PADI Nitrox Course Review

[Thalassamania]

You could learn other techniques like repping up or (gasp) ratio deco.

 

[caseybird]

You could learn other techniques like repping up or (gasp) ratio deco.

What is repping?

 

[Thalassamania]

What is repping?

Repetitive Group (Graver) Method

Dennis Graver further popularized an already widely used table based multi-level calculation in the late 1970s, although this method had been around for at least twenty years and had been discredited in the mid 1960s. The concept behind it is disarmingly simple, one assumes that a given repetitive group represents an absolute quantity of nitrogen, no matter what the dive profile involved in taking up the nitrogen was. This permits the 'sidewise' use of the U.S. Navy Table B-1. The previous dive is simply transferred up the repetitive group column on Table B-1 to the new depth. The diver is assumed to have been at that new depth for the time indicated by the intersection of the repetitive group column and depth row. The diver could continue on until no-decompression time was exhausted. A variation on this technique is used in commercial diving and is called 'Repping Up'.

Despite the sound appearance of the technique, problems develop when dives permitted by this procedure are checked against the U.S. Navy Model. Once a dive is reduced to a repetitive group letter all information except that contained in the 120-minute compartment is thrown away. As a result, Graver suggested that this technique should not permit the entire table to be used, and should be limited to 'less strenuous' dives. Karl Huggins demonstrated a multi-level dive permitted by Graver's method (120 for 15 then 90 for 5 then 70 for 10) that resulted in a 101.5% M0 value for the 20 minute compartment. This value would likely result in bends and is lost when another 'Repping Up' calculation is made.

Repetitive Group/Residual Nitrogen Time Method

This method is similar to the Graver method but the secondary levels of the dive are considered to be repetitive dives without a surface interval. The residual nitrogen time for the secondary levels are obtained by taking the repetitive group produced by the primary level to Table B-3 and comparing the repetitive group to the secondary dive depth to yield a 'corrected' residual nitrogen time, which could be used to calculate the amount of no-decompression time remaining at the secondary depth.

While more conservative than the Graver method, there is still the same basic problem. Since there is no 10 minute surface interval, nitrogen pressures in the 40 minute compartment can build up to unacceptable level which is undetected by the 120 minute compartment that governs the repetitive group designation.

Half and Half Method

This is a patently unsafe procedure that should never be used. It is to spend one-half of the no-decompression limit at a given depth and then ascend and spend one-half of that depth's no-decompression limit at the secondary depth. There is no technique for computing a repetitive dive group after such a dive although some 'native' divers suggest using the repetitive group letter of a dive to the no-decompression limit of the secondary depth.

U.S. Navy Multi-Level Diving Procedure

In order to accommodate the needs of U.S. Navy Combat Swimmers who must remain hidden and use equipment that provides up to 12 hour of life support the U.S. Navy developed a technique that permits 'repetitive dives' without surfacing. The concept is that instead of having a minimum 10 minute surface interval at the surface the diver has a 30 minute 'surface interval' at a depth shallower than 20 feet. According to sources at the EDU, this technique has been tested and is considered safe.

Other Methods

Other techniques have been developed over the years. Most often they have been empirically determined at dive resorts that have a deep wall adjacent to a shallow reef. Profiles that have been used with safety over the years become part of the lore and continue to be used. There are said to be profiles that have been used for over 10,000 man-dives without incident.