My hypoxic incident

[broncobowsher]

Its a tiny hole. 0.0035" or about 0.89mm in a brass orifice. A drop of seawater in there and some green corrosion over time and the flow rate will plummet

0.89mm or 0.089mm?

If corrosion is an issue, does anyone make them from stainless?

 

[lostsheep]

Its a tiny hole. 0.0035" or about 0.89mm in a brass orifice. A drop of seawater in there and some green corrosion over time and the flow rate will plummet

Two questions?

What is the procedure to check the orifice during build up?

If buildup is detected how is it cleaned out?

.0035” is pretty small, I have a lot of tiny drills, I don’t think I have any that small.

Edit: Harvey makes em but they’re not exactly intended for the average user

 

[rjack321]

0.89mm or 0.089mm?

If corrosion is an issue, does anyone make them from stainless?

sorry, correct I missed a zero
0.089mm

 

[rjack321]

Two questions?

What is the procedure to check the orifice during build up?

If buildup is detected how is it cleaned out?

.0035” is pretty small, I have a lot of tiny drills, I don’t think I have any that small.

Edit: Harvey makes em but they’re not exactly intended for the average user

If you own a mCCR you need to own a flow meter

You can put an old orifice in an ultrasonic... BUT corrosion is metal that is oxidized and flaking apart. So it might come out of the ultrasonic larger (with excess flow) compared to its original size. Or not come out corrosion free and clean enough to have the original flow either. They are best just replaced. And you can't make them at home yourself, a bit that small will snap if you look at it funny.

The Revo dual MAV that @nadwidny had on his Kiss classic doesnt have a user servicable (or even inspectable) orifice. You have to replace the whole MAV - its a crappy design that leads to people continuing to use it despite questionable flow rates.

 

[lostsheep]

I’m familiar with tiny drills. Last time we had to make ~.004” hole in acrylic we gave up on the drills and just spun up a guage pin to about 60K and melted through it. WRT actual tiny drills, in my experience, they better be high quality drills and they better be running dead nuts or don’t even bother.

 

[uwxplorer]

Two questions?

What is the procedure to check the orifice during build up?

Not quantitative, but the rEvo build checklist has the user closing the O2 tank valve and monitor the pressure drop (due to the CMF orifice). If that happens "slower" than usual (or faster for that matter), this should prompt an actual flow rate measurement. That should translate readily to any unit with an orifice.
Of course it doesn't help if the problem occurs during the dive, which is why the motto "always know your PO2" is worth every letter it consists of.

 

[nadwidny]

Thanks for sharing. Did you measure the flow rate through the cmf after the dive?

I did not. I did check after to see that it was not completely plugged and that some O2 was getting through. When I took the block apart (I use the Revo-type block) I could see there was some corrosion on the orifice.

 

[nadwidny]

Yeah I know, I'm just interested in the actual flow rate, generally cmf are set to circa 0.7 lpm which is roughly metabolic rate and is checked semi regularly by means of a Dwyer Gauge or similar, I'm just wondering if the actual flow rate with the blockage was measured.

When I got trained we were taught to set the IP so that the flow was somewhat similar to metabolic needs. If I thought I was adding too often, bump it up. If my PO2 was creeping high, bump it down. That's how we learned to measure flow. No mention of Dwyer gauges or similar.

 

[beldridg]

If you own a mCCR you need to own a flow meter

You can put an old orifice in an ultrasonic... BUT corrosion is metal that is oxidized and flaking apart. So it might come out of the ultrasonic larger (with excess flow) compared to its original size. Or not come out corrosion free and clean enough to have the original flow either. They are best just replaced. And you can't make them at home yourself, a bit that small will snap if you look at it funny.

The Revo dual MAV that @nadwidny had on his Kiss classic doesnt have a user servicable (or even inspectable) orifice. You have to replace the whole MAV - its a crappy design that leads to people continuing to use it despite questionable flow rates.

Just a quick clarification. There are two types of rEvo MAVs and two different methods used to implement a constant mass flow (CMF).

1) The orifice is included in the MAV. I think this is the type that rjack321 is referring to above. This is known as the "R217C" type and you can see the indication of the orifice at .0035" on the MAV shunting between the O2 input and the output Picture below:



2) There is a different MAV that my unit has that does NOT have the orifice included in it. These are R217A and R217B (the difference being the type of connector on the output side. See picture below:




So, if the O2 isn't "leaking" into the system in the MAV, how does it happen?

There are, once again, two ways this happens depending if your rEvo is a mCCR or hCCR. In a mCCR, there is a one-way orifice mounted on the bracket in the exhale lung. In a hCCR, the orifice is inside the solenoid.

Therefore, in the case of a unit configured with #2 as above, you don't need to replace the MAV if you have problems with the orifice. I have a mCCR with the R217A MAV and have a spare solenoid in my kit for long, remote trips.

Again, I'm not trying to say the system is either "good" or "bad" but just a quick clarification.

Regards,

- brett

 

[beldridg]

Not quantitative, but the rEvo build checklist has the user closing the O2 tank valve and monitor the pressure drop (due to the CMF orifice). If that happens "slower" than usual (or faster for that matter), this should prompt an actual flow rate measurement. That should translate readily to any unit with an orifice.
Of course it doesn't help if the problem occurs during the dive, which is why the motto "always know your PO2" is worth every letter it consists of.

Correct.

I would also note that I have a pretty good feel for how "fast" the pressure should drop once the O2 cylinder valve is turned off so I can judge (not completely accurately, but it is a good sanity check) whether or not it is clogged.

If it does happen during a dive and you have a unit with a solenoid (i.e., hCCR), it should fire if you drop below your low set point.

But, as you state, you should always know your PO2 regardless.

- brett