What is the criteria used for determining when a 1:1, 1:2, 1:3 time vs. deco needed as referenced in the articles above? Depth, number of gasses, time?
The "setpoints" are based on depth.
There are really only 3 setpoints now
at
150ft BT on 21/35 = DT on 50% (plus some deep stop time) (factor =1)
200ft BT on 18/45 = DT on 50% and DT on O2 (2x BT total plus some deep stops) (factor =2)
250ft BT on 15/55 is 1/3rd deco time on 50% and O2 (factor = 3)
multiple setpoint depth by factor then add 5min for every 10ft deeper or subtract 5mins for every 10ft shallower.
eg
25min at 180ft in 18/45 with 50% and O2 for deco
25 x2 = 50mins
20ft feet shallower than 200ft setpoint tho.
50-10mins = 40mins
40mins deco on deco gas.
20mins on 50% and 20mins on O2
add deep stops based on BT exposure
between 30-45min deco on 50%& O2 thus deep stops are 1min starting at 75% of depth (120ish) and 2 mins starting at 50% of depth (90ft) until the 70ft bottle is intersected.
last step is to shape deco gas time to create a schedule.
Change the BT to 30mins and the schedule would have the same deep stops but there would be 25mins deco on 50% and 25min on O2
30 x 2 = 60mins minus 10mins for being 20ft shallower
Change depth to 210 for 25mins?
25 x 2 = 50 plus 5 for being 10ft deeper = 55mins deco on 50% and O2 split evenly between those 2 gases. (some slight changes to deep stops for this too)
The basic premise is:
"Assuming standard gases, for a particular depth there is a relationship between BT and deco time which can be expressed as a ratio. For small changes in depth the product of this ratio can be adjusted linearly with an addition or subtraction of 5 mins per 10ft."
The ratios themselves were derived from Buhlmann with GF 30/85 with a bit of an eye towards Buhlmanns helium penalty. The ultimate shape of the deco is a bit of magic voodoo.