What depth should my fish finder read when I’m out of the water?
Anywhere between zero and the maximum depth range of the fish finder.
The key question is really "why does one unit read zero while another one reads 1 foot, another one reads 1100 feet and another one constantly jumps around from zero to 1400 feet?"
First, there is always noise associated with any electronics device. There is no such thing as a noiseless electronic. Noise is picked up in our units as a sonar signal. While the unit is working in the water the true sonar signals are hundreds of times greater than a noise signal in our units. The unit easily picks up on the true sonar returns and displays them. The noise from the electronics is so small that it doesn’t even show up on the screen most of the time.
When the transducer is out of the water and the unit is operating in normal mode, the unit is doing its job and trying to find the bottom and report that back as a digital depth reading. Since there are no true sonar returns it starts searching for the bottom. As it searches for a bottom that doesn’t exist since the transducer is out of the water it will pick up on these tiny noise signals and report them back as the bottom since it is the strongest signal available. If that number happens to be correct, it is simply a coincidence.
In summary, there is no digit depth reading that has any significance when the unit is out of the water and it is not recommended to test your product in this manner.