I have always been dissatisfied with how I was taught to do the Negative Test on my rEvo at the end of the Closed Checklist. Pull a negative (how much is enough?), then wait a few minutes (how many is enough?), then open the DSV and I should hear a good rush of air (how much is good?).
That just seems too subjective, to me. And, I feel like a contributing factor to my recent near miss is that I have gotten used to doing negative tests where my unit was only passing with "Barely Acceptable" colors, so then when it probably should have been failed, the difference was so small I didn't recognize it, so I called it another Pass and went diving. And it slowly flooded during my dive.
Based on my other recent thread about that near miss, I learned that pulling a negative and monitoring O2 sensor mV readings is a way to bring some quantification to the process.
This thread is meant to help me work out some reasonable protocol I (and, hopefully, others) can use to track my CCR's ability to hold a negative and to identify when it is acceptable for diving versus not.
I'm really not sure how this protocol should be defined, so this post is really just to get the conversation started and then hopefully somebody that is much more knowledgeable and wiser will chime in and steer this thread down the right track.
As a starting point, and to try and determine if my unit actually has a leak, I performed this test just now.
I used my NERD2 monitor to monitor mV readings from 2 sensors. I closed my unit up, as it would be at the end of my Closed Checklist, and pulled a negative by sucking the air out orally, via the BOV.
Before I pulled the negative, both sensors were reading 10.3 mV.
After I pulled the negative and closed the BOV, they were both reading 9.3 mV.
I checked the readings periodically. At the 30 minute mark, they were still at 9.3 mV. At that point, I opened the BOV and heard a solid rush of air going in. I conclude that, at least on this test, my unit was holding a very good seal.
After the BOV was open, so the unit was fully back to ambient pressure inside, and after waiting a minute or so, my O2 sensor readings only went back up to 10.1.
At that point, I opened the scrubber and pulled the sensor tray out and then the readings went back up to 10.3.
My best guess of what was going on there is that 30 minutes was long enough (with no gas circulating in the loop) for the gas in the sensor tray to lose a little O2 - being consumed by the galvanic reaction in the 5 O2 sensors. So, until I opened the unit and pulled the tray out, the sensors were actually in gas that had a slightly reduced ppO2 from when I started the test, even though absolute pressure was back to where it started.
That suggests that the mV readings should have dropped slightly during the test. The fact that they stayed the same then (I think) means that the unit's seal was not perfect. It allowed enough outside air to leak into the loop for the rise in internal pressure to compensate for the O2 consumed, yielding a constant ppO2.
Even with that theory, I think the overall test result would considered a strong Pass.
So, anyone care to chime in on what a reasonable test protocol might be to determine a Pass/Fail on the Negative test?
- Check mV readings
- Pull negative
- Look for about a 10% drop in mV values
- Wait 5 minutes
- If mV readings have risen by 1% (of original value) or less, it's a Pass, other a Fail
Reasonable? Tweaks?
Example: starts at 10.3mV. Negative yields 9.3. 1% of original is 0.103mV. Call it 0.1. So, at 5:00, if the mV reading is 9.4 or less, it's a Pass. Otherwise, a Fail.
Should it allow for more rise in mV than that 1%? How much? Or is 1% too much?
In the headline, I said "or Positive". I said that because I'm thinking you could do the same thing except instead of pulling a negative, blow into it until the OPV pops, and then close the DSV and check the mV readings. They should be higher. But, how much higher would depend a lot on the OPV design and its spring tension. But, I guess you could still figure out what parameters values are "good" for your unit and use the O2 sensor mV readings in a similar way to test for positive pressure leaks.
However, my primary concern is working out a more objective way to do the Negative test that is called for in my rEvo Closed checklist. I have that and a Pre-jump checklist, and neither calls for ever doing a Positive test. Not that I'm saying one shouldn't be done. Only that the checklists don't call for it. At least, insofar as how I was trained to run the checklist. I do recall a post in my other thread saying that the OPV test in the Closed checklist is meant to be a positive pressure test.
That just seems too subjective, to me. And, I feel like a contributing factor to my recent near miss is that I have gotten used to doing negative tests where my unit was only passing with "Barely Acceptable" colors, so then when it probably should have been failed, the difference was so small I didn't recognize it, so I called it another Pass and went diving. And it slowly flooded during my dive.
Based on my other recent thread about that near miss, I learned that pulling a negative and monitoring O2 sensor mV readings is a way to bring some quantification to the process.
This thread is meant to help me work out some reasonable protocol I (and, hopefully, others) can use to track my CCR's ability to hold a negative and to identify when it is acceptable for diving versus not.
I'm really not sure how this protocol should be defined, so this post is really just to get the conversation started and then hopefully somebody that is much more knowledgeable and wiser will chime in and steer this thread down the right track.
As a starting point, and to try and determine if my unit actually has a leak, I performed this test just now.
I used my NERD2 monitor to monitor mV readings from 2 sensors. I closed my unit up, as it would be at the end of my Closed Checklist, and pulled a negative by sucking the air out orally, via the BOV.
Before I pulled the negative, both sensors were reading 10.3 mV.
After I pulled the negative and closed the BOV, they were both reading 9.3 mV.
I checked the readings periodically. At the 30 minute mark, they were still at 9.3 mV. At that point, I opened the BOV and heard a solid rush of air going in. I conclude that, at least on this test, my unit was holding a very good seal.
After the BOV was open, so the unit was fully back to ambient pressure inside, and after waiting a minute or so, my O2 sensor readings only went back up to 10.1.
At that point, I opened the scrubber and pulled the sensor tray out and then the readings went back up to 10.3.
My best guess of what was going on there is that 30 minutes was long enough (with no gas circulating in the loop) for the gas in the sensor tray to lose a little O2 - being consumed by the galvanic reaction in the 5 O2 sensors. So, until I opened the unit and pulled the tray out, the sensors were actually in gas that had a slightly reduced ppO2 from when I started the test, even though absolute pressure was back to where it started.
That suggests that the mV readings should have dropped slightly during the test. The fact that they stayed the same then (I think) means that the unit's seal was not perfect. It allowed enough outside air to leak into the loop for the rise in internal pressure to compensate for the O2 consumed, yielding a constant ppO2.
Even with that theory, I think the overall test result would considered a strong Pass.
So, anyone care to chime in on what a reasonable test protocol might be to determine a Pass/Fail on the Negative test?
- Check mV readings
- Pull negative
- Look for about a 10% drop in mV values
- Wait 5 minutes
- If mV readings have risen by 1% (of original value) or less, it's a Pass, other a Fail
Reasonable? Tweaks?
Example: starts at 10.3mV. Negative yields 9.3. 1% of original is 0.103mV. Call it 0.1. So, at 5:00, if the mV reading is 9.4 or less, it's a Pass. Otherwise, a Fail.
Should it allow for more rise in mV than that 1%? How much? Or is 1% too much?
In the headline, I said "or Positive". I said that because I'm thinking you could do the same thing except instead of pulling a negative, blow into it until the OPV pops, and then close the DSV and check the mV readings. They should be higher. But, how much higher would depend a lot on the OPV design and its spring tension. But, I guess you could still figure out what parameters values are "good" for your unit and use the O2 sensor mV readings in a similar way to test for positive pressure leaks.
However, my primary concern is working out a more objective way to do the Negative test that is called for in my rEvo Closed checklist. I have that and a Pre-jump checklist, and neither calls for ever doing a Positive test. Not that I'm saying one shouldn't be done. Only that the checklists don't call for it. At least, insofar as how I was trained to run the checklist. I do recall a post in my other thread saying that the OPV test in the Closed checklist is meant to be a positive pressure test.
