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OK, let's see how good you guys really are!  

[Best examiner voice]
An electric (12v) compressor is rated at 45 litres per minute (flow rate).  This compressor is good enough to supply air to a diver down to 7m, after which the compressor can't supply enough.

1) A second compressor is rated at 35 litres per minute.  How deep should this compressor be able to supply?

2) If my SAC is 25, how deep should I be able to go on this compressor?

[You may now begin...]  


What's this all about?  Well, last week I spent an excellent day in Swanage on the Halsewell wreck with some of the Seadart team (Hi Bob and Chris!).  These guys have some great gear (check out Seadart website), including a surface air supply.

In the spirit of DIY, we're both investigating home-builds. Hence the need for some baseline figures, so we can compare the benchmark (the commercial product) with whatever 12v compressors are out there.  


Can anyone help?
 

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[b said:
Quote[/b] (Mdemon @ July 09 2003,19:38)]1) A second compressor is rated at 35 litres per minute.  How deep should this compressor be able to supply?
Mdemon

Is it really that simple?

It may be able to pump 35lpm at surface pressure, but can it pump even 10lpm at 20m?

Is the compressor capable of pumping against the pressure at 20m?

I don't know if my points are relvant. My experience comes from puming water. Pumps have an output curve. Put simply, you might be able to pump 100lpm to 50m, but not 50lpm to 100m. You may only get 40lpm. There is extra weight (water pressure) to work against. Does this make sense?

Adrian
 

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Hi Adrian,

Yes, you're quite right.  These compressors only work in very shallow water for that reason.

I'm trying to find the link between 45 lpm and 7m max depth and then apply that to the 35lpm machine.  Of course, that assumes they output the gas to the same pressure...

The other link I'm trying to find is the effect of depth on SAC, which I'm sure some guys here will be able to help with.  Coming at it from a different angle, if my usual SAC is, for example, 25, then 35lpm should be fine at the surface (ignoring pressure completely). If SAC increases in a predictable way with depth, then I should be able to get a rough idea of how deep I can go before it can't supply me enough (again, regardless of pressure)...

This is all "guesstimates", but will help me work out whether it's worth forking out for the 35lpm model to start experimenting...

Cheers,

John
 

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<font color='#000080'>Well air consumption at any depth should be :-
air consumption = SAC x (depth / 10 + 1)

So if SAC is 25, at 10m it will be -
10/10+1 = 2
25x2=50

or at 7m
7/10+1 = 1.7
25x1.7=42.5

Pretty sure that's it,  but this does not take into account any heavy workload or stress etc.

If I have misunderstood you and you are interested to know if SAC is effected by pressure as far as actual breathing rate increasing due to depth then in my experience no.  I actually get more relaxed and my breathing rate seems to go down a bit when I go past 10m.

Still in the interest of experimentation, side sling a pony for emergency and give it a try
let us know how you get on.

Daz
 

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Why not check out the specs for those floating surface supply systems used for tourists in the Phillipines and places. They look like a life belt with a small generator (2T) on them. Totally vile but "good", IIRC, to c 18m (60').
Might give you a pointer. I think these are still around.

I did use something similar but fixed on a  boat, way back when, it will be well obsolete by now. Can remember sod all about it other than it was vile too. Crap air, restricted movement and a taste of petrol!
Oh joy.

 

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[b said:
Quote[/b] (MATTBIN @ July 10 2003,10:31)]pedantic bit here
sac at depth = SAC x ((depth / 10) + 1)

Sorry
Matt
<font color='#000080'>Smart arse  
 

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Based simply on flow rates and pressures, if 45lpm is good to 1.7 bar, 35lpm should be good to 1.7 x 35 / 45 = 1.3 bar = 3m.  

If your SAC is 25lpm, and its no load SAP is 35lpm, your break even point is 25 x Y = 35 / Y, which gives Y=1.18 bar = 2m

However, bear in mind that this is entirely theoretical and relies on the pump being a perfect engine which is about as likely as finding a perfectly spherical cow of uniform density.
 

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[b said:
Quote[/b] (MATTBIN @ July 10 2003,10:31)]pedantic bit here
sac at depth = SAC x ((depth / 10) + 1)
(super pedantic, nitpciking bit
)

which  is the same thing if you go by basic maths precedence rules:

brackets, division, multiplication, addition, subtraction
(with the left hand having greater precedence)

so from no brackets to full brackets,
 SAC x depth / 10 + 1
expands to
 ((SAC x (depth / 10)) + 1)

but
 depth / 10 + 1
expands to
 (depth / 10) + 1

ergo
 SAC x (depth / 10 + 1)
is really
 (SAC x ((depth / 10) + 1))

(caution though - some basic calculators ignore proper precedence and just use left-right as the order in which to apply the operators instead of doing it properly)
 

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The 45lpm pump may not have the same pump curve as the 35lpm one. They may not even be 'parallel'. The working variable of one should not be used to extrapolate to the other. While the differences between these two pumps may be minimal, it is a dangerous assumption to go down.

Adrian
 

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Richie, do you have a link?  Hopefully, the electric motor should help with the taste of petrol...  The gear Seadart Bob uses is an Australian rig, and just as you describe.  Good too.

Thanks Adrian, you're quite right about the motors.  Just after a "guesstimate".

Daz/Matt et al - thanks for the figures!  All of them!  I remembered my "BODMAS" - I'll put it into Excel.

