VGM implementation in the VRx dive computer
Automatic tissue tolerance adjustments
To allow the VR to give realistic decompression and no stop times across the 10m to 120m range the VGM algorithm automatically modifies tissue tolerances in line with accepted decompression schedules based on stochastic data.
Tissue over pressure tolerances (M values) begin to be modified from standard VR Buhlmann settings above 30m bottom depth and 30min bottom time.
Extra changes to the default values can be achieved using the Fast, Mid and Slow tissue tolerance percentages. Positive percentages give more stop time. Negative percentages reduce stop times.
A warning screen is displayed when exiting this screen if negative values are set:
Choosing the YES option confirms you are happy with the increased decompression sickness risk. Choosing the No option will reset the tissue conservatism to the default values and take you back to the adjustment screen.
Divers can change conservatism up or down, based on their own knowledge of what suits their body chemistry and physical ailments or attributes. The Normal or Default value is 0%. Negative percentage is less safe. Positive percentage is more safe. The Bar Graph of tissue over pressure tolerance compares the current values to the standard default settings. The bar graph transitions from Fast tissues to the left (mainly relevant to deep stops) through to Slow tissues to the right (mainly relevant to shallow stops). Mid range tissues are in the middle!
Fast Tissues can be adjusted between -20% to +40%. The effect of the fast safety setting reduces linearly from Fast to Mid range tissues.
Mid range Tissues can be adjusted between -20% to +40%. The effect of the Mid setting reduces linearly from both mid range to fast, and from mid range to slow.
Slow tissues can be adjusted between -20% to +40%. The effect of the Slow setting reduces linearly from slow to mid range.
To achieve the same setting for all tissues use the same percentage setting for Fast, Mid and Slow.
Custom settings allow precise decompression matching to users known requirements or expectation for the next planned dive.
Mid range tissue adjustment is an extra feature compared to standard gradient factor adjustments. From stochastic dive data, Mid range tissues appear to require longer decompression after prolonged bottom time and/or extreme depths. VGM has a built in system for adjusting this, but the diver may wish to adjust the setting manually as well.
Use Diveplan to check settings for next dive. Fine tuning of Fast, Mid and Slow tissues can be used to achieve user specific decompression profiles.
Preset conservatism settings (0% = normal standard safety value)
Extreme exposure dives
VGM also modifies mid range tissues separately to fast and slow tissues, achieving adaptation of mid range stops separately to deep or shallow stops. Extended mid range stops are a common technique used on deep dives to allow surfacing with a lower bubble count than would otherwise be expected – Britannic 1998 exploration is an early example of this technique.
The Setup screen shows the active algorithm and allows it to be changed from VGM, Buhlmann with micro bubble stops, and VPMB with Buhlmann. If changed to VGM, then when exiting setup the user is shown the Algorithm Safety Bubble Control screen above.
Diveplan and diving
Diveplan screen allows change of preset settings for VGM if VGM algorithm active.
Resulting planned dive:
When exiting the dive plan, the equivalent gradient factor is displayed:
The Equivalent Gradient Factor (EGF) is also displayed in the Dive Log and the Dive summary data:
As an extra warning, the wave on the main dry screen goes red if negative conservatism set:
VGM - Decompression philosophy
The VGM algorithm has been built on recent practical dive planning and diving techniques as well as the scientific and theoretical understanding over the past 100 years. It combines better theoretical understanding of bubble physics together with known diving practices that help decompression and well being during and after decompression diving.
VGM also gives the user the ability to change the conservatism of the decompression to make it more conservative, but also more aggressive. Some technical divers find they feel good after a decompression with less in water time than others. The equivalent gradient factor for the VGM setting is displayed for comparison with other dive planning software and computers.
We feel the default settings will suit many experienced divers, and it is the input of real technical divers in the choice of these settings that has made VR Technology and its team at the forefront of technical diving product design for over 20 years.
Decompression is a physiologically complicated event. There are many factors that affect how well the human body decompresses and how well it is able to withstand pressure exposures. All dive algorithms have been devised to combine the complexity and risk of staying in the water with the risk of decompression sickness after surfacing. Things like hydration before a dive, and rest and hydration and even oxygen after a dive all help reduce the risk of DCS. So bear in mind that as with all decompression planning there needs to be a balance and understanding of the risk of reducing decompression times and the impact of DCS. Please refer to your training agencies’ information and advice on these issues.
VGM incorporates 5 main features:
- Haldane decompression model, based on Buhlmann ceofficients
- Modification of tissue over pressure tolerances or M values for the faster tissues to create a decompression profile similar to a bubble model like VPM
- Further modification of over pressure tolerances for deep or long exposure dives, especially in the fast and middle order tissues
- Automatic adjustment of the above parameters to allow the default settings to give common decompression and No Stop times across the range of diving from 10m to 120m
- User adjustable parameters so the diver can use his/her experience to further modify the decompression to that which suits him/her. The equivalent gradient factors are displayed for a particular dive for ease of comparison with other dive computers and dive tables, although because this system goes beyond gradient factors certain adjustments may only give an estimate of the nearest GF equivalent.
A basic version of PC dive planning software VGM ProPlanner is available free from the web site. This allows a quick way to see what decompression the VRx dive computer will give and allow specific conservatism factors to be tried out on the PC before choosing the right ones for a dive on the VRx. The PC software also gives print outs and an output in common spreadsheet style format for use in creating back-up tables.
The VRx gives an Equivalent Gradient Factor (EGF) after a dive. Because it is an adaptive variable system, the gradient factor cannot be calculated until after decompression has started. The expected EGF is displayed in the dive summary screen accessed by using a long press of the right button from the main dive screen. It is the screen displayed after running through the decompression stop look ahead. The EGF is also shown on the DivePlan exit screen and the dive log in the VRx.
VGM is able to adapt to the dive actually dived – the gradient factor is essentially adjusted on a dive by dive basis automatically. The safety factors can be adjusted from the default settings to adapt a decompression to the required profile based on the diver’s experience though.