9-12 DIVING AT HIGH ALTITUDES

DIVING AT HIGH ALTITUDES

Because of the reduced atmospheric pressure, dives conducted at altitude require more decompression than identical dives conducted at sea level. Standard air decompression tables, therefore, cannot be used as written. Some organizations calculate specific decompression tables for use at each altitude. An alternative approach is to correct the altitude dive to obtain an equivalent sea level dive, then determine the decompression requirement using standard tables. This procedure is commonly known as the "Cross Correction” technique and always yields a sea level dive that is deeper than the actual dive at altitude. A deeper sea level equivalent dive provides the extra decompression needed to offset effects of diving at altitude.

Altitude Correction Procedure

To apply the “Cross Correction” technique, two corrections must be made for altitude diving. First, the actual dive depth must be corrected to determine the sea level equivalent depth. Second, the decompression stops in the sea level equivalent depth table must be corrected for use at altitude. Strictly speaking, ascent rate should also be corrected, but this third correction can safely be ignored.

Correction of Depth of Dive

Depth of a sea level equivalent dive is determined by multiplying the depth of the dive at altitude by a ratio of atmospheric pressure at sea level to atmospheric pressure at altitude. Using millibars (mb) as a unit for expressing atmospheric pressure at altitude equivalent depth is then :

				Pressure at Sea Level (mb)
Equivalent Depth (fsw)	=	Altitide Depth (fsw)	*	----------------------------
				Pressure at Altitude (mb)

Example: A diver makes a dive to 60 fsw at an altitude of 5000 ft. The atmospheric pressure measured at 5000 ft is 843 millibars (0.832 ATA). Atmospheric pressure at sea level is assumed to be 1013 millibars (1.000 ATA). Sea level equivalent depth is then:

1013 mb

Equivalent Depth (fsw)

60 fsw

--------

72.1 fsw

843 mb

Correction for Decompression Stop Depths

Depth of the corrected stop at altitude is calculated by multiplying depth of a sea level equivalent stop by a ratio of atmospheric pressure at altitude to atmospheric pressure at sea level. [Note: this ratio is inverse to the ratio in the formula above

				Pressure at Altitude (mb)
Altitude Stop Depth(fsw)	=	Sea Level Stop Depth(fsw)	*	-------------------------
				Pressure at Sea Level(mb)

Example: A diver makes a dive at an altitude of 5000 ft. An equivalent sea level dive requires a decompression stop at 20 fsw. Stop depth used at altitude is then:

843 (mb)

Altitude Stop Depth(fsw)

20 fsw

-----------

16.6 fsw

1013 mb

To simplify calculations, Table 9-3 gives corrected sea level equivalent depths and equivalent stops depths for dives from 10-190 ft and for altitudes from 1,000 to 10,000 ft in 1000 ft increments.

WARNING

Table 9-3 cannot be used with constant ppO2 diving equipment, such as the MK 16.

TABLE 9-3 Sea Level Equivalent Depth (fsw).

Need for Correction

No correction is required for dives conducted at altitudes between sea level and 300 ft. The additional risk associated with these dives is minimal. At altitudes between 300 and 1000 feet, correction is required for dives deeper than 145 fsw (actual depth). At altitudes above 1000 ft., correction is required for all dives.

Depth Measurement at Altitude

The preferred method for measuring depth at altitude is a mechanical or electronic gauge that can be re-zeroed at the dive site. Once re-zeroed, no further correction of the reading is required.

When using a recompression chamber for decompression, zero the chamber depth gauges before conducting surface decompression.

Most mechanical depth gauges carried by divers have a sealed one atmosphere reference and cannot be adjusted for altitude, thus they will read low throughout a dive at altitude. A correction factor of 1 fsw for every 1000 ft of altitude should be added to the reading of a sealed reference gauge before entering Table 9-3.

Pneumofathometers can be used at altitude. Add the pneumofathometer correction factor (Table 9-1) to the depth reading before entering Table 9-3. The pneumofathometer correction factors are unchanged at altitude.

A sounding line or fathometer may be used to measure the depth if a suitable depth gauge is not available. These devices measure the linear distance below the surface of the water, not the water pressure. Though fresh water is less dense than sea water, all dives will be assumed to be conducted in sea water, thus no corrections will be made based on water salinity. Enter Table 9-3 directly with the depth indicated on the line or fathometer.

Equilibration at Altitude

Upon ascent to altitude, two things happen. The body off-gases excess nitrogen to come into equilibrium with the lower partial pressure of nitrogen in the atmosphere. It also begins a series of complicated adjustments to the lower partial pressure of oxygen. The first process is called equilibration; the second is called acclimatization. Twelve hours at altitude is required for equilibration. A longer period is required for full acclimatization.

