Ratio Deco SCUBA Diving

Ratio Deco- doing the numbers

Example of Ratio Deco for 45 meters set point. 2 Method discussed.
45- 1:1

Reviewing concepts

  • Gas absorption and eliminations by a giving tissue happens exponentially. Some models like the VVAL 18 works with lineal outgas.
  • DCS is cause by the formations of bubbles, quantities and sizes.
  • Gas tension in a tissue must not exceed about twice the ambient pressure, although today is known that exist different limits for different tissues.
  • Activation of the complement system is a large contributor factor.
  • “SAFE” implies no risk and, when we discussed tables, algorithms or deco- procedures there is always a probability of DCS no matter how small, even when diving within the limits stablished.

Individual suceptibility to DCS, enviorement and other factors influences the risk of DCS.

  • Dehydration
  • Age
  • Carbon dioxide
  • Cold
  • Exercise
  • Injuries
  • Patent foramen ovale
  • Fitness
  • Rapid Ascent, Etc.

Other factors that has a major influence on decompression are the ability to determine proper deco, to mantain the deco stops and the ability to responponse any emergency.

Before continuing to the exmaple, take a look at the other posts for review

Evolving thinking

Ratio 1:1

  • Depth Range 24m (80′)- 51m(170′)
  • Bottom time: 10 min to 120 min
  • Gases: Air; 32; 25/25; 21/35 or 18/45
  • Deco gases need it: 1 with Nitrox 50%
  • Set Point: 45m(150′)


Dive to 45 meters with 21/35 deco gas Nitrox 50 for 30 minutes bottom time.

Depth- metersRatio- Minutes
24-35 min.
27-30 min.
30-25 min.
33-20 min.
36-15 min.
39-10 min.
42-5 min.
451:1 set point

Set point refers that for every minute of bottom time you spend you will need to do a minute of deco.

In our example you need to do 30 minutes of deco. The first question to ask is- where to do the first stop? Normally you should do the first stop above the offgassing zone and, following the current procedure it will start approximately around 75% of the depth which is 33 meters.

Remember that The first stop depth is related directly to the pressure change from the maximum dive depth and, we know that first stop location is primarily to limit and prevent bubbling quantities and sizes from growing large early on in the ascent.

Other way to calculate the deco-zone is depth- 2 ata, which is 45m- 10 m (2 ata) = 35 m. It’s possible to Calculate this accurately using bubble mechanics, the excat deco-zone start at 30 meters; thus, our first stop must be above that depth.

As the diver is going to change gases at 21 m; to maximize the oxygen window, is better to do the first stop at this point. The deco- zone concept is important, because if we do our first stop at more depth than that the diver will be on-gassing.

As described above, if the diver is between 0 min and 59 min of deco, the first stop is always at 21m. If the diver is out of this range of deco, for example UTD apply the following set of rules:

Depth0-59 minutes 60- 89 minutes
50 % deep stops1 minute2 minutes
60% deep stops1 minute
UTD stop table

The ascent time up to 21 meters is determinated by the velocity ascent of 9 m/min plus 1 minute, rounded.

Now divide the deco in 2 parts. So, 15 minutes will be spent between 21 meters up to 9 meters. Then you will shape the curve between 5 stops: 21, 18, 15, 12 and 9 meters.

Approach A:

The procedure is to divide 15 minutes into 5 stops. To calculate the deep stop portion of time, dividing 15 minutes by 5 stops is equal to 3 minutes. Thus 3 minutes should be enough for the gas switch. Then you want to spend the least amount of time up until you reach 9 meters, hence at 18 m, 15 m and 12 m you will stay 2 minutes each and, the rest, 6 minutes at 9 meters for a total of 15 minutes.

Approach- S curve:

• 15 min ÷ 5 stops = 3 minutes per stop
• Keep the bottom two stop times the same, halve the two shallower stops
rounding up, and the rest at 9m stop.

DepthInitial timeS curveApproach A
213 minutes 3 minutes3 minutes
183 minutes 3 minutes2 minutes
153 minutes 2 minutes2 minutes
123 minutes 2 minutes2 minutes
93 minutes 5 minutes6 minutes
5 stops15 minutes 15 minutes15 minutes

Shapping the Shallow stops

The remaming time of 15 minutes will be weight toward 6 m and 3 m stops.

DepthS curveApproach A
6 meters15 minutes5 minutes (33%)
3 meters1 min/meter ascent to the surface10 minutes (66%) + 1 min/meter ascent to the surface

Running the tables on VPM

452730 21/35
2432 21/35
240:4033 21/35

Running the table on Cochran EMC-20H

452830 21/35
240:2131:31 21/35
21 1:36 33:1750
18 1:30 34:5750
15 1:21 36:3850

I hope you found this information useful but by no means use these examples for diving. Look for proper training in a recognized diving agency. This post is only for educational propose.

Planning the dive in the field with DiveSoft DC

DC screen: dive table
Parameters of the dive plan
Gas parameters

The table generate by the DC is a little bit more conservative of what we have been discussed above, none the less, this is within the range.

In the parameters’ screen are the surface interval, in case of a second or third dive. The target depth and the bottom mix.

In this screen is a warning regarding the amount of gas in relation of what you put in reserved, in this case 50 bars. Thus, in the next screen shows the predictive consumption in litters and bars.

The information presented is quite clear and useful. It is known the NDL at the target depth (No- deco), the Total Time to Surface (TTS), the run time, the CNS and the no-fly time (Bühlmann).

So with this DC you don’t need anything else to plan your dive, and you may use Ratio Deco to check the numbers or the other way around.

What is lack of is the ability to plan multilevel dives but you may use average depth to compensate some of the inputs.

One reply on “Ratio Deco- doing the numbers”

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s