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

[stuartv]

Yup, been there done that...

As have I, actually.

And a point related to that is, unlike some SPG failures, when rMS fails, it doesn’t cause you to start losing gas.

 

[Geo7]

Good point...

Can we assume that if a temp stick shows a good distribution of temperature through the scrubber bed, it works and can be trusted? And if something goes wrong (breakthough, water ingress, what else?), then the reading goes fuzzy and the diver knows to ignore it? Or are there any "misleading" failure modes?

 

[Wibble]

Good point...

Can we assume that if a temp stick shows a good distribution of temperature through the scrubber bed, it works and can be trusted? And if something goes wrong (breakthough, water ingress, what else?), then the reading goes fuzzy and the diver knows to ignore it? Or are there any "misleading" failure modes?

It’s an indication of the scrubber state which must be interpreted by the human in the loop.

At all times the human must be aware of the status of the entire system including themself, taking into account all inputs.

 

[Geo7]

Agreed, as is interpeting O2 sensor information. But as I wrote above, I am not very familiar with the temp stick with regards to practical interpetation of the bar that is displays...

 

[TimPB]

Good point...

Can we assume that if a temp stick shows a good distribution of temperature through the scrubber bed, it works and can be trusted? And if something goes wrong (breakthough, water ingress, what else?), then the reading goes fuzzy and the diver knows to ignore it? Or are there any "misleading" failure modes?

On the Inspiration the temp stick shows as a bar across the top of the handset. When it works properly, the bar starts at the left as an empty box and over time fills the box as the scrubber warms up. Once the box is full the bar starts to empty in the same way as a fuel gauge. Once the box has emptied this indicates the scrubber is used up.

The most common issue I've heard of is that, as it's a series of temperature sensors built into a rod that goes through the middle of the scrubber, one or more of the sensors can fail. This gives the reading a break or gap in the bar. This could also be interpreted as channeling through the scrubber, indicating that part of the scrubber isn't working.

I have a temp stick on my inspo and find it a useful tool. I prebreathe the unit until at least 1 bar is showing on the tempstick and it is a handy indicator that the scrubber is behaving as it should during the dive. I don't use it as a fuel gauge, but have used it to show that I can go a bit beyond the manufacturers recommended scrubber duration. My unit has a 3 hour scrubber and I've only gone half an hour beyond that. It wasn't anywhere close to the end of the bar. I'm still MOD1 and most of my dives are an hour to 90 minutes, so that works for me. If I waste a bit of sorb, no big deal.

If it breaks, I would probably replace it. I do find it useful, but not strictly necessary. I would have no qualms about buying a unit without one.

 

[rjack321]

However, I am not convinced that exceeded scrubber duration was the cause in all these hypercapnia cases (think of failed flapper valves, overbreathing, etc.), so the temp stick addresses only one issue (duration/capacity).

Yep

My dilemma is this: If your scrubber is rated for 3 hours and the temp stick on a given dive deviates a little and allows for e.g. 3.5 hours, I guess you'll trust the temp stick. But if the temp stick goes way beyond, say 4 or 5 hours, at what point do you draw the line and distrust the stick?

Unless you dive in very cold water with a very high workload, the reported temp stick duration will always be longer than the quoted 'standard' test duration (which is almost always 40F and 1.6 lpm workload)

So it's an extra source of info and you still use your brain to determine the duration. Personally, I am ok with simplifying that decision-making process and using the conservative manufacturers rating. However if the temp stick can warn about a wrongly packed scrubber, a missing o-ring (or missing scrubber), that adds safety (if reliable).

Temp sticks don't do any of these things.

 

[TTPaws]

I dive the APD with a temp stick. I also exceed the rated time on the scrubber, sometimes by a significant amount (for short & shallow dives), I also understand that the CE rating for a scrubber is based on extremely cold water and high workloads. I've also read all of the papers about temperature and scrubber use.

Here in the tropics where it usually doesn't fall below 79° F (26° C) and I don't have a high workload on dives. When I were plan on a 200-250 Ft (60 - 75m) dive I do start with a new scrubber, but for a 40ft (12m), 2 hour dive (repetitive) I rely on the temp stick.

I did notice that the people that are nay saying the stick seem to be the ones that don't have one. Is this a case of 'get off my lawn' ? I'd like to hear from someone that HAS a temp stick but doesn't use it at all.

 

[lermontov]

My general rule of thumb is when my O2 is out, so is the sorb.

what size ( kg) sorb canister are you working your guidline on?

If you are experienced enough to be doing a 100m+ dive then presumably you know your unit fairly well

Id be curious to know what people do that dont have RMS? do you exceed the limits or do you strictly adhere -or do you adjust your time depending on conditions

and if you exceed it what are the parameters that you are determining the limits you put on your time

I think it’s kinda funny that a number of people here knocking the value of rMS are also (I believe) regularly diving their units to scrubber durations that exceed the manufacturer’s spec.

Id suspect they do -but are coy to say

 

[tbone1004]

what size ( kg) sorb canister are you working your guidline on?

Id be curious to know what people do that dont have RMS? do you exceed the limits or do you strictly adhere -or do you adjust your time depending on conditions

and if you exceed it what are the parameters that you are determining the limits you put on your time


Id suspect they do -but are coy to say

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.

 

[Wibble]

(@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)