Tracking maintenance

[joshk]

the cell will only give a mV value that correlates to 1.3

You mean the cell is current limited or you mean it's not responding linearly?

My confusion with what @DA Aquamaster wrote is around the “displayed PPO2” phrase.

 

[DA Aquamaster]

Let me clarify it for you.

The O2 content and pressure in the pot is a known quantity, and thus you can compute the actual PPO2 of the gas the cells are exposed to. The cells are calibrated at the start of the test at 1 ATA and then the pressure is in the pot is increased incrementally and both the actual PPO2 and the measured PPO2 produced by the cells is displayed along with the percentage difference. The PPO2 as read by the cells is normally very close at low PPO2s and starts to lag by an increasing percentage as the PPO2 increases.

A reading of 1.17 or 1.18 would be pretty normal for an actual PPO2 of 1.2 and would not be a significant error. However a reading of 1.23 at an actual PPO2 of 1.3 is potentially a problem as your loop PPO2 isn't 1.2 with a 1.2 set point but rather just short of 1.3. Similarly, if you the actual PPO2 is 1.4, and the cells are only displaying 1.31, that's a problem.

It can all be very linear, but the slope of that line can be quite wrong as the cells age.

I think many CCR divers have the impression that cells become current limited and just won't display a PPO2 above 1.4, 1.5, or whatever. That certainly happens to cells when they are way past their prime, but the bigger issue is when they start to lag by .1 or more in the range where you're actually running the loop during the dive. And that happens long before they fail to ever exceed a maximum of 1.4 to 1.6.

The issue I had with checking at 20' is that you needed a perfect O2 flush to get 1.6, and that's very O2 intensive at best, and it's hard to achieve when you are actually on the loop. If you're reading 1.54 then, you're never quite sure if that reflects cells that are lagging or an imperfect flush. Even with that 1.54 reading, you may have 1.27 at 10' and 1.0 at the surface and it may look very linear. Yet, you may still not be reading accurately at the 1.0-1.2 range where you're normally maintaining the PPO2 during the dive.

Consequently, I bought a cell checker (at wholesale cost thankfully, rather than retail) and I know with a high degree of confidence that my cells are reading accurately within the critical range.

 

[joshk]

Got it. You're saying if the cell doesn't behave linearly (within some boundary) between 1.3-1.6 it's going to be replaced.

 

[tbone1004]

Got it. You're saying if the cell doesn't behave linearly (within some boundary) between 1.3-1.6 it's going to be replaced.

correct.
Some units can perform 2-point calibration *meg and liberty*, and that will allow you to calibrate for linearity, but it is only considered accurate within the range of calibration. Unfortunately we operate CCR's outside of the range of calibration at the surface, so you have to perform some check to validate current limiting at the ppO2's you intend to use, as well as validate linearity up to those points.

the extent of allowable linearity is something that you have to decide as a diver to what you will allow based on your risk analysis

 

[joshk]

Some units can perform 2-point calibration *meg and liberty*, and that will allow you to calibrate for linearity

Which unit do you dive?

 

[tbone1004]

Which unit do you dive?

meg 2.7 that I inherited, but if I was choosing a new backmount unit it would be a tossup between the X-CCR and the Liberty with it likely going to the Liberty due to being $3k less and having some nice head features including a built in cell checker

 

[joshk]

meg 2.7 that I inherited, but if I was choosing a new backmount unit it would be a tossup between the X-CCR and the Liberty with it likely going to the Liberty due to being $3k less and having some nice head features including a built in cell checker

Favoring the electronics features even though it's not a Shearwater controller?

 

[tbone1004]

Favoring the electronics features even though it's not a Shearwater controller?

Pros of Liberty over X-CCR
$3k cheaper
redundant electronics
fully field serviceable with the modules
helium sensors to validate O2 sensors
Head pressurization kit to validate O2 sensors *if you want to do that with something else you're in for minimum $500 for a cell checker

Cons
"non standard" scrubber-the X uses the meg/hammerhead/CisLunar scrubbers which is super nice
no availability of baby hobo canister to tailor dives
unit is physically very heavy due to the plate design
not Shearwater, and the tilt/tap function to change gases is really annoying

 

[northernone]

O2 cells:

I wasn't satisfied with the linearity check on 02 at 20 ft and started using a DE-OX cell checker. I'll check them periodically and note the changes over time. Once I start to see either differences between the actual PPO and the displayed PPO2 at 1.3, or excessively long response times, the cell gets retired.

Tanks:

We currently have 38 tanks including primary sidemount and back mount tanks, stage bottles, O2 bottles and rebreather bottles, plus some that are used for O2 storage and a half dozen tanks that are on loan at the moment. They are fairly evenly spaced out regarding hydro test. It's not hard to keep track of what is due for hydro with the active tanks - it's a bit harder with the inactive tanks or with tanks that we store in FL.

Regulators:

Between two divers with back mount, sidemount, dil, O2 and stage regs, we have a lot of regs to keep track of. The less used regs (back mount regs, sidemount regs, and some of the stage regs) are inspected before a trip where they will be used and are serviced based on condition. With good selection of the design, decent post dive maintenance, proper storage, and thorough condition inspection regulators, and in particular first stages, can have a long shelf life. The more heavily used regs are serviced annually or more often, again based on condition.

----

I guess in short, I focus as much on inspection as I do preventative maintenance and reserve the service work for when a condition inspection shows need.

I have no idea what people do in similar situations if they are not doing their own tank, regulator and rebreather inspection and maintenance. I suspect they have more failures. I personally would never want to rely on a calendar to tell me when something needs to be maintained. Not all cells are created equally and not all regulators will go a full year between services - especially when used frequently and in demanding conditions.

I too go with condition based maintenance schedules.

The only thing calender based is hydro tests, I'll get them tested when I have a pile heaped up big enough to be worth finding a testing facility. Only around 25 tanks in the collection (give or take due to being spread out between borrows and loans and a few locations.) Again, need based hydro testing. For example: prior to the start of the therapeutic dive program come March, I'll check all my student suitable tanks and hydro what I discover.

It's probably more time consuming doing ongoing equipment inspections but I like the results and find the process satisfying. It works well for my mindset.

When I did aircraft maintenance as a subcontractor we had maintenance logs and I loathed that aspect of the work... glad there's another way in my personal hobby.

Regards,
Cameron

 

[kensuf]

Some units can perform 2-point calibration *meg and liberty*, and that will allow you to calibrate for linearity, but it is only considered accurate within the range of calibration. Unfortunately we operate CCR's outside of the range of calibration at the surface, so you have to perform some check to validate current limiting at the ppO2's you intend to use, as well as validate linearity up to those points.

the extent of allowable linearity is something that you have to decide as a diver to what you will allow based on your risk analysis

You can manually do a two point calibration on every unit you may remotely consider diving. I'm 99% positive your CCR instructor will teach you how. He will also teach you how to track the health of the cells over time.