Problems At Depth

In order to understand better some of the problems encountered at depth by a diver, it is necessary that we understand first the basic components of air, the gas we breathe.

Air is not a single gas, but is rather a combination of gases. This combination of gases is of primary importance to us, for reasons which will be explained later. However, suffice it to say that air is a mixture of primarily two gases, oxygen and nitrogen. It is composed of approximately 80% Nitrogen and 20% Oxygen, the other gases being unimportant amounts and inert.

Daltons Law

Coupled with this knowledge of the composition of air, we must now take into consideration two laws of physics which deal with the reaction of these gases under pressures. The first of these laws we will consider is DALTONS' LAW OF PRESSURES.

Essentially, Daltons' Law states that the partial pressures of gases in a mixture remain constant and act independently of each other. In other words, let's apply this to our air. We have said that air is a mixture of gases, 80% Nitrogen, and 20% Oxygen. According to Dalton's Law, this means that at surface pressure, (14.7 psi) we have in our air 11. 76 psi of Nitrogen (80% of 14. 7) and 2.94 psi of Oxygen. (20% of 14. 7). Now suppose we increase the pressure of this air to 100 psi. As we apply Daltons Law, we see that now we have 20 psi of Oxygen and 80 psi of Nitrogen.

Henry's Law

Now let us consider the second of these laws, Henry's Law, which states that the amount of gas that a fluid will absorb under pressure varies in direct proportion to the pressure and the partial pressure of the gases.

What this means is that in our air at surface pressures, the blood of the diver will absorb in solution a given amount of the component gases and as we increase the pressure from the surface (14. 7) to two atmospheres, (29. 4 psi) it will absorb twice as much of these gases in proportion to their partial pressures.
This can be illustrated if the reader will think for a moment of a bottle of carbonated beverage, still sealed in the bottle. Take for instance champagne, a graphic example. As long as the cork is in place, keeping the fluid and the gas (Carbon dioxide in this case.) under a high pressure, the gas remains in solution in the wine. However, as soon as the cork is removed, the pressure is reduced dramatically, and the gas, no longer able to be contained in solution in such large quantity at the reduced pressure, boils or fizzes out in the form of bubbles

Nitrogen Narcosis

Because the diver, as he descends, is continually increasing the pressure of the gases he is breathing in accordance with ambient pressure, he is also increasing in his blood stream the quantity of Nitrogen in solution. Nitrogen, in quantity in the blood stream acts as an anesthetic and as the pressure and quantity of nitrogen in the blood stream increases, so does its anesthetic affect upon the diver, until he comes into a condition known as Nitrogen Narcosis, or Rapture of the Depths. This condition makes itself known somewhere in the vicinity of the fifth atmosphere absolute, or fourth atmosphere, gauge, (approximately 125 feet.) However, this effect is extremely variable, and one diver may be affected at one depth, and another at a different depth, or even the same diver at different depths on different occasions.

The individual divers reactions to Narcosis may also differ; for some it is a joyous, free feeling; to others it brings on depression, melancholy, and unreasoning fear. In all cases it reduces logical thinking. The important concept here is to realize that Nitrogen Narcosis is a progressive thing, becoming gradually more pronounced as the partial pressure of nitrogen increases in the blood stream.

Treatment for a person suffering from Nitrogen Narcosis consists of bringing the victim up to a level where the concentration of nitrogen in his blood stream is no longer great enough to create this narcotic effect. Likewise, prevention of Narcosis is simply do not dive below 120 feet.

Oxygen Poisoning

Another malady brought about by the affects of Henry's and Dalton's Laws, is a condition known as Oxygen poisoning.

When Oxygen is present in the blood stream at a partial pressure of or approaching two atmospheres absolute, (29.4 psi) it is toxic and cannot be tolerated by the human body. Oxygen at this partial pressure causes visual distrubances, twitching, nausea, and convulsions.
Obviously, if a diver were to use a rebreather type scuba, or were to charge an open circuit tank with oxygen instead of air, as he approached two atmospheres absolute, he would be approaching the toxic level of oxygen. However, a diver breathing air can also reach a depth where the partial pressure of oxygen is toxic, and suffer from oxygen poisoning.

Recalling Dalton's Law, that the partial pressures of gases are directly proportional to pressure, and remain in constant proportion, consider the partial pressures of the component gases of air at 306 feet fresh water. The absolute pressure at 306 feet, or 10 atmospheres, would be 147 psi (10 x 14. 7) If the partial pressure of Nitrogrn is 80% and the partial pressure of Oxygen is 20% it can readily be seen that the partial pressure of Oxygen is now 29.4 psi, the toxic level.

