Why aren’t scrubber monitors (aka TempStick) standard on all rebreathers?

[tbone1004]

(@tbone1004 — you mentioned an oxygen sphere. What sort of unit do you dive? Just curious as cylinders are normal and never seen a sphere in the flesh)

Kisskat is my primary rebreather these days. Not my picture or my rebreather but close enough. You can see the valve sticking out of the holder that @billgraham 's buddy used to make. Spheres are out of US Navy rebreathers and the valves have to be custom machined. It's a bit of a faff honestly but it is very effective for these sidemount rebreathers.

 

[Garth]

I would gladly have a scrubber monitor that was accurate on my rebreather but don’t find it essential. The scrubber can be affected by many factors and the ratings account for these for the most part. I think it’s more of the individual and the trust you have for a given rebreather in how much someone is willing to push the time limits.

If I’m not in an extreme cold environment I am not generally worried about the sorb though as long as it stays dry in my loop. I will do a repetitive dive on sorb but not a big dive where it would run close to the limit stack time. Only reason to have a monitor is if you are planning to push the limit. In that case the monitor might cause more of a concern than a benefit.

 

[billgraham]

Kisskat is my primary rebreather these days. Not my picture or my rebreather but close enough. You can see the valve sticking out of the holder that @billgraham 's buddy used to make. Spheres are out of US Navy rebreathers and the valves have to be custom machined. It's a bit of a faff honestly but it is very effective for these sidemount rebreathers.

View attachment 760840

The valve is just a regular inline valve, machined to match the thread on the Inchonel sphere. A good machinist can make that happen, you just don't want to mar the threads on the sphere!

 

[GiraffeMarineSalvage]

Sofnolime 797 is rated to absorb 150L of CO2/kg of sorb. A 3L bottle of O2 filled to 2400psi/165bar which is standard O2 pressure will hold ~500L of O2 and if you remove the 40bar or so that you try not to use you get around 400L. A standard scrubber for many mfg's is 2.5kg's or just under 400L of CO2 to scrub. There will always be some losses with calibration, O2 flush at 1.6, O2 flush when you get to deco, etc. so it comes out practically perfect. If you fill to 3000psi, if you have a tiny scrubber, etc it's not ideal but I do know that with my 3L sphere and a starting pressure of 2400psi there is no way that I have to worry about sorb if I am still at 1500psi on the gauge.

Most of the really experienced CCR guys I know base their scrubber times on their O2 consumption since that is what actually determines the reaction. There are flub factors put in there for temperature and what not and each scrubber design is different in terms of how much sorb is unusable before a breakthough but most will have some sort of experiments they've done pushing the scrubber on deco to figure out when it becomes a problem.

Carrying the above one step further, I can't help but think that means a dive computer which accurately knew oxygen consumption rate could dramatically increase the accuracy of remaining stack time calculation as purely a software update? (After plenty of manufacturer specific testing/calculations/programing of course...)
ie: both Petrel 3 and NERD2 with Swifts (or other savy dive computer company) could conceptually be really really dialed in on stack time? Throw in the fact computers already know ambient temperature and if unit is submerged or not it seems with a ton of R&D on specific scrubbers internal temps relative to surrounding (or throw in a couple Bluetooth temp sensors in the scrubber itself if not limiting oneself to purely a software update, which would be far more ideal) and it seems like an absolutely ideal way to go?
(Note, I'm speaking purely conceptually, getting all the experimentally determined algorithms dialed in would be a massive undertaking unique to each unit under varied conditions and I don't have a clue if even the larger CCR manufacturers sell enough units to make it worth it, let alone if they need CE cert...)

 

[tbone1004]

Carrying the above one step further, I can't help but think that means a dive computer which accurately knew oxygen consumption rate could dramatically increase the accuracy of remaining stack time calculation as purely a software update? (After plenty of manufacturer specific testing/calculations/programing of course...)
ie: both Petrel 3 and NERD2 with Swifts (or other savy dive computer company) could conceptually be really really dialed in on stack time? Throw in the fact computers already know ambient temperature and if unit is submerged or not it seems with a ton of R&D on specific scrubbers internal temps relative to surrounding (or throw in a couple Bluetooth temp sensors in the scrubber itself if not limiting oneself to purely a software update, which would be far more ideal) and it seems like an absolutely ideal way to go?
(Note, I'm speaking purely conceptually, getting all the experimentally determined algorithms dialed in would be a massive undertaking unique to each unit under varied conditions and I don't have a clue if even the larger CCR manufacturers sell enough units to make it worth it, let alone if they need CE cert...)

they would be silly to take on that liability by backing into stack time from O2 pressure. Same reason they only give gas consumption in psi/min because the divers can't be trusted to give them an accurate cylinder size.
Like I said in my original post though, you can back into it quite easily. If you have a 2L bottle and fill it to 200bar/3000psi then you change the scrubber when you fill the O2 bottle. If you dive a 3l and only fill it to 2200/2400pi then the same applies. Any inefficiencies in your O2 use will add to scrubber time conservatism so you're good to go there.

The other thing with a measured stack time based on anything other than the location of the reaction front in the scrubber itself is predicated on the diver not accidentally resetting the stack timer. Way too much liability for IMO a near 0 useful benefit.

 

[GiraffeMarineSalvage]

they would be silly to take on that liability by backing into stack time from O2 pressure. Same reason they only give gas consumption in psi/min because the divers can't be trusted to give them an accurate cylinder size.
Like I said in my original post though, you can back into it quite easily. If you have a 2L bottle and fill it to 200bar/3000psi then you change the scrubber when you fill the O2 bottle. If you dive a 3l and only fill it to 2200/2400pi then the same applies. Any inefficiencies in your O2 use will add to scrubber time conservatism so you're good to go there.

