January 23, 1999

Today's intended mapping mission was scrubbed due to bad weather. A strong weather front moved through and the area was surrounded by lightning from around 10 a.m. to 3 p.m. The heavy rain was not a deterent to John Zumrick and Paul Heinerth's mapping mission to C-Tunnel, but the thought of the decompression chamber (floating at the cave mouth) being hit by lightning while a crew was on-board was grounds for an abort. Fortunately, the gear is already prepped, the mapper and scooters are charged and ready, and the mission will get an early start tomorrow.

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The Approach to Saturation

As bottom times (time spent inside the cave at depth) grow longer during radio location and mapping missions, a curious phenomenon takes place. Bodily tissues and organs begin to "saturate" with the inert gas (helium) being breathed under pressure. The time required to safely offgas the helium (known as decompression) rapidly builds up during the first two hours spent at depth (typically between 81-91 meters (265 to 300 ft) in Wakulla Spring). As mission duration grows longer, however, the rate at which the decompression penalty rises for every minute spent within the cavern diminishes. This means, for example, that Jason Mallinson and Rick Stanton, on their January 21st mission to O-Tunnel, incurred only 18% more decompression than Matthes and Kakuk's January 18th mission, even though they spent nearly 50% more time inside the cave. As we proceed even farther into the cave we can expect this phenomenon of saturation to work to our favor, such that exceptionally long duration missions will be able to be conducted without an unmanageable decompression obligation. "Unmanagable" is a relative term. We will shortly be switching to the new dual MK5 rebreather system which will significantly reduce the stress of being at exceptionally remote distances from the entrance (largely due to having a spare 8 hour life support system available on an instants notice). Thus far, thermal control has not proved to be a problem and the propulsion vehicles and underwater lighting systems are not being taxed. Our safety procedures for long missions already involve having two MK5 rebreathers available independently for emergency decompression (in addition to the dual units that will be carried by each diver). These procedures optimize the in-water portion of the decompression (through active oxygen control). Pulmonary oxygen toxicity during the final phases of the chamber decompression profile is an issue at this stage. Our chamber decompression approach (which has produced no bends incidents to date) is as follows:

1) The returning divers doff their MK5 rebreathers at the 24 meter (80 ft) level and enter the PTC with assistance of the support dive crew. The dive is terminated on the rebreather onboard computer system by means of a software command while underwater (a special software feature developed for the MK5 for the Wakulla 2 expedition).

2) On the surface, the chamber team receives the MK5 rigs and interrogates the onboard computer for its decompression information. Meanwhile, the returning exploration team has typically reached the surface (inside the PTC) by this time and has locked into the habitat.

3) Using the uploaded decompression status for each diver, a conservative schedule for the chamber is generated. The schedule is controlled by the diver with the maximum exposure, assuming compressed air on bibs (with manual bell vent) inside the PTC during travel to the surface; Nitrox 50/50 on bibs (overboard dump) from 21 to 9 meters (70 to 30 ft) inside the chamber; and pure oxygen on bibs (with overboard dump) from 9 meters (30 ft) to the surface.

4) Parallel sets of decompression tables are automatically generated which consider an abort to compressed air (normal chamber mix) at any point at and above the 9 meter (30 ft) decompression stop. This option may be exercised either at the request of the dive team, or taken independently by the chamber operator if it is believed that pulmonary oxygen toxicity is becoming a risk to the divers. Thus far, this option has not been exercised. Because the oxygen exposure (OTU) count is typically high on these missions, dive teams are required to take 3 off days between missions. Again, thus far, this approach is working and our mission manifest has been able to sustain a schedule of 3 consecutive missions followed by a team-wide day off (generally used for open circuit missions of limited range or gear maintenance).

Bill Stone

January 24, 1999

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