All gases can be narcotic and then toxic to us whilst breathed under pressure and the depths of gas narcosis in recreational diving can be very personal.
Most people rarely experience it at depths shallower than 30 metres but some people have experienced it in mus#ch shallower water and to varying results.
It is a temporary experience and can be revered by simply ascending to shallower depths if an individual wants the sensation to abate.
Due to turbidity, suspended particles in the water, light diffuses erratically and can make objects appear further away than they really are.
I experienced this only a few time and usually at depths between 35-50 metres while conducting Deep Specialty and Technical training and it's truly fascinating.
Good surface procedures are imperative in every diving situation and are emphasised during the PADI Open Water Diver Course - have a fully inflated BCD, retain your mask and air source or as I always say to students, see, breath, float until you're on the boat!
If for any reason a diver cannot gain positive buoyancy via the BCD ditching weight is the fastest and safest method of increasing buoyancy in an emergency.
Remember, the most important feature of a weight system is its quick release mechanism for this very reason.
Definitions for many terms and situations within the PADI system can be found in Training Standards in the PADI Instructor Manua.
It isn't a specific depth of water for the reason that it depends on the height of the student you are teaching. The water must be shallow enough for them to stand in so if you're conducting a confined open water session near the beach or shore position the students so they can stand as you may be in slightly deeper water as the Instructor.
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It's important to find the type and volume of each cylinder prior to diving. Each one will react differently and can affect buoyancy during the dive and required stops and although 2 cylinders may be filled to the same pressure, they may contain a differing amount of gas based on their capacity.
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