UNITS OF MEASUREMENT

In the BSAC we normally use the metric system for our diving calculations. This is not the only system which can be

used however. When diving abroad and hiring equipment locally, or when diving with some of the older divers here in

the UK, you may come across items of kit like depth gauges, computers, & cylinders which are not calibrated in metric

units. This article is intended to review & explain relationships between some of the units of measurement currently in

use by divers around the world.

CYLINDER VOLUMES AND PRESSURES

When we describe a cylinder we normally do so in terms of its water capacity and working pressure (for example, a

cylinder having 12l WC and 232 bar WP). We could give an equal valid description in terms of how much air the

cylinder contains when charged to its working pressure (i.e. 12 x 232 = 2784 litres). In the United States, and in other

countries still using imperial measures, cylinders are often described using the latter description, but in cubic feet of air

contained in the cylinder when fully charged ( 1 cubic ft = 28.32 litres). Standard cylinder sizes are: 80 cubic feet, 100

cubic feet and 120 cubic feet (which correspond approximately to our 10, 12 , & 15 litre cylinders respectively).

Cylinder working pressures are normally expressed in 'bars' in the UK. Many compressors however still have gauges

calibrated in pounds per square inch (psi). In countries still using the imperial system of measurement, cylinder working

pressures are also usually expressed in psi (1 bar = 14.5 psi). Typical cylinder working pressures which you may come

across when diving overseas are 2400 psi (165 bar), 3000 psi (206 bar), 3300 psi (227 bar) and 3500 psi (241 bar).

DISTANCE, DEPTH AND PRESSURE

When diving, we always carry a device for measuring depth (either a depth gauge or a dive computer). In the UK, such

depth measuring devices are normally calibrated in metres of seawater. In many other countries the same devices are

calibrated in feet of seawater (1 metre = 3.28 feet). Most of the charts which we use for dive planning in the UK show

depth of water in metres. Other charts which you may come across (including charts for UK waters) however show

depths in feet or fathoms (1 fathom = 6 feet).

As members of the BSAC, we will probably carry out the majority of our dives using the BSAC '88 decompression

tables. These are not however the only decompression tables available. In fact, probably the most commonly used

decompression tables in the world are the US Navy tables. All of the available decompression tables use a combination

of depth and time to compute the diver's decompression requirements. Many of these tables (including the US Navy

decompression tables) express the depth in feet of seawater.

Many of the calculations that we carry out for diving require the absolute pressure at the dive depth. In the metric

system, we simply divide the depth (in metres) by ten and add 1 bar to obtain the absolute pressure at that depth. When

we are using the imperial system of measurement, depths will normally be expressed in feet. To obtain the absolute

pressure P ( in atmospheres) at a given depth D (where D is in feet), we use the formula:-

{NB - Another use of distance measurements in diving is in underwater photography. Since an object

underwater appears to be 1/3 larger than it really is (i.e. 4/3 actual size) the camera focus will need to be set

to 3/4 of the actual distance (in metres or feet) from the object}.

WEIGHT

Underwater, the weight of an object is affected by buoyancy (Archimedes Principle). The effects of buoyancy were

covered in an earlier article in this series. Divers compensate for this buoyancy by wearing a weight belt and a buoyancy

compensator device (an ABLJ or stab-jacket). Buoyancy will also effect the weight of objects in calculations relating to

underwater lifting operations. In the metric system weights are expressed in grams, kilograms and metric tonnes. In the

imperial system of measurement, weights are expressed in ounces, pounds and tons. Equipment which you may come

P =(D+ 33)

33ats

across (such as weights for weightbelts or lifting bags, for example) may use either imperial or metric units which may

be converted thus:-

1 ounce = 28.35 grams

1 pound = 0.45 kilograms

1 ton = 0.9078 metric tonnes

Some conversion factors for commonly used units are listed in the table below. The list, though far from

comprehensive, should provide a useful reference when confronted with unfamiliar units in diving:

To convert into Multiply by bars pounds/sq in 14.50 Centigrade Fahrenheit (oCx 9/5) +32 centimetres feet 0.0328 inches 0.3937 cubic centimetres cubic feet 0.00003531 cubic inches 0.06102 cubic metres 0.000001 litres 0.0001 cubic feet cubic cm 28320.0 cubic metres 0.02832 litres 28.32 cubic inches cubic cm 16.39 cubic metres 0.00001639 litres 0.01639 cubic metres cubic cm 1000000.0 cubic feet 35.31 cubic inches 61023.0 litres 1000.0 Fahrenheit Celsius 5/9(oF - 32) fathoms feet 6.0 metres 1.8288 feet centimetres 30.48 kilometres 0.000348 metres 0.3048 miles (nautical) 0.0001645 miles (stat) 0.0001894 gallons cubic cm 3785.0 cubic ft 0.1337 litres 3.785 pints 8.0 grams ounces 0.03528 inches centimetres 2.54 metres 0.0254 kilograms pounds 2.205 kilometres miles 0.6214 knots km/hr 1.8532 mph 1.1516 league miles (nautical) 3.0 litres cu feet 0.03531 cu metres 0.001 gallons 0.2642 metres feet 3.281 miles (nautical) 0.0005396 miles (statute) 0.0006214 miles (nautical) feet 6080.20 kilometres 1.853248 miles (statute) 1.1516 miles (statute) feet 5280.0 kilometres 1.609 ounces grams 28.349527 pounds 0.0625 ounces (troy) 0.9115 pints gallons 0.125 litres 0.4732 pounds grams 453.5924 kilograms 0.4536 ounces 16.0 pounds/ cu in grams/cu cm 0.01602 kg/ cu metre 16.02 pounds/ sq in bars 0.06894 tonnes (metric) kilograms 1000.0 pounds 2205.0