1At a glance
- Purpose
- To adjust decompression model conservatism
- Origin
- Bühlmann decompression model
- Components
- GF Low (depth), GF High (surface)
- Range (typical GF Low)
- 30% to 90%
- Range (typical GF High)
- 70% to 99%
- Default settings
- Often GF Low 40%/GF High 85%
- Application
- No-decompression and decompression diving
2How gradient factors work in dive computers
Gradient factors essentially modify the 'M-values' of the Bühlmann algorithm. M-values represent the maximum inert gas pressure a tissue compartment can tolerate without suffering from decompression sickness (DCS). A GF Low value (e.g., 30%) means that a diver is allowed to load their tissues only to 30% of the M-value at maximum depth, effectively requiring deeper stops. A GF High value (e.g., 85%) means that at the surface, the diver is allowed to off-gas to 85% of the M-value, indicating the conservatism of the shallowest stops and overall surfacing.
When a dive computer calculates a decompression profile using gradient factors, it interpolates between the GF Low and GF High values. As the diver ascends, the permitted tissue pressure gradually increases from GF Low's conservatism towards GF High's. This linear interpolation ensures that decompression stops are prescribed at depths that allow for efficient off-gassing while maintaining a buffer against DCS.
For example, a GF setting of 30/85 (GF Low 30%, GF High 85%) would be more conservative than a setting of 40/90. The 30/85 setting would likely result in deeper and/or longer decompression stops compared to 40/90, providing an increased safety margin, particularly for repetitive or aggressive dive profiles. Divers can adjust these settings based on their physical condition, dive intensity, and environmental factors.
3Gradient factor options compared
| Setting | Conservatism | Dive Profile Impact | Typical Use |
|---|---|---|---|
| GF 50/80 (High Conservatism) | Very High | Longer and deeper decompression stops, shorter no-deco limits. | Aggressive multi-day diving, less fit divers, colder water, technical diving with significant bottom time. |
| GF 40/85 (Moderate Conservatism) | High | Deeper and slightly longer decompression stops than default, reduced no-deco limits. | Repetitive liveaboard diving, early post-DCS dives, divers seeking extra safety margin. |
| GF 30/85 (Standard Technical) | Standard Technical | Standard for technical diving, balances conservatism with reasonable dive times. | Most technical decompression diving, extended bottom times. |
| GF 40/95 (Less Conservative) | Moderate | Similar to traditional recreational profiles, shorter and shallower stops. | Single dives, divers with high tolerance, specific training scenarios where efficiency is key. |
| GF 90/90 (Bühlmann ZHL-16B equivalent) | Low (Least conservative) | Minimal decompression, essentially mirrors original Bühlmann, potentially aggressive surfacing. | Rarely used directly by recreational divers, often a baseline for comparison. |
4What gradient factors mean for your liveaboard trip
On a liveaboard trip, divers typically perform multiple dives per day for several consecutive days. This repetitive diving creates a significant accumulation of inert gas in the body, making conservative dive planning essential. Adjustable gradient factors on a dive computer allow liveaboard divers to tailor their decompression obligations to this demanding schedule, enhancing safety.
By selecting more conservative gradient factor settings (e.g., 40/85 or 50/80), divers can proactively mitigate the risk of decompression sickness. These settings will typically prescribe slightly deeper and/or longer safety stops or mandatory decompression stops, ensuring more thorough off-gassing of nitrogen between dives and across the entire trip. This is particularly beneficial on trips with frequent deep dives or in situations where divers may feel fatigued or slightly dehydrated.
While liveaboard operators and dive guides ensure adherence to safe diving practices, divers are ultimately responsible for understanding and managing their personal dive computers. Customising gradient factors can provide peace of peace of mind and contribute to a safer, more enjoyable multi-day diving experience, allowing divers to maximise their time underwater without compromising their safety.
