The invention disclosed herein employs an algorithm, the Gas Formation Model ("GFM"), to calculate the formation of free gas in a human body. The GFM is based on a novel theory of the formation of free gas relative to the physiology of the human cardiovascular system. Additionally, the GFM utilizes a novel means for the solution of integro-differential equations, the type of equations that derive from the introduction of physiological parameters. GFM-based dive computers utilize novel inputs, including a measure of exercise at depth to reflect the state of an individual's cardiovascular system. GFM-based dive computers also produce novel outputs, including the actual volume of free gas present in a diver's cardiovascular system. The GFM is implemented as a practical computational tool by means of a incorporating the algorithm into a dive computer.