Overview

The Physical Aspects of Hyperbaric Medicine
Courtesy: Dick Clarke, National Baromedical Services, South Carolina & Handbook of hyperbaric medicine, Daniel Mathieu & Physiology and medicine of Hyperbaric Oxygen Therapy, Neumann & Thom.

We are constantly exposed to the surrounding atmospheric pressure, often referred to ‘Sea level’ pressure. This pressure is the sum of the height of the atmosphere above us, the extent of which extends upwards some 100,000 feet. Pressure decreases as one ascends. Pressure increases as one descends, down a mine shaft for example, or within the ocean. Because water is much denser than
air, pressure changes experienced by divers are much greater than those encountered by mine workers

Describing Pressure

Pressure is measured by a variety of units and from one of two reference points. It can be expressed with respect to a vacuum, i.e. zero pressure. This would be a measurement described in absolute terms. A second method measures differences in pressure above or below the surrounding (ambient) pressure. These values are referred to as ‘gauge pressure’.

Atmospheric Pressure: Universally applied: it does not register on a standard pressure gauge.

Barometric Pressure: Essentially the same as atmospheric pressure, but varies slightly with changes in the weather patterns (often
referred to as areas of high or low pressure)

Gauge Pressure: Measures only the pressure difference between surrounding atmospheric atmosphere and that within a
pressurized vessel. Hyperbaric chamber pressure gauges read ‘Gauge’ pressure.

Absolute Pressure: Is the combination of atmospheric pressure and gauge pressure (the sum of what is inside and outside a
hyperbaric chamber, for example). Absolute pressure will only register on an ‘absolute’ pressure gauge.

Ambient Pressure: The term used to describe that pressure which one is exposed to at any given time.

At sea level, absolute pressure is referred to as 1 atmosphere absolute (1ATA). A standard pressure gauge will read zero at this point, while an absolute pressure gauge will read 1 ATA or the equivalent pressure scale. Upon descent (compression) through the water or within a hyperbaric chamber, pressure increases. For each additional 1 atmosphere of pressure increase there is a corresponding increase in both ‘gauge’ and ‘absolute’ pressure.

Measuring Pressure

A. Metric
B. Standard ( imperial )

For hyperbaric operations in North America treatment doses (pressures) are ordered in atmospheres absolute (ATA)

Multiplace hyperbaric chambers have pressure gauges calibrated in feet of sea water (fsw) equivalent. Monoplace hyperbaric chambers have pressure gauges calibrated in pounds per square inch, gauge (psig). India we follow ATA/PSIG while using mono chambers & metres/FSW/ ATA in some Multi chambers

Pressure Conversion Scale