So, I really need a motor >=45lpm, and preferably capable of decent pressures.  Oh, and it has to be 12v, unless I use a 240v with a transformer, but I think the batteries would sink the boat!  Next question - anyone know of a good supplier?!

 

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[b said:
Quote[/b] (Mdemon @ July 10 2003,22:38)] Next question - anyone know of a good supplier?!
<font color='#333399'>Hi John
I'm working on that one.
Just waiting on a price for one I've found on the internet.
Cheers
Bob
 

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[b said:
Quote[/b] (hideo @ July 10 2003,17:49)]
[b said:
Quote[/b] (MATTBIN @ July 10 2003,10:31)]pedantic bit here
sac at depth = SAC x ((depth / 10) + 1)
(super pedantic, nitpciking bit
)

which  is the same thing if you go by basic maths precedence rules:

brackets, division, multiplication, addition, subtraction
(with the left hand having greater precedence)

so from no brackets to full brackets,
 SAC x depth / 10 + 1
expands to
 ((SAC x (depth / 10)) + 1)

but
 depth / 10 + 1
expands to
 (depth / 10) + 1

ergo
 SAC x (depth / 10 + 1)
is really
 (SAC x ((depth / 10) + 1))

(caution though - some basic calculators ignore proper precedence and just use left-right as the order in which to apply the operators instead of doing it properly)
<font color='#000080'>Well bugger me sideways with a fish fork.  

Didn't any of you have a misspent youth spent bunking off school drinking cheap cider from a bottle.

There is far too many of theses edumacated (sic) people around here.  


Daz
 

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Alternatively, you could do it the way I do:  depth in metres, knock the zero off, add 1, multiply by your rate on surface. Bingo, Robert's your mothers brother...
Any newbie looking in on all these maths figures is gonna get a nasty shock

Chee-az
Steve
 

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[b said:
Quote[/b] (Steve W @ July 11 2003,11:49)]Alternatively, you could do it the way I do:  depth in metres, knock the zero off, add 1, multiply by your rate on surface. Bingo, Robert's your mothers brother...
Chee-az
Steve
Ah, but what if there isn't any zero to knock off (e.g. 25 m)?
 
 

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Hi John
Got a few more facts and figures.

Point we haven’t mentioned is we are using a reservoir and are compressing that to about 20psi.
The reservoir is about the size of a 10 Lt tank
Also the DV is of a very low pressure.

Compressor supplying 45 l/min to a reservoir pressured to 20 psi will put a diver to 7 mts.

Compressor supplying 90 l/min to a reservoir pressured to 20 psi will put a diver to 12 mts.

Compressor supplying 160 l/min to a reservoir pressured to 40 psi will put a diver to 22 mts.
These figures are tried and tested.

I think our concern is more volume of air rather than pressure.
As the air is removed from the reservoir it’s how quick can it be replaced !!

Like the idea of putting two compressors in line.
I have 2 x 45Lt compressors, I’ll rig them up and see what depths that will give me.
Although I wouldn’t be able to carry out trials on them before Aug.

Can you give me any more info on the 35 l/min compressors you saw in Newton Abbott?
Make etc, the price looks very reasonable.

Found this one on the internet
www.seadart.net/images/comp01.jpg
Still waiting for the price.
Will perhaps bell them on Monday.
[looks an expensive piece of kit to me though]

Cheers
Bob
 

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[b said:
Quote[/b] (John Gulliver @ July 11 2003,14:06)]
[b said:
Quote[/b] (Steve W @ July 11 2003,11:49)]Alternatively, you could do it the way I do:  depth in metres, knock the zero off, add 1, multiply by your rate on surface. Bingo, Robert's your mothers brother...
Chee-az
Steve
Ah, but what if there isn't any zero to knock off (e.g. 25 m)? :devil:  
Ah say, ah say pilgrim, this'n here board ain't big enough fer two pedants, no way no how,  reach for yer weapons of mass destruction pardner...


Ok Ok, "depth in metres, insert decimal place after first digit and treat accordingly ..."

 

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Bob,

I like the compressor you've found - interested to hear what the price is too.  I notice it will go up to 8 bar - I'm sure my regs work at 10 to 12.  If I've got my figures right, then maybe they'll work unmodified...?


I'm off to Plymouth again tomorrow with DougP, so I'll stop in at Trago and get some more details.

As far as using two compressors, I guess you would have them in parallel, rather than series?  I wonder if you could set a simple pressure switch to kick in the second compressor only if the reservoir pressure drops below x psi.  A diaphragm system would do it, like the terrorists use!  This would save a few minutes of battery life...

If size of battery wasn't an option, there are many 240v units which could work with an inverter...!  
 

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[b said:
Quote[/b] (Mdemon @ July 12 2003,20:55)]so I'll stop in at Trago and get some more details.
OOOHHH

Diving kit kit from Trago. That'l be a first.

(and probably a last)  


Sorry, just can't stand the place. I wonder how good a comp from them will be.

Adrian
 

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Adrian, a comp from Trago will be CHEAP!!!  

I hate the place too - but it has its uses...

Bob, I put your figures into a spreadsheet and it looks like we should get 4.7m out of a 35lpm unit.  The stats on the box are as follows: Made by Carpoint, Art.Nr 06.232.17. It is rated at a max pressure of 300psi/21Kg, 69dB and will fill a tye 0-30psi in 3.5mins, for what that's worth...
 
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