Measure scrubber runtime?

[Big-swede]

"My scrubber last 4h in warm waters and 3 in cold"
"I get up to 5 hours out of my scrubber"

There is a lot of different bids on the runtime of scrubbers that are not from the manufacturer and they seem to be very individual.

I know that some rebreathers have their own measuring system (revo rms for example) but how do the rest of you decide?

Do you go by trial and error?
Or, "my friend told me that it should be fine for another hour and it worked", or is there a more "safe and scientific" to test it?

 

[Wibble]

Revo RMS is great. It is intrinsically accurate as there's two separate scrubbers in series: the critical measurement is when the second scrubber starts to be used.

All single-scrubber implementations have no simple A ==> B test.

 

[jale]

Hi
As a "benchmark" from a study (sorry but I lost the link so don't believe me ), 90 minutes per kilogram is a "safe" good base for me.
I do of course go beyond that if I think the conditions are fine with my risk assessment.
But starting with manufacturers numbers is good.
What is your unit?

 

[broncobowsher]

Temperature is a true variable in scrubber performance. Warm scrubbers go further than cold scrubbers. Some will even add insulating foam to improve cold water performance.

The manufacturer's ratings are based on bench testing with a fairly strong CO2 production. So if they test it will last 3.5 hours with someone on the top end of CO2 production, the average Joe is safe since they fall under that.

So where does your CO2 production compare to the test stand? RMS, temp sticks are ways to gauge the performance of a scrubber during a dive. Because things change. Even doing the same dive time and time again. A little colder one day, the hood didn't tuck in as well, a little more current, your buddy is in a little bit more of a hurry than usual since last night's dinner is ready to leave.

Is it possible to get 5 hours of life from a 4 hour rated scrubber? Absolutely. Keep your CO2 production down below 75% of how the scrubber was rated. Since there isn't much in the consumer grade, dive rated, CO2 measuring to calculate what you produce during a dive, the manufacturer guidelines are generally a good, and safe, guideline to follow. Because once you run out of scrubber, game over. Hopefully you catch it soon. And hopefully you have enough bailout.

 

[Gareth J]

The AP has the temp stick, and the optional CO2 sensor (which I don't use). In addition each Scrubber size, has specified operational limits, i.e. 3hrs maximum on the larger scrubber (once above 20m).

I do tend to 'use' the temp stick, when doing shallow recreational dives (30m, minimal deco'). If the temp stick is happy, I will occasionally push beyond the 3 hour limit. Mainly this occurs on weekend club trips.
As a general rule, because the 'scrubber cost' is minimal I change the scrubber early so as to avoid running beyond the 3hour limit. In all the years I have had my unit, I have never had a temp stick warning. Changing the scrubber early, does also mean that a change in site is not likely to violate the scrubber rules.

One nice thing about the temp stick is that when doing the pre-breath its very reassuring to see the display react. Beyond the normal test protocols, it's part of my 'yes, the units is behaving normally' final check.

P.S. Scrubber material is cheap, violating scrubber limits is not a bright idea. People died proving this, hopefully we have all learnt and absorbed this lesson!

 

[RedSix]

GUE has a somehow more "scientific" way to calculate it:
Estimating Your Scrubber Duration

 

[rjack321]

GUE has a somehow more "scientific" way to calculate it:
Estimating Your Scrubber Duration

yet it seems to be missing any allowance for temperature

 

[rjack321]

Do you go by trial and error?
Or, "my friend told me that it should be fine for another hour and it worked", or is there a more "safe and scientific" to test it?

I go by how the owner of the company dives his and his recommendations (which are more lenient than the CE test conditions)

There is no way to test it yourself, in fact there is considerable variation in duration even within the exact same conditions.
How Long Will Your Rebreather Scrubber Canister Last? | John Clarke Online

 

[Jeremie Guichard]

I will not say I never pushed it a bit, but seriously the size of the risk is quite big compared to the win of making the economy of few minutes of scrubber time...

The real calculation depends on so many factors that (unless being on a test banc) as common joe, forget about getting it even close. The "scientific" formula presented above is interesting for mind play but most likely missing key elements... Here are but a few: temperature, different breathing rate and pattern between RMV measured on OC and when being in the loop, dencity of gaz, etc...

I think that giving a formula like this is actually contra productive because of giving a false sense of precision and safety. You want to be certain? just follow constructor recommendations...

I'd even say that if I go for serious deep or challenging dive I change my scrubber even it did not or would not reach constructor limit during the dive. On the other hand if I do easy short dives in warm waters, no current then I allow myself to push it a bit...

I know what I'm about to say is at the opposite of any scientific approach but here is what work for me: I go dive to enjoy my time; with this scrubber will I enjoy or will I spend half of my dive thinking about it and over reacting to any strange feelings? As soon as I have the slightest impression it may be the second.... New scrubber!!!

 

[GF99/99]

I know that some rebreathers have their own measuring system (revo rms for example) but how do the rest of you decide?

Do you go by trial and error?
Or, "my friend told me that it should be fine for another hour and it worked", or is there a more "safe and scientific" to test it?

This is a pretty simple answer you go by what your unit manufacture specifies. If your unit is rated for 4 hours then you change your scrubber after 4hr. There is a reason why the manufacture has a recommended scrubber duration because they already did the scientific research for you so you don't have to ask the question your asking and the answer your manufacture gives you is safe.

Yes your manufacture may base the rating off of a high SAC rate under working load with high Co2 production in cold water but who cares. Why risk it. If you can afford a rebreather then you can afford scrubber. The minimal money your saving to get that extra hour out of your scrubber is not worth a Co2 hit. The whole reason why you dive is because you enjoy it and its something fun. So why not continue to enjoy it for a life time not a short time not to mention if your over diving your scrubber your putting your whole dive team as risk not just your self.

For example (key word here is example, everyone's unit is different and maybe I am dumb and got the math wrong so sorry in advance if I made an error) My unit has holds 3kg of 797 and the manufacture rates it for 4hr. I just looked on DGX and as of today 20kg tube of 797 is $170 (and obviously if you buy a pallet and split with some friends you can get it even cheaper)so 20kg will give me 26.66hr of diving for $170. Now if I were to over dive my scrubber and push it to 5hr per scrubber that would give me 33.33hr of diving per tube. Scrubber cost of diving safe is $6.39 per hour scrubber cost of diving unsafe and getting an extra hour out of each fill is $5.10 per hour. Now you ask yourself is your life worth saving $1.29 per hour of diving. Lets say you average 200hr a year on your unit all you would be saving is $258 for a years worth of diving. I'm pretty sure if CC diving is your hobby then you can afford to dive responsibly and spend the extra $258