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ContinuousWave: Small Boat Electrical
Speed of Sound Travel
|Author||Topic: Speed of Sound Travel|
posted 09-17-2013 01:43 AM ET (US)
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.
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?
posted 09-17-2013 07:07 AM ET (US)
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."
posted 09-17-2013 08:19 AM ET (US)
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.
posted 09-17-2013 09:21 AM ET (US)
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.
posted 09-17-2013 09:28 AM ET (US)
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?
posted 09-17-2013 10:40 AM ET (US)
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.
posted 09-17-2013 11:41 AM ET (US)
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.
posted 09-17-2013 02:19 PM ET (US)
Using the figures I gave in the initial article, the speed variation is 4.2-percent.
posted 09-17-2013 05:54 PM ET (US)
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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