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Been mulling something over this evening. I've been looking into minimum gas and running some examples. I have a question I fear may be silly and embarrasing.

If you have a dive planned to say 35m for 40 mins and next gas source is at 21m, you have a 2 min travel time from 35m to 21m, is it possible to work out beforehand what the average depth would be so you can work out your gas consumption rate?
 

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Been mulling something over this evening. I've been looking into minimum gas and running some examples. I have a question I fear may be silly and embarrasing.

If you have a dive planned to say 35m for 40 mins and next gas source is at 21m, you have a 2 min travel time from 35m to 21m, is it possible to work out beforehand what the average depth would be so you can work out your gas consumption rate?
Hope I'm not missing the question BUT there's two ways you could do this.

1) plan the whole dive until the 21m switch as 35m and that way you have a highly conservative estimate or

2) plan 40min at 35m plus 2 minutes at 28m (mid point between 21 and 35m).
 

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Been mulling something over this evening. I've been looking into minimum gas and running some examples. I have a question I fear may be silly and embarrasing.

If you have a dive planned to say 35m for 40 mins and next gas source is at 21m, you have a 2 min travel time to 35m to 21m, is it possible to work out beforehand what the average depth would be so you can work out your gas consumption rate?
In terms of minimum gas, the average depth fo the dive is irrelevant. You need to know the max depth, as sod's law this is where you would have a problem. Additionally, the bottom time is also irrelevant - at the level we are talking about.

A la dir....

so, let's say we have a 33 metre dive ( Is saturday 33 metres?)

1 minute for the problem
1 minute to get to 80% of the ATAs (24 metres)
1 Minute to get to 21 M
1 Minute for the gas switch

So it's a 4 minute ascent time.

We assume 30 litres per minute for both divers. so thats 30X2X4 = 240 litres. Now we multiply that by the average depth. Given our ascent, I would use probably 25 metres for this. That's 3.5 bar. 240 X3.5= 840 litres, or 35 bar. We can't really tell 35 bar on a guage so call it 40 bar. So minimum gas is 40 bar. I'd probably say 50 bar tbh. However, bear in mind that this is the DIR approach, so it's assuming you have a deco gas to swap to at 21 Metres.

for the dive on Saturday, the numbers also work out that the 50 bar would still allow you to deco out on your backgas if you had a deco gas failure. However, this is not always the case, especially if you move away from standard gases, so you need to consider this as well.
 

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Utrinque Paratus
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i would keep it at 35 meters :), the idea of average depth is you start the dive at the bottom say 40m for 10 minutes upto the deck 30m for 10 minutes and top of bridge 23m for 10 minutes average depth is approx 31 meters.

your plan is for the max depth all the time so you'll be conservative if you used that and came up on the wreck

Graham
 

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Utrinque Paratus
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Been mulling something over this evening. I've been looking into minimum gas and running some examples. I have a question I fear may be silly and embarrasing.

If you have a dive planned to say 35m for 40 mins and next gas source is at 21m, you have a 2 min travel time from 35m to 21m, is it possible to work out beforehand what the average depth would be so you can work out your gas consumption rate?
you have an E mail (PDF) :wink:

Graham
 

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Been mulling something over this evening. I've been looking into minimum gas and running some examples. I have a question I fear may be silly and embarrasing.

If you have a dive planned to say 35m for 40 mins and next gas source is at 21m, you have a 2 min travel time from 35m to 21m, is it possible to work out beforehand what the average depth would be so you can work out your gas consumption rate?
Yep and assuming it's linear - half it.

So eg. If i go from 40m to 20m and it takes 2 mins then work out the gas
calcs for 2 mins @ 30m.

So assuming 25 lts/min thats 4x 25x 2 = 200 lts

Total used to get from 40m > 20m = 200lts

Simple, BTBH do what Graham says and assume 2mins @ 40m, much safer.
 

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Finless: You couldn't invent him...
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If I understand your question correctly the answer is yes but you need to make 2 calculations. First is for gas consumption at 35 mtrs. Second is the average of your ascent time which means 2 mins at 28 mtrs (of course this, to be precise, depends on an even ascent rate).

Alternatively, PM me your email addy and I will send you my 'gas consumption spread sheet' - works quite well, if I say so myself + others have confirmed it is quite useful. One page to calculate your average SAC and another to plan a dive.

Returning to your original example, for gas planning I would recommend you calculate 42 mins at 35 mtrs. A. It is simpler. B. It builds in a bit of gas useage margin.
 

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A short fat well off crap cave diver. Likes wrecks
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Been mulling something over this evening. I've been looking into minimum gas and running some examples. I have a question I fear may be silly and embarrasing.

If you have a dive planned to say 35m for 40 mins and next gas source is at 21m, you have a 2 min travel time from 35m to 21m, is it possible to work out beforehand what the average depth would be so you can work out your gas consumption rate?

Only if you know the dive site.


As an example my first dive on the Moldavia I was told it was a 50m wreck so I gas planned and cut tables for 25 at 50m. Turned out to be avg 45m and you have to try relay hard to get 50. Worst was the Illinois which I was told was 70m. So guess what I planned for. Turns out the Illinois is a 55m dive. Thats a massive difference.

Lat week we did a 70m wreck that was 62-65 on the decks and we didn't touch the sea bed. Then we did a 55m wreck and spent the entire dive at 54-55m because it was flat.

This is what DOTF or Log books are for and its the main reason I hate tables.

Either cut generic tables or learn DOTF. Personally I would opt for DOTF and minimum gas if i were OC now.

ATB

Mark Chase
 
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