runsongas
Contributor
The chart on page 11 is more applicable if you are going directly from interstage pressure into the loop with the leaky MAV. Outlet would be less than 50% of inlet until 100m.
Needle valve spreadsheet?
- Thread starter rjack321
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kotik
Contributor
Good point, agreed.
kotik
Contributor
Gotcha, there's my ignorance of the application front and center.
That Jan Willem Bech article is pretty awesome, he went over and above writing that. I'm gonna pipe down now, since even though your original question clearly stated choked flow, I went right into talk of changing delta P's and other irrelevant stuff. This whole subject is interesting, I never knew the choked flow phenomenon was put to use like this in rebreathers.
Download the Pelagian DCCCR manual off rebreatherlab.com. There's a chart on page 65 that's basically a flow table, and it's got some math on it that might help.
I had to do all of the math for my Mod 1, but in all practicality, it's pretty useless considering you WILL have to futz with the needle if you want to take advantage of the benefit of having a needle valve in the first place. (as opposed to CMF & blocked 1st or the Fathom wünderkombo)
This is why the blocked 1st stage of the Fathom is a rather silly solution to a problem that doesn't really exist in actual diving. Yeah, it's great that you set it on the surface, but as soon as your workload increases, you've gotta play with it. As soon as you get back on the scooter, you've gotta play with it. As soon as you start deco'ing out, you've gotta play with it. You've gotta play with it anyway, so measuring it with a micrometer is kinda useless when you're marking it with railroad chalk and going at it with a dull beaver.
From a purely academic standpoint, it's an interesting discussion, from an actual dive standpoint it's really splitting hairs. I find there's more benefit to playing with it over the course of a few dives to get a feel for it, rather than trying to be ultra precise with your surface setup of your flow rate.
I had to do all of the math for my Mod 1, but in all practicality, it's pretty useless considering you WILL have to futz with the needle if you want to take advantage of the benefit of having a needle valve in the first place. (as opposed to CMF & blocked 1st or the Fathom wünderkombo)
This is why the blocked 1st stage of the Fathom is a rather silly solution to a problem that doesn't really exist in actual diving. Yeah, it's great that you set it on the surface, but as soon as your workload increases, you've gotta play with it. As soon as you get back on the scooter, you've gotta play with it. As soon as you start deco'ing out, you've gotta play with it. You've gotta play with it anyway, so measuring it with a micrometer is kinda useless when you're marking it with railroad chalk and going at it with a dull beaver.
From a purely academic standpoint, it's an interesting discussion, from an actual dive standpoint it's really splitting hairs. I find there's more benefit to playing with it over the course of a few dives to get a feel for it, rather than trying to be ultra precise with your surface setup of your flow rate.
I've been diving a needle valve longer than the Fathom has been around, so I'm intimately familiar with its use. I fully understand the reasoning behind the blocked first stage and the needle valve combination. I'm not complaining about it, I just think it's a solution to a problem that doesn't actually exist.
Tell me, when you dive your Fathom, do you adjust your needle valve during the dive? If the answer is yes, then you've already negated the benefit of the blocked first stage. If you set it on the surface and don't touch it, then there's no point in having a needle valve in the first place, just use a leaky valve like a KISS.
Don't get me wrong, I like the Fathom, I like its construction, I like the head closure mechanism and the ability to use a radial scrubber. The first one was a Meg, and the pedigree shows. I just disagree with the idea that a blocked first stage and a needle valve is any benefit over just a needle valve. You have to adjust the needle valve anyway, and I've heard all of the "task loading" excuses Fathom proponents like to throw out there, but If you get task loaded tweaking a needle valve, you probably shouldn't be on a rebreather anyway. Even with scooters and a truck full of bailout, it's just not an issue after a couple hours on a unit.
Hell, I wouldn't mind having a unit, but I wouldn't bother with a blocked first stage. It just means if you get to the dive site and have a problem, you're taking a reg apart on a picnic table to swap parts instead of just throwing on another first stage. A 2 minute reg swap becomes an annoying deal.
I've got right under 50 hours on the Fathom and don't have any issues with the needle valve. Johnny's right that it is imprecise, but when you get it dialed in for the workload you don't have to make any other changes. Yesterday I spent 60 minutes scootering around Ginnie, I don't think I manually added O2 or DIL more than 2 or 3 times. Not having to adjust flow rate when you have depth changes is nice, especially in places with wide changes like Manatee or Cathedral.
I get your point a little more now. Hadn't really put too much thought into the difficulty in swapping regs if there's an issue. I can see that as a problem when you're travelling. I don't see it as a huge problem though (I guess not until it happens in a remote location far from home).
OP
I have a kiss now and the orifice in the cmf is pretty big - just one of those things with trying to make really tiny holes, sometimes they aren't as tiny as one would like. The bottom line is that my consumption is usually slight and hence my IP is around 130-135psi. Which is fine for most of my diving but not very versatile and is barely working at 250ft.
I have debated either:
1) changing to a stronger tec-me spring and a mav with a 0.0030" orifice (or even 0.0025") so I can get my IP up around 200psi and basically have no practical depth limit on the CMF (I'm not really a 400ft diver and if I wanted to do that deep a dive I would use my eCCR Meg)
2) putting the depth compensation back into the o2 reg and getting a needle valve.
But I'd was hoping to get some sense of how actual flow rates would vary as the IP rises. Yes some of this comes with just fiddling with it in use. But especially if I am doing a 280ft wreck dive with a drop straight down the shot line - how do you set the needle valve on the surface so its not going crazy fast by the time you hit the bottom?
I have debated either:
1) changing to a stronger tec-me spring and a mav with a 0.0030" orifice (or even 0.0025") so I can get my IP up around 200psi and basically have no practical depth limit on the CMF (I'm not really a 400ft diver and if I wanted to do that deep a dive I would use my eCCR Meg)
2) putting the depth compensation back into the o2 reg and getting a needle valve.
But I'd was hoping to get some sense of how actual flow rates would vary as the IP rises. Yes some of this comes with just fiddling with it in use. But especially if I am doing a 280ft wreck dive with a drop straight down the shot line - how do you set the needle valve on the surface so its not going crazy fast by the time you hit the bottom?
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