5Common dive computer gradient factor defaults and recommendations
| Dive Computer Brand | Typical Default GF | Common Recreational Recommended GF | Common Technical Recommended GF |
|---|---|---|---|
| Shearwater | GF 40/85 (Adjustable) | GF 40/85 to 50/90 | GF 30/85 to 40/85 |
| Suunto | Varies by model (often P+ settings), not direct GF | P+2 (more conservative) | Not directly applicable, uses proprietary algorithm |
| Mares | Varies by model, uses proprietary RGBM | Conservative setting | N/A |
| Garmin Descent | GF 40/85 (Adjustable) | GF 40/85 to 50/90 | GF 30/85 to 40/85 |
| Scubapro | Varies by model (ZHL-16 GF or ZH-L16 ADT MB) | Lesser MB Levels (more conservative) | N/A |
6Common misconceptions
Myth: Gradient factors only apply to technical divers. Fact: While widely adopted in technical diving for precise decompression planning, many recreational dive computers now incorporate adjustable gradient factors or similar conservatism settings. Understanding them is beneficial for any diver seeking to fine-tune their dive profile safety, especially on repetitive or demanding dive trips like liveaboards.
Myth: Higher gradient factors always mean safer diving. Fact: This is partially true for GF Low but inverse for GF High. A lower GF Low value (e.g., 30% vs 50%) means more conservatism at depth, requiring deeper stops. A lower GF High value (e.g., 70% vs 90%) means more conservatism at the surface, requiring longer shallow stops. It's the combination that defines the profile, and overly conservative settings can lead to unnecessarily long decompression, increasing gas consumption and exposure.
Myth: Setting gradient factors to 90/90 provides maximum conservatism. Fact: A setting of 90/90 is actually the least conservative gradient factor setting in the Bühlmann ZHL-16B model, essentially matching the maximum allowable tissue pressure throughout the ascent. This setting results in the shortest decompression obligations but offers minimal safety margins compared to more common, conservative gradient factor combinations like 40/85.
Myth: Once set, gradient factors never need to be changed. Fact: Divers should periodically review and adjust their gradient factor settings based on their physical condition, hydration, dive type (e.g., deep, cold, strenuous), and the specific demands of a liveaboard itinerary. More conservative settings are advisable for multi-day repetitive diving, colder water, or if feeling fatigued.
FAQ
What is the difference between GF Low and GF High?
GF Low (Gradient Factor Low) dictates the maximum allowed inert gas supersaturation at deeper depths, determining when the first decompression stop starts. GF High (Gradient Factor High) dictates the maximum allowed inert gas supersaturation at the surface, influencing the length and depth of shallower decompression stops and the overall conservatism of surfacing.
How do gradient factors affect my no-decompression limit (NDL)?
More conservative gradient factor settings (e.g., lower GF Low and/or GF High values) will result in shorter no-decompression limits (NDLs). This means your dive computer will indicate a need for decompression stops sooner than it would with less conservative settings, providing an increased safety margin.
Can I adjust gradient factors on all dive computers?
No, not all dive computers allow direct adjustment of gradient factors. Many recreational models offer pre-set conservatism levels (e.g., 'Conservative', 'Normal', 'Aggressive') that indirectly adjust the underlying algorithm. Higher-end and technical dive computers more commonly provide direct gradient factor control.
What is a good starting point for gradient factor settings for liveaboard diving?
For repetitive liveaboard diving, a common and often recommended starting point for gradient factors is 40/85 (GF Low 40%, GF High 85%). This provides a good balance of conservatism without excessively prolonging dive profiles, offering an enhanced safety margin for multi-day diving.
Do gradient factors replace safety stops?
No, gradient factors do not replace the standard three-minute safety stop typically performed at 5 metres (15 feet). While more conservative gradient factor settings might extend or deepen your decompression obligation, the safety stop remains a prudent practice for all dives, even those well within no-decompression limits.
How do temperature and exertion affect gradient factors?
Colder water and higher exertion during a dive can both increase inert gas uptake and reduce off-gassing efficiency. In these conditions, it is advisable to use more conservative gradient factor settings (e.g., lower GF Low and GF High values) to provide an additional safety buffer against decompression sickness.