GF99 and Surface GF Explained

Both numbers come from the same place: gradient factors, the conservatism dial on a Bühlmann computer (the 40/85 you set). GF is short for gradient factor, and a gradient factor is just how much of the model's permitted supersaturation you're actually using, on a scale from 0 (the surrounding pressure) to 100 (the raw M-value, where the model says you're risking bubbles). If gradient factors are new to you, start with Gradient Factors; the M-values underneath are covered in How Decompression Models Work.

GF99 and Surface GF are just that scale, read live, from two different vantage points.

Watch them both move across a dive

Set a depth, a bottom time and your gradient factors below, then scrub the whole dive: descent, bottom, the ascent, the stops. Watch GF99 (where you are now) and Surface GF (where you'd be if you bolted up) move together.

The shape tells the story. On the bottom, GF99 reads On Gas while you load up, but Surface GF is already high, because a direct ascent from depth is the most aggressive thing you can do. As you come up, GF99 climbs and Surface GF falls, and they meet near the surface. You've earned the surface when Surface GF reaches your GF High.

GF99: where you are right now

GF99 is your gradient factor at this instant: the overpressure in your leading tissue, as a percentage of the distance from the surrounding pressure up to that tissue's M-value. The "99" is just the name for the current value, not a score out of 100.

• On Gas: you're still loading; no compartment is supersaturated yet, so there's nothing to express as a percentage.
• 50: your leading tissue sits halfway between the surrounding pressure and the line where bubbling gets likely.
• 100: you're sitting right on the raw M-value. Past that, hold or drop down slightly.

While you load on the bottom, GF99 shows On Gas. It starts showing a number and climbing once you ascend and a compartment goes supersaturated, usually peaking somewhere on the way up, then drops as you hang shallower and breathe the nitrogen back out. Treat it as a live speedometer for decompression stress, not a speed limit.

Surface GF: the number that decides your ascent

Surface GF takes the same idea and asks it at the surface: if I went straight up right now, what gradient factor would I hit up there? It projects your current loading to surface pressure and reads out the result.

• Surface GF well above your GF High: a direct ascent would blow past your limit. Stay down, or keep ascending slowly.
• Surface GF at your GF High (say 85): you've earned the surface at the conservatism you chose. On a Shearwater it sits yellow above GF High and red above 100, so the colour alone tells you where you stand.
• Surface GF below GF High: you're carrying margin to spare.

This is the honest go/no-go number, because it always assumes the worst case: a straight bolt up. GF99 can look reassuringly modest at 30 m while a direct ascent would still hurt you, since surfacing dumps far more pressure than staying put. Surface GF closes that gap.

Which one to watch

Use them for different jobs. GF99 is your state, moment to moment: watch it to understand what the dive is doing to you. Surface GF is your decision: watch it to know when you can leave. Read GF99 to follow the dive; read Surface GF to decide the ascent.

How to read them in the water

• On the bottom, expect GF99 to read On Gas and Surface GF to be high. Both are normal; you haven't started clearing yet.
• On ascent, let Surface GF set your pace. If it's falling toward GF High, you're decompressing as planned.
• Treat GF High as the line, not zero. Surfacing at GF High means surfacing at the conservatism you chose, not cutting a corner.
• After a cold, long or hard dive, bank a little extra by letting Surface GF land below your GF High. Safe ascents covers flying the stop on a real computer.

For the live, in-the-water side of these numbers (ascent rate, what the algorithm can't see), see Dive computers in real time.

References

1. Shearwater Research. Surface GF and Other Teric Musings. Shearwater community blog.
2. Shearwater Research. Teric / Perdix Operating Instructions — GF99 and SurfGF display definitions (On Gas behaviour; SurfGF yellow above GF High, red above 100%).
3. Bühlmann AA. Decompression–Decompression Sickness. Berlin: Springer-Verlag, 1984.
4. Baker EC. Understanding M-values. Immersed. 1998;3(3).
5. Doolette DJ. Gradient Factors in a Post-Deep Stops World. InDEPTH. 2019.

This makes sense fastest when you watch the numbers move on a real dive, with a computer on your wrist and an instructor beside you. Our technical and CCR training builds decompression literacy from the gauge up. Get in touch and tell us where you're at.