If a diver begins a dive at altitude within 12 hours of arrival, the residual nitrogen left over from sea level must be taken into account. In effect, the initial dive at altitude can be considered a repetitive dive, with the first dive being the ascent from sea level to altitude. Table 9-4 gives the repetitive group associated with an initial ascent to altitude. Using this group and the time at altitude before diving, enter the Residual Nitrogen Timetable for Repetitive Air Dives (Table 9-7) to determine a new repetitive group designator associated with that period of equilibration. Determine sea level equivalent depth for your planned dive using Table 9-3. From your new repetitive group and sea level equivalent depth, determine the residual nitrogen time associated with the dive. Add this time to the actual bottom time of the dive..

Example: A diver ascends rapidly to 6000 feet in a helicopter and begins a dive to 100 fsw 90 minutes later. How much residual nitrogen time should be added to the dive?

From Table 9-4, repetitive group upon arrival at 6000 feet is Group E. During 90 minutes at altitude, the diver will desaturate to Group D. From Table 9-3, sea level equivalent depth for a 100 fsw dive is 130 fsw. From Table 9-7, residual nitrogen time for a 130 fsw dive in Group D is 11 minutes. The diver should add 11 minutes to bottom time.

Table 9-4 can also be used when a diver who is fully equilibrated at one altitude ascends to and dives at a higher altitude. Enter Table 9-4 with the difference between the two altitudes to determine an initial repetitive group.

Example: Divers equilibrated at a base camp altitude of 6000 feet, fly by helicopter to the dive site at 10,000 feet. The difference between the altitudes is 4000 feet. From Table 9-4, the initial repetitive group to be used at 10,000 feet is Group C.

WARNING

Table 9-3 Altitudes above 10,000 feet can impose serious stress on the body resulting in significant medical problems while the acclimatization process takes place. Ascents to these altitudes must be slow to allow acclimatization to occur and prophylactic drugs may be required. These exposures should always be planned in consultation with a Diving Medical Officer. Commands conducting diving operations above 10,000 feet may obtain the appropriate decompression procedures from NAVSEA 00C.

TABLE 9-7 Residual Nitrogen Timetable for Repetitive Air Dives.

TABLE 9-3

TABLE 9-4

Diving At Altitude Worksheet

Figure 9-22 is a worksheet for altitude diving. To determine Sea Level Equivalent Depth (SLED) and corrected decompression stops for an altitude dive, follow these steps:

WARNING

FIGURE 9-22 Worksheet for Diving at Altitude.

Corrections for Depth of Dive at Altitude and In-Water Stops

Line 1. Determine dive site altitude by referring to a map. From Table 9-3, enter the altitude in feet that is equal to, or next greater than the altitude at the dive site.

Line 2. Enter the actual depth of the dive in feet of seawater.

NOTE.

Refer to paragraph 9-12.3 to correct divers’ depth guage readings to actual depths at altitude.

Line 3. Read Table 9-3 vertically down the Actual Depth column. Select a depth that is equal to or next greater than the actual depth. Reading horizontally, select the Sea Level Equivalent Depth corresponding to an altitude equal or next greater than that of your dive site.

Repetitive Dives

Repetitive dives may be conducted at altitude. The procedure is identical to that a sea level, with the exception that the sea level equivalent dive depth is always used to replace the actual dive depth. Figure 9-25 (on page 9-48) is a Repetitive Dive at Altitude Worksheet.

Example: Fourteen hours after ascending to an altitude of 7750 feet, divers make a 82 fsw 60 min MK 21 dive using the Standard Air Table. Depth is measured with a pneumofathometer having a depth gauge adjustable for altitude. After two hours and 10 min on the surface, they make a second dive to 79 fsw for 30 min and decompress on the Surface Decompression Table Using Oxygen. What is the proper decompression schedule for the second dive?

The altitude is first rounded up to 8000 feet. For the first dive, a depth correction of +1 fsw must be added to the 82 fsw pneumofathometer reading. The divers actual depth on the first dive is 83 fsw. Table 9-3 is entered at an actual depth of 85 fsw. The Sea Level Equivalent Depth for the first dive is 120 fsw. The repetitive group designation upon completion of the 60 min dive is Group O. This decays to Group H during the 2 hour 10 min surface interval.

The actual depth of the second dive is 80 fsw (79 fsw plus a 1 fsw penumofathometer correction). Table 9-3 is entered at an actual depth of 80 fsw. The Sea Level Equivalent Depth for the second dive is 110 fsw. The residual nitrogen time for Group H at 110 fsw is 27 min. The equivalent single dive time therefore is 57 min. The appropriate decompression schedule from the Surface Decompression Table Using Oxygen is 110 fsw for 60 min. A 26 min stop at 40 fsw in the chamber is required by the schedule. This stop is taken at a chamber depth of 40 fsw.

Figure 9-26 shows the filled-out Repetitive Dive at Altitude Worksheet for these two dives. Figure 9-27 and Figure 9-28 shows the filled out Diving Charts for the first and second dives.

TABLE 9-3 Sea Level Equivalent Depth (fsw).

FIGURE 9-25 Worksheet for Repetitive Dive at Altitude.

FIGURE 9-26 Completed Worksheet for Repetitive Dive at Altitude.

FIGURE 9-27 Completed Chart for Dive at Altitude.

FIGURE 9-28 Completed Chart for Repetitive Dive at Altitude.