Rescue exercise 4 may be performed in confined water prior to being completed in open water. When performed in open water the exercise which is essentially a three-part skill; responding to a diver making an uncontrolled ascent, being overexerted and signaling out of air, it has a maximum depth as listed in the 2018 PADI Instructor Manual of 18m. 

There is a printing error on some slates indicating that the maximum depth for Rescue 4 is 6m. This was corrected in a Training Bulletin in 2016. 

Slack water is when the tide is 'unstressed'. meaning it is neither coming in going out.

This means there is a drop in water movement causing currents, making entries and exits simpler for shore dives. Slack high tides can also bring upwelling of clearer water meaning improved visibility too. 

Each tissue compartment contains 6 halftimes. Every compartment is named for the time it takes for each halftime to fill 50% of the available space while loading.

This means 1 halftime to become 50% saturated, 2 half times to become 75%, 3 to become 87.5 and so on.

When a diver inhales a pilot valve, such as a tilt lever, is triggered on the diver's inhalation. This allows intermediate pressure from the hose through the pilot assembly to the second stage. The diver then breathes this gas at ambient pressure.

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The passive movement of gasses from areas of high concentration to areas of lower concentration is important for us to understand in regards to inert gas loading and first aid for decompression incidents.

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