Negative (or Positive) Test using O2 sensors

[Jack Hammer]

Manufacturer Technical specification. Expected life-42 months. Used in hospitals, these are often in 100% O2 environments. Either way, if you believe the 12 month replacement cycle, you'll never "wear a cell out" by leaving it in 100% O2. It's the same horsesh!t that some of the instructors teach about not doing a 1.6 check for current limiting because it wears the cells out.
PSR-11-39-MD | AII Inc

Current limiting affects higher PO2 readings. Rebreathers see and are used in higher than 1.0 PO2 environments. Perhaps you can post the manufacturer's life expectancy for cells used in a highly humid 1.3 environment.

 

[tbone1004]

Current limiting affects higher PO2 readings. Rebreathers see and are used in higher than 1.0 PO2 environments. Perhaps you can post the manufacturer's life expectancy for cells used in a highly humid 1.3 environment.

The cells aren't made for that environment so they aren't going to quote it. The point of that was sitting for a week in dry O2 at surface conditions is not going to cause them any appreciable wear.

 

[beldridg]

First, a thanks to @tbone1004 for the comment on the original thread about monitoring cell mV when doing pos/neg checks (at least from memory I think he mentioned it). I've added that check to my list when assembling my rEvo.

In terms of this thread OP from @stuartv and a methodology, I think the simple approach is better.

After doing this check a few times, I've landed on a very simple approach. It might not be perfect, but I think it balances cost vs. benefit.

Similar to @stuartv, I have a Petrel controller and a NERD secondary. For this test, I only do it on the two cells connected to the NERD because the PO2 usually falls below the 0.19 threshold and I don't want the Petrel controller constantly trying to fire the solenoid.

My "simple" version of this test is

1) Check the PO2 & cell mV on the NERD
2) Pull a negative and watch the mV drop.
3) Note the mV readings on the negative.
4) If they change by more than 0.1 over a 3+ minute period, then investigate.

Thinking about percentage changes and mixes, etc. all seems too complex. I like simple.

More than 0.1 change, investigate.

- brett

 

[rjack321]

Both of my CCR courses (with wildly different instructors and agencies) had us watching for the mV readings to stabilize before hitting the calibrate button. How else would you confirm the head or unit is full of whatever you are calibrating with? (98% is what I typically enter per Shearwater's suggestion).

 

[beldridg]

Both of my CCR courses (with wildly different instructors and agencies) had us watching for the mV readings to stabilize before hitting the calibrate button. How else would you confirm the head or unit is full of whatever you are calibrating with? (98% is what I typically enter per Shearwater's suggestion).

Sorry, just to be clear, this isn't about calibrating sensors.

- brett

 

[rjack321]

Sorry, just to be clear, this isn't about calibrating sensors.

- brett

I was replying to this

The best thing I learned in my sidewinder class from Edd was to watch MV and wait for them to stabilize before calibrating. I wasn't taught that initially, I was just taught to "eyeball" when you felt the loop was fully flushed with oxygen. Now I use the MVs to tell me when the cells are fully flushed with oxygen

 

[beldridg]

I was replying to this

Ahhhh. Okay, now I understand.

- brett

 

[AdamSa]

I think you missed my point.

I'm saying to check the mV readings with the unit closed - i.e. just prior to pulling a negative. Make sure they are stable. It doesn't matter what is in the loop (air or trimix).

Then you pull a negative and check the mV readings.

Then let it sit and check the readings again.

The only thing that matters is how much the readings changed in that few minutes. So, the only "aging" of the sensors that comes into play is the 5 minutes (or however long) of that actual test.

In my test, I had air inside the loop and my sensors showed 10.3mV. I sucked a negative down to 9.3mV.

If you do the same thing, you might have TX10/50 inside your loop. Before you pull the negative, you sensors might be showing 6.1mV. After you pull the negative, you might be seeing 5.4mV. Using my proposal of 1% of the original value means you'd be looking for a change of 0.06mV or less. Assuming the computer only shows a resolution of 0.1mV, that means you'd be looking to see that your sensors did not go above 5.5mV during the negative test.

And yes, I have a rEvo. If your JJ doesn't need the same kind of test, then, well, this kind of test is not for you.

I did not miss your point, I was just simply saying that there are a number of variables which may make the test a little unreliable and unnecessary. However I might have been somewhat biased by the fact that on my unit, the ADV diaphragm is a very good indicator of the negative test state. Admittedly, I have no other experience with other units. As such, I gave the idea a test, results below:

• Pulled a negative with exhaled air. Cell Readings: 9.7 10.5 10.3
• Opened the DSV very slightly until the ADV Diaphragm started to wobble. If I get this mechanical result I would investigate. Cell Readings: 9.8 (+0.1) 10.7(+0.2) 10.4(+0.1).
• Removing the negative, Cell readings: 10.3 11.3 11.0

I Repeated the tests with loop flushed with O2

• Pulled a negative. Cell readings: 44.2 48.2 47.4
• Opened the DSV slightly until i got the same wobble. Results: 44.8(+0.6) 48.6(+0.4) 47.8(+0.4)
• Removing the negative, Cell readings: 46.3 50.5 49.5

So I guess it works, and at least in my case, works better with o2. I did not repeat the tests and there is not guarantee that I opened the DSV the same amount between the 2 tests. That being said, for very small leaks, the voltage change is going to be very small and even perhaps negligible/ rounding error. So I guess I would suggest leaving the test for a long time so that any small leak will have enough time to effect the voltage measurement.

Can a revo be held negative for extended periods? I know that this may cause damage to certain units, but not sure which (for the JJ as an example, the positive shouldn't be left long, but fine for the negative).

 

[stuartv]

Can a revo be held negative for extended periods? I know that this may cause damage to certain units, but not sure which (for the JJ as an example, the positive shouldn't be left long, but fine for the negative).

I have only been told that a negative for a few minutes is all it needs and that I should avoid leaving it negative overnight.

So, I think 30 minutes is probably fine. It also seems like it is more than adequate for testing the negative. If it holds a reasonable amount of negative pressure for that long, then there is no reason to leave it longer.

Of course, the problem is that subjective "reasonable amount".

I've checked mine a couple more times. I'm still feeling like if the sensor mV readings do not increase by more than 0.1mV in 5 minutes, that is adequate indication of a good (enough) negative for diving.

As has now been observed to me in an off-ScubaBoard conversation, the ability to hold a negative only needs to be stronger than what it takes to crack the ADV and get it to fire. And the ability to hold a positive just needs to be enough that the OPV opens instead. I'm not saying I would take those as minimums and be fine. I'm just saying that holding a good negative for 5 minutes is actually far more stress than the unit's seal would experience during a dive.