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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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.
This question can be answered using only Table 3 from your PADI RDP Table.
Intersect C along the top with C on the left-hand side and you'll find 2 boxes containing differing numbers.
The key below the table shows what each of these means. The blue box is the maximum allowed bottom time for a repetitive dive to this depth. The white is the Residual Nitrogen Time or RNT.
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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Carbon Monoxide forms a reversible bond with Haemoglobin much the same way Oxygen does. Unfortunately, it bonds 200 times more tightly then Oxygen forming Carboxyhaemoglobin.
Carboxyhaemoglobin has a half-life of several hours and impedes the transportation of Oxygen from the lungs to the rest of the body which is why ciggerette smoking prior to dives should be avoided.