The NASA Stingray Echo Sounders come in a slimline case with an easy to read extra bright LED display on a 4 inch dial. They both have a dual audible alarms for shallow and deep water which can be set to give two ranges either 1-25 metres or 1-100 metres, and are simplicity itself to operate. They come complete with transducer and stirrup mounting bracket and only require connection to the ships 12v power supply. How They Work Very simply, they make a noise and measure the echo. Knowing that sound travels at 144 metres a second through water allows us to calculate the depth by measuring the interval between the sound being made and the echo being heard. The NASA Stingray Echo Sounders have a transducer with a high quality Lead Zirconate Titanate Piezodisc crystal which creates ultrasonic 'sound' pulses at a frequency of 150kHz (imperceptible to the human ear). These pulses pass through the water and bounce back off the bottom. The returning echo pulse is then sensed by the transducer crystal. On these rotary flashing LED type sounders, a pulse is generated as the lED flashes at the 12 'o' clock position. The rotor arm is calibrated to run at a speed whereby when it flashes on receiving the returned pulse this flash will be in the correct position on the dial to indicate the correct depth.