When planning a dive which takes you from your home at about 600 feet in elevation (Dallas/Fort Worth area) to the dive spot like Balmorhea, which is 3205 feet. Balmorhea, no more than 25 feet deep, as that is the max depth of the lake/pool.

So herein is the issue you may perceive ... In the afternoon we'd like to visit some area attractions (Fort Davis, McDonald Observatory, etc.) However, these other places are about another 2000 to 3000 additional feet in altitude. Will it be safe for us to take these little excursions in the afternoon or should we plan on bringing lots of board games and staying in?

Answer:
The NOAA Diving Manual would say that you should consider that your dives will be in salt water (allowing you to leave out figuring in the difference in fresh water and salt water). They would also allow you to subtract 600 feet (starting altitude in DFW) from the 3205 feet (projected altitude) of the dive. This becomes a dive at about 2605 feet. You would round this off to the nearest thousand, or 3000 feet.

In their Table 4.1 on page 4-25, (see attachment) a 25 foot dive at 3000 feet would be a sea level equivalent depth of 30 feet.
However, correcting for pressure equivalents, it would closer to 27.75 fsw (feet of salt water), placing you in Repetitive Group B on your dive tables. Repetitive dives then have to be figured as if the dives are from the sea level equivalent depth dives, possibly ending up as a group G (as an example).

NOAA also has a table 4.3 on page 4-28, (see attachment) that gives the surface interval in hours: minutes before making a further ascent to altitude. The surface interval depends on upon the highest repetitive group designator obtained in the previous 24 hour period at the altitude of the dive. Ascending to 6500 feet would be an increase of 3500 feet (rounded to 4000 feet), this entered on the table at Repetitive Group G and the wait would be 1 hour: 23 minutes. There would be no wait with groups A-F.

Please feel free to call us with any questions or concerns you might have. We are always happy to take the time to give you all the information you need when making the big decision on which dive shop to trust your training to.