Dalton's Law of Partial Pressures states - In a mixture of non-reacting gases, the total pressure exerted is equal to the sum of the partial pressures of the individual gases.

As the approximate percentage of Oxygen in air is 21% that transfers from percentage to the decimal 0.21ppO2 at sea level - 1ATM.

If we multiply the ppO2 by the 0.8ATM - 0.21 x 0.8 - we get the new partial pressure of the gas when breathed at this new pressure. 

You'll find definitions in the Training Standards section of your PADI Instructor Manual - this section is also ordered alphabetically to assist in your search.

A free upgraded copy of the PADI Instructor Manual is available to all Active Status PADI Members at the padi.com pro section.

I prefer the digital manual because it is easily searchable for keywords and sections.

Although rarely used in actual diving now and in fairness, a better section for it would be the history of diving, the J-Valve reacted to the internal pressure of the cylinder.

As the diver consumed more gas throughout their dive pressure would begin to drop. When the internal pressure dropped to approximately 35-50 bar the valve would slowly close the gas supply off.

This restriction of gas would serve as a warning device in lieu of the now standard SPG of the dropping of cylinder pressure. 

The diver would then manual reopen the valve and begin their ascent to the surface safely with remaining pressure in the cylinder. 

Way back in 1687, Sir Isaac Newton explained how the relative position of the Moon and Sun and their gravitational effect cause the large bodies of water in the oceans to rise and fall.

Without tides life on Earth as we know it would not be possible and tidal energy is now being used to generate green energy around the world. 

There are three processes on the PADI eRDPml - Dive Planning - Surface Interval & Max Depth.

Choose this mode from the menu and then simply key in the dive time and let the eRDPml do all the work. This can be a useful mode to play with when you're trying to make the most of the time you have remaining of your diving day.