As a result of OP Brian Sharpe's question I got to exploring this SAC issue in a little more detail. I emailed Suunto and asked how their computers perform the calculations. It is a little more involved than I expected, but for those of you interested in the details, here is their response:
July 21, 2010
Dear Sir/Madam,
thank you for contacting Suunto.
Please see the extracts from the Dive Manager help file below for a description of how SAC is calculated in SDM:
SAC calculation method:
To calculate Surface Air Consumption (SAC), Suunto Dive Manager first determines the average depth of the dive. It is the average of all depth reading shown on the depth graph in the dive log entry.
It next determines the total amount of air used on the dive. Dive Manager uses information you provide about cylinder size and about pressure gauge readings. Enter this information on the Air consumption tab in the properties window. Dive Manager then calculates the total volume of air used for the dive.
Dive Manager then determines the volume of gas used per minute and adjusts the result to sea level from the dive's average depth. Finally, it adjusts the result from ambient water temperature to zero degrees Celsius.
The result is a volume of gas per minute at Standard Temperature and Pressure (STP). Because Dive Manager uses volumes at STP, you can directly compare SAC numbers from one dive to another, regardless of depth and water temperature. Dive Manager reports SAC in metric or imperial units. Metric units are Standard Liters per Minute (SLM), and imperial units are Standard Cubic Feet per Minute (SCFM).
Accuracy of SAC calculations:
Surface Air Consumption (SAC) is the average volume of gas breathed per minute of dive time expressed in standard liters per minute or standard cubic feet per minute. It is an estimated average for the dive as a whole. It is most accurate for long, square profiles with a constant breathing rate.
The following factors have an effect on the accuracy of Dive Manager's calculated SAC:
accurate cylinder pressure readings
Accurate pressure readings improve SAC accuracy. You must manually record cylinder pressures at the start and end of a dive for dive computers that are not air-integrated. When practical, read gauge pressures after the cylinder warms or cools to water temperature.
changes in breathing rate
A constant breathing rate improves accuracy. SAC is an average for the dive. Changes in breathing rate will skew the average.
rate of depth change
Slow depth changes improve SAC accuracy. Fast changes sometimes skew the average depth for the dive, especially if your computer is set for a long sampling rate.
water temperature changes
Uniform water temperature improves SAC accuracy. Cylinders gain or lose pressure as they warm or cool. This has a minor effect on the amount of air available for use during the dive. The SAC calculation assumes one water temperature for the entire duration of the dive.
Instrument errors
Accurate measurements improve SAC accuracy. The cylinder pressure gauge is the most subject to error. For best results, use a gauge whose accuracy at both high and low pressures is known.
Your dive computer measures depth, dive time, and water temperature. As with any instrument, these measurements have some amount of error. Modern dive computers are accurate enough, and do not have a considerable effect on SAC accuracy.
With best regards,
Suunto Helpdesk
