Complex sentence: The design of the level indicator meant that it only read a length of 3 metres, anything above that didn't register, worst still the design meant any level above 3 metres could show on the screen as a drop in level.
Split into simpler sentences: The design of the level indicator meant that it only read a length of 3 metres; anything above that didn't register. In addition, the design was such that any level above 3 metres could show on the screen as a drop in level.