter  Send Email to Peter     
The Garmin chartplotter/sounder combos do not have a fresh or salt setting that I can find but I wonder whether intelligence has been built in to know, based on position, whether the water is salt or fresh.
Jefecinco posted 09-17-2013 09:21 AM ET (US)     Profile for Jefecinco  Send Email to Jefecinco     
In coastal areas salinity changes with the seasons. A lot of rain can also have a large impact, especially in our large delta region of Mobile Bay.

Perhaps the speed of echo returns tells the sounder how to adjust for salinity.

Butch

fishgutz posted 09-17-2013 09:28 AM ET (US)     Profile for fishgutz  Send Email to fishgutz     
Interesting question on a theoretical basis, but I have to ask, why does it matter?
I look for rises and drops on the bottom and decreases in depth coming into shallow areas. The numbers really don't matter that much. At least not by tenths of a foot or even feet in deep water.
With differences in salinity, surface waves, turbidity, temperature, tides, etcetera, how really accurate is sonar in differing conditions?
tmann45 posted 09-17-2013 10:40 AM ET (US)     Profile for tmann45  Send Email to tmann45     
The difference between the two would be about 8.2%, or 2.7 ft at 10-meters (~33 ft). If the average speed was used, that would half the difference, so 1.35 ft at 33 ft. At 10 ft depth the error would be 5 inches, and at 100 ft the error would be 4 ft, not a lot to worry about in my opinion for recreational use. Do you add in the depth of the transducer below the water surface when not moving, and adjust this depth when on plane?

Now on a HDS unit, the selection of the 83/200 transducer could use the fresh water speed and selecting the 50/200 transducer could use salt water speed, since that is the general recommended use for those transducers. Or, you could contact Lowrance and ask them, instead of getting opinions/speculations from the internet sonar experts.

fishgutz posted 09-17-2013 11:41 AM ET (US)     Profile for fishgutz  Send Email to fishgutz     
I get a difference of 4%. Am I doing something wrong?
I used this: http://www.onlineconversion.com/percentcalc.htm

An another note, I use my boat in the St. Johns River in Florida where fresh water has some salt in it and varies with tides so no matter what, readings are never accurate.

jimh posted 09-17-2013 02:19 PM ET (US)     Profile for jimh  Send Email to jimh     
Using the figures I gave in the initial article, the speed variation is 4.2-percent.
tmann45 posted 09-17-2013 05:54 PM ET (US)     Profile for tmann45  Send Email to tmann45     
You are both right depending on which way you do the calculation, 4.0% or 4.2%. So, if the average speed is used the error would be only 2.1%.

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