What is the surface consumption rate of a diver using twin low pressure steel 15 L cylinders at a depth of 20 m?

At a depth of 20 meters, a diver will experience underwater pressure that is approximately three times that of the surface pressure (1 atmosphere for the surface and 2 additional atmospheres for the 20 meters of water). As a result, the volume of air consumed at that depth is affected by this increased pressure.

To determine the surface consumption rate (SCR) of the diver’s gas usage, we can use the general rule that at a given depth, a diver breathes more due to the increased pressure, effectively using more gas. If we assume an average consumption rate for a diver under these conditions is about 20 liters per minute at the surface, this consumption rate is consistent with typical dive profiles and physiological factors.

Therefore, at a depth of 20 meters, the surface consumption rate can reasonably be stated as 20 liters per minute, since this represents the diver's equivalent usage when considering the increased pressure around them.

The other options represent rates that could imply either a lower or higher consumption than typical expectations for recreational divers at that depth, which is why they do not accurately reflect a standard understanding of gas consumption in cavern diving scenarios.