These sonar instruments do not directly measure depth, they time how long it takes the ping to return. That time is then converted/displayed as a depth thru water for us (the user). Sound travels faster thru water than it does thru air.
The speed of sound thru air is ~343 m/sec. The speed of sound thru water is ~1480 m/sec. The ratio should be ~.232 = (343/1480). So to test the depth reading thru air --> Hold the transducer puck steady & level at an elevation of 25 inches over a hard surface, the unit should read a depth thru air of approximately 9 feet (b/c the unit is designed to read thru water).
I perform this test each year before I hit the ice - these specific test numbers (25 inches & 9 feet) are just easy to recall. You could do the math to test other distances.
