VGM - Decompression philosophy
The VGM algorithm is 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 knowledge of bubble physics together with known diving practices that help decompression and well being after and during decompression diving.

VGM also gives the user the ability to change the conservatism to increase or decrease decompression times. 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.

The default settings have been implemented based on feedback and dive records from 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, rest 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.

WARNING!
Even if your dive computer or PC generated software allows for less conservatism and a reduction in decompression times this should not be arbitrarily undertaken without first researching data available from other divers/agencies, conducting controlled trials and understanding that you may be undertaking a level of experimentation in order to adjust the algorithm for your specific needs.

VGM incorporates 5 main features:

1. Bühlmann decompression model
2. 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
3. Further modification of over pressure tolerances for deep or long exposure dives, especially in the fast and middle order tissues
4. 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
5. User adjustable parameters - 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. However, because this system goes beyond Gradient Factors certain adjustments may only give an estimate of the GF equivalent.

Background to the VGM algorithm

Below is a brief background to the rationale behind the VGM decompression algorithm and its validity in technical diving.

Decompression from common sports and technical dives
We know from experience that common dives to around 30m do not require the type of decompression that VPM or gradient factors of say 20/80 would advise. Bühlmann and Navy tables are reasonably reliable to these depths and short durations.

Contrast this with an 80m technical dive. With VPM and modern research, it has been shown there is now a need to do deep water stops.

So why is there a difference? There are scientific papers on this, but my feel for this has, as much as anything, come from a chance remark when talking to Phil Short. We were looking at a typical VPM decompression for an 80m 30min trimix dive. He instinctively described the first half of the stops as decompressing the bottom time, and the second half of the stops as decompressing the first part of the decompression. It was as though there were two dives to be decompressed. One might consider even deeper dives to have even more pseudo dive sections.

So the 30m dive only really has one decompression. The extra decompression that would be required using VPM is not necessary as the body and bubble growth is stabilised shortly after the initial decompression, because the diver gets to the stable condition of the surface relatively quickly.

However, for the 80m dive, the diver has to manage the initial micro bubble growth more properly, as his body will be subject to considerable further tissue pressure difference exposures as he/she ascends through each of the decompression stops. There is no swift stabilization of pressure. Bubbles have to be managed much more carefully because of this instability and continued exertion. (Micro bubbles, arterial bubbles and large bubbles all have to be treated carefully).

It is as though the decompression path becomes not only longer and longer, but also narrower and narrower with increased depth and bottom time. The fact that it becomes narrower suggests a need for more conservatism in the decompression. You might run down a wide road with a precipice either side, but would you run down a narrow path in the same conditions? A decompression for an 80m dive may now be relatively common and even standard, but the diver still has to be careful not to over exert himself after the dive. Rest is the best. We know the body may feel alright when surfacing after one of these dives, but DCS can be brought on by exertion etc..

So when doing deeper and longer dives, the body is exposing itself to a higher potential for triggering DCS. Thus extra conservatism is needed to give flaws in the body, or exertion or stress the chance of being coped with without DCS. Deeper dives in themselves are more stressful, so decompression tables cannot just be extrapolated. They have to take into account that the likelihood of creating conditions for DCS to manifest itself, are increased as well. Not just more gas loading, but also more chance of that gas loading causing DCS..

One can extrapolate the path analogy such that at some stage the path becomes a tightrope. The care required to traverse the decompression increases.

And fitness does not make the need for decompression diminish. Paraphrasing Billy Deans – decompression is not something you can train for – you cannot train your body to require less decompression. (You can however damage your body so that it requires more..)

So what does VGM do?
VGM takes account of the reliable data and experience of the sports and technical divers. It uses this data to create a change in style and increase in conservatism of decompression stops, moving from what we know works for shallow dives through to what we know works reasonably well for common technical dives down to around 120m.

For a few years, divers have used different gradient factors to achieve this change in style, manually adjusting gradient factors of Haldane models to suit the dive they are planning. VGM does a similar thing, but modifies the over pressure tolerances of the different tissues that control the various stop depths and durations. VGM modifies these tolerances based on increased depth and bottom time so that the diver does not have to. The equivalent gradient factor is shown during and after each dive. This gradient factor will change depending on the bottom depth and time for each dive. The user can further manually adjust the factors modifying the different tissues to achieve a decompression that suits his or her experience and body conditions.

Note – 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.

Because VGM can modify midrange tissues, the equivalent gradient factor, which considers a linear change from the first stop to the last stop, will not be wholly accurate. It will however be a good indication.

Summary
VGM takes the over pressure tolerances acceptable for shallow, short dives and modifies them to those required for 120m or long dives, as the bottom depth and duration are increased. This technique has achieved decompressions already in common use for all dives in the 0 to 120m range.

So, with VGM, we now have one decompression model that fits all dives in this range. (Note: Diver specific adjustments must still be made as suggested for unfit or smokers etc..) VGM allows further safety to be adjusted by the diver to create a schedule that suits his/her experience.

Custom VGM
We are currently working with navies and other diving agencies to further develop and customise the capabilities of VGM to fit specific decompression experience and requirements into a VGM model for diving outside the sports and technical diving domain. If you are a diving agency or navy, and require a solution specific to your needs, please contact us at our head office. Phone +44 1 202 624 478.