Gas Mixtures

The obvious question that now comes to mind is, if approaching 306 feet causes oxygen poisoning in open circuit scuba, how was the record dive of 475 feet set? The answer lies in the fact that the user was not breathing air, but rather a mixture of Oxygen and Helium, in which the percentage of oxygen was regulated to 10%, giving the diver a partial pressure of Oxygen of 23. 7 psi at 500 feet, his goal. (500 x . 445 + 14. 7) x . 10

Incidentally, this breathing a mixture of gases has another use too; the prevention, or at least partial prevention, of Nitrogen Narcosis. In this instance, because the gas breathed contains no Nitrogen, the effects of Nitrogen Narcosis are much delayed. They are not completely eliminated, however, because the diver still retains some Nitrogen in his system from the surface.

U. S. NAVY STANDARD AIR DECOMPRESSION TABLE

Instructions For Use

I. "No decompression" limits

This column shows at various depths greater than 30 feet the allowable diving times (in minutes) which permit surfacing directly at 60 ft. a minute with no decompression stops. Longer exposure times require the use of the Standard Air Decompression Table (Table 1-5).

II. Repetitive group designation table

The tabulated exposure times (or bottom times) are in minutes. The times at the various depths in each vertical column are the maximum exposures during which a diver will remain within the group listed at the head of the column.

To find the repetitive group designation at surfacing for dives involving exposures up to and including the "no decompression limits": Enter the table on the exact or next greater depth than that to which exposed and select the listed exposure time exact or next greater than the actual exposure time. The repetitive group designation is indicated by the letter at the head of the vertical column where the selected exposure time is listed.

For example: A dive was to 32 feet for 45 minutes Enter the table along the 35 ft. depth line since it is next greater than 32 ft. The table shows that since group "D" is left after 40 minutes exposure and group "E" after 50 minutes, group "E" (at the head of the column where the 50 min. exposure is listed) is the proper selection.

Exposure times for depths less than 40 ft. are listed only up to approximately five hours since this is considered to be beyond field requirements for this table.          

Table 1-6. -"No decompression" limits and repetitive group designation table for "no decompression" dives.

General Principles Of Diving Repetitive Dive Worksheet

I. Previous Dive:
minutes see table 1-5 or 1-6 for Group
feet repetitive group designation

II. Surface Interval:
Hours minutes on surface see table 1-7 Group
Group  (from I.) for new group

III. Residual Nitrogen Time:
feet (depth of repetitive dive) see table minutes
Group  (from II.) 1-8

IV. Equivalent Single Dive Time:
minutes (residual nitrogen time from III.)
(add) minutes (actual bottom time of repetitive dive)
(sum) minutes

V. Decompression For Repetitive Dive:

minutes (equivalent single dive see table time from IV.)
feet (depth of repetitive dive) 1-5 or 1-6

I I No decompression required or Decompression stops:
Feet minutes
Feet minutes
Feet minutes
Feet minutes

U. S. NAVY DIVING MANUAL
 
Table 1-7. - Surface interval credit table.

Instructions For Use

The bottom times listed in this table are called "residual nitrogen times" and are the times a diver is to consider he has already spent on bottom when he starts a repetitive dive to a specific depth. They are in minutes.

Enter the table horizontally with the repetitive group designation from the Surface Interval Credit Table. The time in each vertical column is the number of minutes that would be required (at the depth listed at the head of the column) to saturate to the particular group.

For example - the final group designation from the Surface Interval Credit Table, on the basis of a previous dive and surface interval, is "H". To plan a dive to 110 feet, determine the "residual nitrogen time" for this depth required by the repetitive group designation: Enter this table along the horizontal line labeled "H". The table shows that one must start a dive to 110 feet as though he had already been on the bottom for 27 minutes. This information can then be applied to the Standard Air Decompression table or "No Decompression" Table in a number of ways:

(1) Assuming a diver is going to finish a job and take whatever decompression is required, he must add 27 minutes to his actual bottom time and be prepared to take decompression according to the 110 foot schedules for the sum or equivalent single dive time.

(2) Assuming one wishes to make a quick inspection dive for the minimum decompression, he will decompress according to the 110/30 schedule for a dive of 3 minutes or less (27 +3= 30). For a dive of over 3 minutes but less than 13, he will decompress according to the 110/40 schedule (27 + 13 = 40).

(3) Assuming that one does not want to exceed the 110/50 schedule and the amount of decompression it requires, he will have to start ascent before 23 minutes of actual bottom time (50 - 27 = 23).

(4) Assuming that a diver has air for approximately 45 minutes bottom time and decompression stops, the possible dives can be computed: A dive of 13 minutes will require 23 minutes of decompression (110/40 schedule), for a total submerged time of 36 minutes. A dive of 13 to 23 minutes will require 34 minutes of decompression (110/50 schedule), for a total submerged time of 47 to 57 minutes. Therefore, to be safe, the diver will have to start ascent before 13 minutes or a standby air source will have to be provided.

Table 1-8. - Repetitive dive timetable.

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