The other thing with a measured stack time based on anything other than the location of the reaction front in the scrubber itself is predicated on the diver not accidentally resetting the stack timer. Way too much liability for IMO a near 0 useful benefit.

You know far more than me on this topic so mainly me thinking through concept, I just can't help but think with giant disclaimers not to exceed most conservative stack times given by manufacturer might in time make a difference in the still safe but much more efficient use of sorb someday in the future similar to vehicle gas mileage and OC trimix diving shift to CCR if (when?) the price of sorb gets significantly higher? (Especially with wireless temp sensors using standard protocols placed within the scrubber..)

 

[tbone1004]

You know far more than me on this topic so mainly me thinking through concept, I just can't help but think with giant disclaimers not to exceed most conservative stack times given by manufacturer might in time make a difference in the still safe but much more efficient use of sorb someday in the future similar to vehicle gas mileage and OC trimix diving shift to CCR if (when?) the price of sorb gets significantly higher? (Especially with wireless temp sensors using standard protocols placed within the scrubber..)

I would be surprised if they changed, the duration tests for the scrubbers are standardized and tested in a lab setting which helps shed some liability. With experience you will learn your body and your unit which will allow you to choose whether or not you want to actually exceed the ratings set by the manufacturers.

 

[Brad_Horn]

Only two popular rebreathers have scrubber monitors; AP Inspiration and Revo.

These monitors provide excellent information about the status of the scrubber time left for the diver to make decisions based on facts rather than assumptions.

Why is this? It’s not like voodoo as the scrubber chemistry is exothermic.

In a nutshell, unless you design and engineer it inhouse, 'patents'... and the sale/maintenance cost this adds combined with limited to no verification of their accuracy in use.

Marten Silvanius http://bth.diva-portal.org/smash/get/diva2:1470427/FULLTEXT03.pdf sums it up nicely "Canisters are usually of radial or axial design and from a design perspective it becomes easier to monitor an axial scrubber as the area of heat is more concentrated.
Today there are mainly two carbon dioxide scrubbers with heat monitoring available. One is designed and patented by Dan Warkander and manufactured by rEvo rebreathers, Bruges Belgium, whereas the other one is designed, patented and manufactured by AP Diving, Water-Ma-Trout U.K.
Both these are evaluated in paper I. It is shown that surface testing could cause the CO2-exothermic heat monitor prediction of scrubber life-time to be deceiving; however at depth they become more reliable."

Both above examples are from axial scrubbers, though you can do thermal front monitoring with EACs as well...

 

[Wibble]

In a nutshell, unless you design and engineer it inhouse, 'patents'... and the sale/maintenance cost this adds combined with limited to no verification of their accuracy in use.

Marten Silvanius http://bth.diva-portal.org/smash/get/diva2:1470427/FULLTEXT03.pdf sums it up nicely "Canisters are usually of radial or axial design and from a design perspective it becomes easier to monitor an axial scrubber as the area of heat is more concentrated.
Today there are mainly two carbon dioxide scrubbers with heat monitoring available. One is designed and patented by Dan Warkander and manufactured by rEvo rebreathers, Bruges Belgium, whereas the other one is designed, patented and manufactured by AP Diving, Water-Ma-Trout U.K.
Both these are evaluated in paper I. It is shown that surface testing could cause the CO2-exothermic heat monitor prediction of scrubber life-time to be deceiving; however at depth they become more reliable."

Both above examples are from axial scrubbers, though you can do thermal front monitoring with EACs as well...

Don't have access to Zukerburg's tracking empire.

Obviously radial scrubbers won't work as the front is massively larger than axial scrubbers. However, most scrubbers are indeed axial.

Pity that the temp stick principle, even a modified one such as the Revo's excellent implementation, cannot be retro fitted to most rebreathers.

 

[Gareth J]

The first rebreather i dived was an Inspiration classic, i.e. no temp stick.
My first proper unit was an inspiration vision, with a temp stick.

As someone else stated, I pre breath the unit until the temp stick reacts. If it doesn't, then there is something wrong so I abort the dive. I've had to do this twice, once for what looks like dodge batteries, once because the tempstick wasn't plugged in properly (so I also checked the scrubber, scrubber O-ring, Mushroom valves etc.

The temp stick display should "fill" from left to right smoothly, with the left going out again once this layer of material is exhausted. If the display has holes in it I immediately become very cautious, because, (if the temp stick is not damaged,) then there is channeling occurring. The first time this occurred, I wasn't cautious enough, and end the dive with a raging headache, due to CO2 breakthrough. The second time I cut the dive short and got shallow as soon as possible, flushing the loop regularly, and expecting to have to bailout.

The inspiration has a 3 hour scrubber. If I am club diving, i.e. basic recreational diving (no deeper than 40m, decompression under 30 minutes - say upto 90 minutes runtime). I can do a weekend trip without changing the scrubber (if I pay attention to the temp stick), i.e. two dives a day, two days diving. I can run beyond the 3hour limit. I am quite conservative, if in doubt, change the scrubber, or skip the last dive.
Note (most of the decompression is for my OC buddies).

I can remember people saying bailout valves where not required, if they had been they would come as standard. The best investment I have made is fitting a proper bailout valve. From all those I have talked to who have CO2 incidents, by the time they knew they had a CO2 problem, they couldn't get off the loop to switch to OC. They had no control over their breathing.

Gareth