Although the first aid steps should remain the same; O2, call for help and monitor the patients' lifeline, regardless of the time of symptom onset, taking down details for the next stage of medical care can give them insights into the type of treatment that they provide.

The approximate make of air is 21% oxygen and 79% nitrogen. This will be true whether the air is at 1 ATM or at an altitude of 0.7 ATM. 

Don't get the physical amount of the gas confused with its physiological effect when breathed at various atmospheric pressure. If the question was asking for the partial pressure of oxygen in air when breathed at 0,7 ATM the answer would have been 0.147ppO2.

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One of the three knots taught in the PADI Search and Recovery Specialty Diver program and the PADI Divemaster, the sheetbend is a stronger alternative to the square knot and is effectively bowlining two lines of dissimilar diameter together. 

It's secure, quick to tie and easily untied if the lines have been put under pressure.

Always use the thicker of the two lines you joining to make sure it is at it's most secure.

This question often throws people as they get focused on the certification level offered at the end of the program rather than the course the student is enrolling in.

Everyone enrols in the PADI Open Water Diver course but may be limited with maximum depths and certification offered at completion based on their age.

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If cylinder markings are unfamiliar don't assume they are unimportant. The (+) making on steel cylinders denotes their overfill capacity and is important for someone filling them.

As compressed gas is pumped into the cylinder it will generate heat which displays itself as an increase of internal pressure. Once the tank begins to cool the internal pressure will drop also. If a diver wishes to have 200 bar available for a diver the compressor operator will have to fill the cylinder over 200 bar to account for this pressure drop once the cylinder begins to cool.