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FHOF max effective depth....

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New Member
Aug 28, 2002
Hi ya

did some more tests and experiments with FHOF at the SETT tank this weekend. I wonder if someone can do the maths to answer these questions?

the FHOF system in its current set up is a 9L bag with a 0.2L cylinder

at a depth of 100m will there be enough lift with this setup to lift a diver from the bottom.

If the answer to the above is no what would be the failure depth of this system with say a 85kg diver ?

what would be the desired bottle size to provide ample lift at depths of 100m or more?

Thanks in advance
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assuming the cylinder is filled to 230bar, it could fill the bag at depths of up to about 40m. beyond that it would only be partially filled. at 100m the bag would be about half full. you'd need a 0.4-0.5litre bottle to fill it at that depth.
i think that amount of buoyancy would be enough to lift a sensibly weighted freediver from that depth, but i'm not sure what the initial ascent speed would be like.... could be rather slow?.... you could get a reasonable estimate by running an experiment in shallow water.

um.... weren't these used in Cyprus? haven't they already been tested down to 90-100m?

I think the best way to do this is to run some more tests.

A good place to do them would probably be the 10m line. This will be at 2 bar pressure and makes some of the maths a bit easier. ( Make sure the top of the bag ( after inflation ) is equal to the line, not the diver ).

Ideally, the 85 kg diver should have exhaled enough so that he / she has empty lungs at 10m.

Attach the FHOF, and slowly add air, until the bag has enough lift to start the ascent ( and then close the valve ).

Back at the surface, attach an SPG to the cylinder, and measure the pressure. ( You should also have measured the pressure beforehand. ) From the difference ( in bar ), you can then work out the volume. This figure then needs to be divided by 2 to give the volume needed at 10 metres.

From there, you can easily work out the quantity of gas needed at each 10 metre depth, until you reach the point where it exceeds the cylinder contents.

If you wanted to err on the side of caution, and say that you needed a full bag of air at any depth to lift the diver, then things are a bit easier. If your cylinder is charged to 200 bar, then you have ( 200 * 300 ) / 1000 = 60 litres.

At 50m depth, there is 6 bar of pressure, so the volume scales down to ( 1/6 ) * 60 = 10 litres - more than a bag full.

At 60m depth there is 7 bar of pressure, so this works out at ( 1/7 ) * 60 = 8.6 litres - less than a bag full.

If you are testing in the SETT tank, then you do need to be generous with the amount of lift needed. Although a constant bag volume will give the same amount of lift at ant depth, the bouyancy of the diver will decrease at deeper depths, as air spaces are squeezed further.

I'm not qualified to comment on this, it is simply my opinion based on the diving physics I know. Before anyone's life is put on the line, someone who is more knowledgeable about physics and maths should validate this.

Thanks for keeping an eye on us all on Saturday :)

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Alun / Mark

There has never been a test done with a real diver from 100m not by us anyhow! Tests have been done using weights at various depths and other people have worked out that in theory it works.

Theory is all good but until someone has tested it out from that depth ?

Ideally we would like to see it lift someone from 100m but hey.... there was a lack of volunteers

My concern is with the initial asscent rate, the current system willl lift two divers plus a small weight belt from 28m fresh water but the asscent starts very slowly.

Think the plan is for the FHOF for the deep dives to use bigger bottle to ensure the ascent is quicker.

just thinking of you guys
i didn't mean a test on a real person - but an object with similar drag and buoyancy to a freediver at 100m.... shouldn't be too difficult to build some form of crash test dummy.

yes, i would say it should work in theory. i think a half filled (4-5 litre) bag would shift a freediver at those depths - but again - the speed is the critical thing... and that's hard to estimate without doing a realistic experiment.

there isn't a great deal of difference in the buoyancy of a freediver between 50 and 100, so half-filling the bag at 50-60 will give you a reasonable idea of what will happen at 100.

i was going to ask if it's possible to attach a larger bottle - i should think it is - it must have a standard fitting.

i have been thinking about buying one, but i think i'd want to fit a bottle that holds more than 200ml.
Alun - Hadn't seen your reply before mine went out, but it looks to work out fairly similar. I think I'd stick to 200 bar maximum for my calculations, as this allows a pressure difference contingency.

