Prism2 vs rEvo?

[kensuf]

@kensuf still trying to understand this and not trying to be an ass because there is likely something I'm missing.

Are the guys on the Fathom using the needle with a blocked first stage or with a normal one?

If blocked, why are they OK with adjusting the needle based on workload but are not ok with adjusting it based on depth and workload? The same risks exist with either scenario in terms of accidental adjustment, over adjustment, and failure.

They dive it blocked. It comes stock with a fancy spring and a fixed IP Apeks first stage.

My understanding is the adjustment is once or twice a dive max as opposed to continuous with a non-IP blocked first stage.

Think: scooter and deco -- one adjustment because you're basically at rest the whole time. If your flow rate was set to at rest on the last dive, then you really don't need to adjust it.

Think: swim and deco -- two adjustments, one for swimming (working) and once for deco.

When showing me the first prototype unit two years ago, Charlie warned me that it's possible to dork up a needle valve by over-tightening it -- something I'd think would be a risk if you're continually fiddling with it.

I've never dove a Fathom but I did dive with the designer last week and I slept in a Holiday Inn Express this month.

 

[Doby45]

I slept in a Holiday Inn Express this month.

Good enough for me!

 

[JohnnyC]

Eh, guys have been diving Pelagians and Chris Kennedy's needle valves with normal first stages for quite a while. I don't have any issues on my Pelagian and I just don't see the issue diving unblocked first stages. Using a flow meter you set the valve for about 0.8 liter and if you need to give it a bit of a twist you do. Takes all of about 2 minutes total, between setting it on the surface and tweaking it when you get to depth. It's a fully manual CCR and a little squirt of O2 to even out the consumption for your metabolic rate isn't a big deal. Depending on how tight you're able to get it, once every fifteen or twenty minutes isn't unreasonable. Set it for depth with the flow meter and whack it open for deco.

It would take a lot of ham-handed manipulation to screw up the valve, and it just really isn't much of an issue. Obviously don't store it fully closed, and don't wrench on it.

The plural of anecdote isn't data, but I don't think it's as big a deal as it's being made out to be. There are enough of them on the market that we'd be hearing about issues if it were a thing. Hell, the Fathom is basically just an evolved Pelagian, something built in a machine shop as opposed to a garage in Thailand. If I'd have kept my Meg I'd have put a needle valve on it, and I'm planning on doing it to my SF2 after I get about 100 hours on it post-crossover. Neither will have blocked 1st stages and I don't anticipate any problem with either.

Quoted from the Pelagian manual as it's what I have handy:
"As the increase / decrease in mass flow of O2 is 1% per meter it is easy to predict the flow at the planned depth before the dive. The diver needs to know his/her metabolic rate at light work, the planned depth and an easy to remember equation. The flow including the increase is calculated as a fraction so for example at 50 meters depth there would be a 50% increase in mass flow compared to the surface. If the surface flow was measured to 0,5 lpm the flow should be multiplied with 1,5 and the mass flow at 50 m would be the surface equivalent to 0,75 lpm."

As you get a feel for how much you need to turn to get the flow where you need to, it becomes even less of an issue.

 

[Michael Guerrero]

The thing is, with this strategy, any change in your atmospheric pressure will require you to adjust the needle valve.

Not sure this accurately reflects the way you dive these systems (the Pelagian for example). You usually have two settings, surface and at depth. You have it open more at the surface to ensure you have sufficient O2 in the loop at low pressure, and before you descend you close it down to the at-depth setting. During descents and ascents, you manage the ppO2 with the MAV. You don't actively manage the ppO2 with the needle valve, you manage it with the MAV, and you use the needle valve to keep a baseline and extend the length of time between MAV pushes.