A dive computer measures pressure and time, and runs a decompression model on those measurements throughout the dive. It shows how long you can stay at your current depth without a stop, which stops you need once you have passed that limit, and how long the ascent will take.
The model inside is of the same kind as the one a planner uses. Most technical computers run Bühlmann’s ZHL-16C with gradient factors; some manufacturers use their own models. The difference from a planner is the input: a planner calculates a dive you describe in advance, while a computer calculates the dive you are actually doing.
- Depth
- …
- Dive time
- …
- NDL
- …
- TTS
- …
- CNS
- …
- Tissues
What a dive computer measures
The main sensor is a pressure sensor. The computer converts the water pressure to depth using a water density, which is why computers have a salt or fresh water setting. It also has a clock and usually a temperature sensor.
Some computers receive cylinder pressure by radio from a transmitter screwed into the regulator’s first stage. Rebreather computers also read the oxygen cells in the loop, which measure the partial pressure of oxygen you are breathing.
The no-decompression limit
Every few seconds the computer updates the gas loading of each tissue compartment for the depth and gas it has recorded. From those loadings it calculates the no-decompression limit (NDL): the minutes you could still spend at the current depth and ascend directly to the surface.
On this dive, planned with ZHL-16C at GF 85/85, the NDL falls by about 1 minute for every minute on the bottom. Ascending to a shallower depth increases it.
Stops and time to surface
When the NDL reaches zero, a compartment would exceed its limit on a direct ascent. The model now has a ceiling, a shallowest depth you may ascend to, and the computer shows the first stop instead of the NDL.
It also shows the time to surface (TTS): the ascent and every stop at the planned ascent rate, if you left the bottom now. TTS grows the longer you stay.
During the stops, and CNS
On the ascent the computer shows the depth and the remaining time of the current stop, and moves to the next stop when the ceiling allows. The tissue bars show each compartment’s loading as a percentage of its M-value.
The CNS figure is the oxygen exposure so far, as a percentage of the allowed limit for the oxygen partial pressures breathed. On air at 30 m it stays low. On nitrox and on oxygen-rich decompression gases it can become the limit of the dive; the oxygen guide explains the calculation.
Conservatism settings
A computer running ZHL-16C lets you set gradient factors. GF 100/100 is the model with no margin. Lower values keep every compartment further below its M-value: the NDL is shorter, the first stop is deeper and the stops are longer. Other models have their own conservatism settings under names such as personal factor or safety level.
Both the computer and the plan have to use the same settings. A dive planned at GF 70/70 and dived on a computer set to GF 85/85 will show a different NDL and different stops.
What a dive computer does not know
A computer knows only what it measures. It does not know how cold, dehydrated, tired or fit you are, or how hard you worked on the bottom. The model gives a calculated limit for an average diver; decompression sickness can occur on a dive that the computer showed as within its limits.
A computer can also fail: a flat battery, a flooded case or a failed sensor. Technical divers therefore carry a second computer, or a written runtime schedule from a planner, so that they can complete the ascent if one computer stops.
Try it
Move the slider to any moment of the dive and read the screen at that moment. Change the gradient factor and the engine replans the same dive: at a lower setting the NDL reaches zero sooner and the stops are longer.
- Depth
- …
- Dive time
- …
- NDL
- …
- TTS
- …
- CNS
- …
- Tissues
What to remember
- A dive computer measures pressure and time, converts pressure to depth, and runs a decompression model on the recorded profile every few seconds.
- Before a stop is required it shows the NDL. After that it shows the first stop and the TTS for an ascent started now.
- CNS shows oxygen exposure as a percentage of the allowed limit.
- Gradient factors set the computer’s conservatism. The plan and the computer should use the same values.
- The computer knows only what it measures. Carry a second computer or a written schedule in case it fails.
The same readings in DiveLogic
A DiveLogic plan shows the ceiling, TTS and CNS at every step of the profile, from the same engine as the figures above. The NDL tool gives the no-decompression limit for any depth, gas and gradient factor. Downloaded dives are described in the Handbook.
Sources
- Bühlmann A. A., Völlm E. B., Nussberger P. (2002). Tauchmedizin, 5th edition. Springer, Berlin. (ZHL-16C compartment table.)
- Baker E. C. (1998). Understanding M-values. Immersed 3(3).
- Naval Sea Systems Command (2016). U.S. Navy Diving Manual, Revision 7. SS521-AG-PRO-010. (Appendix 2B, Navy Dive Computer.)
- NOAA (2001). NOAA Diving Manual: Diving for Science and Technology, 4th edition. Best Publishing, Flagstaff, AZ. (Oxygen exposure limits.)