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The oxygen clock is a method of monitoring oxygen exposure over time. When diving at oxygen partial pressures above 0.5 atmospheres absolute (ATA) for long periods of time it becomes important to monitor oxygen exposure. At sea level the pO2 of 0.5 ATA of oxygen is 380 mmHg. At a depth of 33 feet, 0.5 ATA of oxygen equals 760 mmHg. At a depth of 99 feet, 0.5 ATA of oxygen equals 1520 mmHg. Over time, the constant exposure to elevated partial pressures of oxygen is detrimental to the pulmonary and central nervous system of a diver. The theory behind the oxygen clock has been around for about 30 years and concerns cerebral and pulmonary oxygen toxicity. The oxygen tolerance unit (OTU) measures the degree of exposure to dangerous levels of oxygen. The OTU is based on empirical data from which the following best fit formula has been derived:

OTU = t [ [ (PO2 – 0.5) / 0.5 ] ^0.83 ]


t = the exposure time in minutes

PO2 = the partial pressure of oxygen in ATA

0.5 = the threshold below which no significant pulmonary oxygen toxicity has been observed

0.83 = the exponent which gives the best fit to experimental observations.

Perfusion Theory is an educational platform for the Oxygen Pressure Field Theory (OPFT). August Krogh’s theoretical concept of the oxygen pressure field is explained and then applied to clinical applications in perfusion practice.