rEvo - O2 at more than 100 m ?

[grantctobin]

Or if you want to retain the CMF function, upgrade to an Apollo A320 Oxy; factory set by Open Safety to 19bar IP

You going to go replace the non replaceable, pressed in orfice inside the seal-beating-sized Revo triple MAV when they do?

 

[kensuf]

Any reason you can't use a needle valve on the rEvo?

The issue isn't the orifice nearly as much as it is the blocked first stage. In the case of the Fathom, the stock config comes with a needle valve but is depth limited to around 400' because of the first stage.

 

[jale]

Hi
Just a needle valve with a normal reg. Still don't get the idea of needle valve with a blocked reg... it is not that much to do adjustments on the fly...
Or just use your DIL as it should have a good enough ppo2...how long are you going to stay that down anyway?

 

[Raf BE]

Hi
Tx all for the answers
I want to keep the cmf working / Choice can be discussed but that's my preference.
Somehow the 90 % of the dive is between 60 and surface ;-)
I used to plug a BO but I like the idea of an extra small 02 tank and I'm now reassure that some did with success - Will try within 2 weeks.

Rgds

 

[J-Vo]

Theoretically, not at all. I've talked to a few different divers about it.

Which would you recommend?

- brett

I'm running a Fathom on my KISS. I have the first stage blocked @ 200psi. I don't think I will personally ever want to go deeper than that will allow me.

 

[boriss]

I'm running a Fathom on my KISS. I have the first stage blocked @ 200psi. I don't think I will personally ever want to go deeper than that will allow me.

For anyone wondering what that translates to: 200psi is just under 14bar. So the zero flow point would be <140meters. Paul Raymaeker recommends staying 20m above that point (and even there the flow-rate is reduced significantly). So in essence a 200psi IP is good to around ~120m / 390ft. Definitely plenty for almost everyone.

This also happens to coincide with the IP set by the Fathom MK2.5 CCR from the factory. They run 205psi IP with a depth limit of 395ft / 120m.

Lastly, I'm sure anyone who'd attempt this would already be aware, but just in case: don't just turn up the IP without upgrading to (or ensuring you have the) components rated for the additional pressure.

 

[Wibble]

Can I ask a dumb question about a CMF please...

The requirement is for a constant amount of oxygen to flow into the loop as the body metabolises the same amount regardless of depth.

My understanding of a CMF (constant mass flow) "orifice". There's a load of physics regarding gas flows through a tight hole. It's about the absolute pressure of the gas behind pushing "a certain amount of molecules" through the orifice. The pressure differential on the output side doesn't matter much, at least within a couple of bar. If the input pressure varies, the amount of molecules of gas shoved through the hole will drastically change.

Q: how much would the output differ for different absolute pressures on the input side of the orifice? Is it very non-linear?

 

[boriss]

@Wibble and anyone else who's interested, here's some info about CMF which may be useful.

To answer your question, if you increase the pressure on one side the velocity won't change, but due to the increased density more molecules will flow. It's linear and this is where a needle valve can be useful. You can increase the pressure of P1 and reduce the orifice to maintain the flow at your base metabolic rate. So even if you double the density (or pressure) of IP, the total number of molecules won't exceed your metabolic rate at a shallower depth. Hopefully that makes sense... definitely recommend the article and video.

Understanding Constant Mass Flow ADVANCED DIVER MAGAZINE By Paul Raymaekers

Great video by Richard Harris:

 

[cathal]

Can I ask a dumb question about a CMF please...

The requirement is for a constant amount of oxygen to flow into the loop as the body metabolises the same amount regardless of depth.

My understanding of a CMF (constant mass flow) "orifice". There's a load of physics regarding gas flows through a tight hole. It's about the absolute pressure of the gas behind pushing "a certain amount of molecules" through the orifice. The pressure differential on the output side doesn't matter much, at least within a couple of bar. If the input pressure varies, the amount of molecules of gas shoved through the hole will drastically change.

Q: how much would the output differ for different absolute pressures on the input side of the orifice? Is it very non-linear?

To keep things very simple (which is what you want below 100M) I just use a Dil that will give me a 1.2 SP, while I am at the bottom as 90% of the run time will be well above 100M.

 

[Wibble]

@Wibble and anyone else who's interested, here's some info about CMF which may be useful.

To answer your question, if you increase the pressure on one side the velocity won't change, but due to the increased density more molecules will flow. It's linear and this is where a needle valve can be useful. You can increase the pressure of P1 and reduce the orifice to maintain the flow at your base metabolic rate. So even if you double the density (or pressure) of IP, the total number of molecules won't exceed your metabolic rate at a shallower depth. Hopefully that makes sense... definitely recommend the article and video.

Understanding Constant Mass Flow ADVANCED DIVER MAGAZINE By Paul Raymaekers

Great video by Richard Harris:

Thank you for those very informative links.