IP drop in MK2 scubapro

[reefrat]

Has anyone looked at the IP drop in a late model MK2 variant first stage during inhalation with say 2000 psi supply pressure? How responsive is the piston to an IP drop, much does the IP drop and how fast is the recovery to the set IP?
Cheers
RR

 

[halocline]

You can't really measure IP drop during inhalation accurately with a standard IP gauge. Keeping that in mind, the MK2 has what I would call excellent IP recovery and stability. Its a really under-rated 1st stage IMO.

 

[The Chairman]

You can't really measure IP drop during inhalation accurately with a standard IP gauge.

Sure you can. But IP drop is not the whole story. A high volume second stage can and will cause a quicker drop. I am not familiar with this reg, but I wonder what seems to be the issue? What are you seeing?

 

[reefrat]

No issue here, at least not on a Sunday afternoon at the beach!
I am rationalizing (for lack of a better word) our gear with the goal of having less diversity in our main regs to service myself due to the lack of ANY scuba services on this island. I want to keep it as simple as possible without sacrificing performance and I understand that, being unbalanced, the shift from full tank to 500 psi would be about 20 psi, which shouldn’t be an issue for me!
In my messing around with regs I have seen a bigger IP drop and slower recovery from piston first stages than diaphragms when inhaling or purging the second stages. I know there is nothing scientific about my observations, however all the pistons have shown significantly bigger drops and slower recoveries, this is a known phenomena I think, judging on search results on a scubaboard search.
What interest me most is that the biggest discrepancy between diaphragms (all of which drop barely 10 psi and bounce back) and the piston first stages when the second stage is just inhaled gently and slowly- which is how I breath when diving. So, since I am using a non-pnuematically balanced second stage most of the time I would prefer a first stage to mate with that second stage that AFAIK provides a minimum IP drop during the breathing cycle, along with simplicity and absolute reliability.

 

[halocline]

I'm telling you that the standard IP gauge on a LP inflator hose is not giving you accurate numbers while gas is flowing. The flow of the gas past the port creates a venturi effect and lowers pressure in the hose. Faster air flow, more effect. I have no idea how to quantify this effect but I know it exists. I've seen MK2s drop less under purge than MK15s and 20s, both of which are much higher flow regs. They can't be flowing more air with lower pressure. The two are proportional.

Even an inline IP gauge, teed off the LP hose near a 2nd stage, will be subject to these same venturi effects. I've tried it! I think the only way to get an accurate reading of the actual pressure in the line while air is flowing is with a flow meter or something that is designed to measure the pressure of moving gas.

I would trust your senses and simply evaluate the regs by breathing on them.

Now, if by some reason I've misunderstood you and you're concerned about the IP drop over the supply range in the MK2, it's about 20 PSI from 3000 to 300. That's enough to notice a sizable increase in breathing resistance with an unbalanced 2nd stage. When I use mine, I start to feel it below 500 PSI, which I don't mind at all. I'm usually back on the boat by then, or damn close.

 

[The Chairman]

I'm telling you that the standard IP gauge on a LP inflator hose is not giving you accurate numbers while gas is flowing.

It's not that it's inaccurate, it's simply not the right tool for the job. You don't have to "trust your senses", as flow gauges will give you a better idea of what's happening in regards to volume. Magnahelic gauges will also help diagnose other breathing issues. Use the right tool for the job. If all you own is a hammer the entire world becomes a nail. Think about it.