Nomos Glashutte Watch Ahoi Datum Signalblau Sapphire Crystal 554. Diving in style, At 40 millimeters in diameter, this Ahoi is very large, and very blue too, siren blue. It also tells time in the dark, and the days date is easily visible even in murky waters, all quite automatically so as NOMOS caliber DUW 5101 is at work within. Glance through the sapphire crystal glass back and you can watch it as it divides time. The blue luster comes from the tempered blue balance spring of the NOMOS swing system, the watchmaking companys proprietary escapement. We think its just as beautiful as the blue dial.