If one bag lifts 2 divers from 28 metres, then a half bag should lift one. I'd add another 50% for contingency / decreased boyancy ( i.e 6.75 litres ).

The max pressure equation should be:

( 1 / ambient pressure ) * cylinder capacity = bag fill volume, i.e
( 1 / ambient pressure ) * 60 = 6.75, so:
1 / ambient pressure = 6.75 / 60
ambient pressure = 1 ( 6.75 / 60 ) = 8.89 bar

To convert this to depth ( in metres ):

( pressure in bar - 1 ) * 10 = 78.9 metres

This would make me feel uneasy about using one 300ml cylinder on an FHOF below 75 metres.

Same disclaimer as before.

I think the fitting is standard so you could fit any size bottle you wanted.

James, thanks for that.

this is how i reached my figures...

0.2l x 232bar = 46.4 litres at the surface
divide by asbolute pressure to give volume available at depth
40m.... 46.4/5 = 9.3litres.... full
100m....46.4/11 = 4.2litres.... half full

working on 200bar...40litres at surface
40m...8l... 90% full
100m...3.6l.... 40% full
one thing i did forget is that the system needed to be as small as possible because the safety diver needs to carry it somewhere, this was why the original design uses a 0.2L bottle.

cheers guys
I think your calculations are a bit clearer than mine, Alun. I was working on the assumption of a 0.3l cylinder being used ( which some of the more recent FHOF's have ).

Maybe a dual cylinder set-up is worth considering. 2 x 0.3l ??

Then there is some redundancy as well. The 1 x 0.2l configuration still looks usable, but perhaps not beyond 40-50m.
I understood that the FHOF is a standard AP Valves SMBCi delayed surface marker buoy, which is used for scuba diving. They are available with 100ml or 200ml bottles but the Freediver.co.uk site says the bottle is 400ml.

Something to consider is that you don’t want the bag to be too full at depth. There is a small chance that the expanding gas won’t vent fast enough through the dump valve and that the bag will rupture. The AP Valves DSMBs have a worldwide reputation for being exceedingly tough but a friend of mine has ruptured 2 of them by inflating them too much at depth (using a regulator).

When technical divers send them up from the bottom (to show the boat skipper that they are off the wreck and doing their drifting decompression) they will only fill them to a certain point, knowing that the bags will be completely full and sealed against the baffle on the surface. I assume that the SMBCi systems are designed to be used with the 100ml – 400ml bottles and using a bigger bottle could increase the risk of the bag rupturing. If you used a bottle that had the capacity to inflate the bag fully at 100m, I don’t know if the bag will remain intact until the surface.

I am sure that AP Valves could answer any questions easily enough.

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we were discussing this at the weekend and Donna mailed me the following, calculations done by Martin

Following the discussions at the tank on Saturday regarding the FHOF at depth -

Gas in a 0.4 cylinder at 210 bar = 84 litre/bar, but assume a hot fill at 40 C and dive at 7 C then 313 degrees K / 280 degrees K * 210 bar = 188 bar.

So 0.4 x 188 = 75 litre/bar

If SMB is 25 litre then maximium depth at which it will be filled is 20m (3 bar)

@ 50m or 6 bar, the SMB will occupy a volume of 75/6 or 12.5 litre, which will lift 12.5kg
@ 100m, looking at a lift of around 6kg
The lift values are a bit more difficult to assess, because it then comes back to how much lift you need for your heaviest diver. More testing in the SETT would help with this.

There is a big difference though, between James' estimate of 9 litres capacity for the FHOF, and Martin's of 25 for the SMB.

Something else that really ought to be measured accurately.
Don't know where Martin got 25l from
According to the Buddy website the SMBCi have 20 lb (c 9kg) if buoyancy at the surface so I guess James's 9l is right
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