When PADI ask this question it is off the information given in PADI's Encyclopedia of Recreational Diving, and PADI Divemaster materials stating that there are three different forms of heat transmission - Conduction, Convection and Radiation.

The simplest way to remember them in order of most to least affecting a diver whilst underwater in to remember them in alphabetical order - Conduction be the most and Radiation being the least.

Although the standards for individual PADI Specialty programs such as Wreck Diver are not in the PADI Instructor Manual, general guidelines such as Instructor Rating required to teach the speciality course, age requirements and certification prerequisites along with ratios are found in the Key Standards section of the PADI Instructor Manual.

This section is a great quick reference guide to all PADI programs from Bubblemaker to CCR Instructor Trainer.

Different manufacturers often use varying algorithms in their dive computers and may give different NDL's to their user for the same planned dive profile. You should always follow the most conservative model when diving in a team.

Another reason you should compare NDL's is when you are completing multiple dives on the same day. Even if you were on the same previous dive as your buddy and are wearing the same make and model of computers, NDL's can vary based on the actual dove profile that you followed as individuals

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Waste entering the water and causing an excess of plankton blooms is often called a red tides because of the discolouration caused to the water.

Red Tides can deplete oxygen in the water and cause illness in animals due to toxins released.

The M-Value or maximum Gradient Value is linked to a tissues compartment's halftimes. 

When you are diving tissue compartments load in relation to the pressure exerted against them as well as the time you are diving. Fast tissue compartments have high M-values and are therefore the controlling compartments for deep, short dives.

Slow tissue compartments have low M-values and are the controlling compartments for short, long doves.