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August 2000 Vol. 15, No. 8   RSS Feed for Undercurrent Issues
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Rescue Devices

a new study points to improved diver visibility

from the August, 2000 issue of Undercurrent   Subscribe Now

With increased interest in more extreme diving offered by both liveaboards and land-based operations, itís imperative for a diver to carry some device to increase the chances he can be spotted from afar. But not all devices are equally visible, and not all are suited for all conditions.

Englandís Heriot Watt University, under contract to the governmentís Health and Safety Executive (roughly equivalent to Americaís Occupational and Safety Health Agency), recently studied the visibility and effectiveness of safety sausages, telescopic flags, dive lights, and strobes, among other devices. They wanted to learn whether any device was significantly better than others and which shape, size, and color was most readily sighted by observers on full-sized boats or rescue boats.

At sea, they used a variety of strategies to simulate real search conditions under comparable conditions, with sea states from calm to marginal and a broad range of light intensities.

Typically, a diver in the water can see a recovery vessel before it is close enough for spotters to see him. In fairly calm seas, with waves less than three feet, divers could see inflatables up to .8 miles away. They could see larger boats up to 2+ miles away.

However, the spotters in boats had to be much closer to see a diver without a visual locating device. Under ideal daylight conditions, they could spot a diver bobbing on the surface at 250 yards. The distance tripled to 750 yards if the diver fully extended his arm.

Flags. Surface markers remarkably raised a diverís chance of being spotted. Folding flags were by far the most reliable and cost-effective location device tested, particularly a day-glo yellow pennant, which was consistently spotted at more than 1.2 miles to 1.8 miles. Yellow was the most conspicuous color in all sea states, even with breaking waves, and could be seen in deteriorating light when seeing pennants of any other color was impossible. Red and orange flags were less visible, seen up to a mile away. The flags are attached to a mast that folds into sections. It is normally strapped to a tank with elastic cords.

Sausages and Bags. Bags ó or sausages ó can be self-inflating or require air from a regulator or oneís lungs. A spotter could see bags of a height similar to the flag at roughly 3/4 of a mile. A day-glo orange/red bag was visible at a greater distance than red bags and wider and taller devices were easier to locate.

The researchers concluded that sausages become more visible the taller and wider they are. They should retain their form for some time under adverse sea states. They would be seen at greater distances if they were the same day-glo yellow as the folding diverís flag.

Dive Lights. Lights that produced very bright beams were visible 2.5 miles in daylight and 5.4 miles in darkness. They best saw them when moved slowly but steadily in a scanning motion (both horizontally and vertically), rather than pointed at the search vessel (pointing may be inaccurate and the light can temporarily blind the searcher). A light is such a critical signaling device that one ought to keep a spare at night, because the primary light may have lost power.

Strobes. Strobes attached to the shoulder of a diverís BC were only visible intermittently because of waves lapping over and around. Strobes attached to the top of folding flags were not observable until daylight was very low, and then could be seen about 1.2 miles away. A high-intensity strobe is very useful in low light levels, but it needs to be mounted as high as possible.

The Observer. Observers varied considerably in their skill in locating divers, especially if they had any visual impairment. The ability of many observers to spot different devices in varying conditions improved with practice. However, they need to know what they're looking for, so they should be briefed beforehand about what devices divers carry. Under adverse conditions, at least two people should remain on watch. As conditions deteriorated, they found that location distances of devices at sea level that did not provide an artificial light source also decreased.

What is particularly interesting is that two observers had red/green colorblindness and had difficulty even seeing red and orange sausages, particularly in intermediate light conditions. In fact, up to ten percent of the male population either cannot distinguish red from green or see red and green differently than most people. Obviously, someone with red/green colorblindness may be an ineffective spotter. Some people may not know they are colorblind ó or deny it ó so spotters should be checked for colorblindness before being handed the job

. -from Diver

ADDITIONAL RESOURCES ON SURFACE LOCATION: For a variety of colorblindness tests and information, go to the web at

The full report, Diver Emergency Surface Location Devices, can be viewed on

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