Overbreathing the scrubber

[TSandM]

Sorry if this has been discussed before, but I searched and couldn't find it:

Is it possible to "overbreathe" the scrubber? In other words, if you found yourself in heavy current or otherwise having to exert yourself heavily, can you increase the flow rate through the scrubber to the point where it's no longer effective? Or, in the alternative, does the scrubber create enough resistance where, after a point, you just can't draw more gas through it, so your work of breathing goes way up?

I have never dived a rebreather, and I can't ever remember seeing this issue discussed anywhere.

 

[Randy43068]

It's ironic that you bring this up as I was just now thinking the same thing. I'd imagine the "load" on the scrubber would vary as a factor of diver workload. Maybe the scrubber can't "scrub" fast enough when under high demand??

 

[Meng_Tze]

You can over breathe a scrubber to the extend that CO2 scrubbing capabilities are diminished/gone. You are then basically waiting to build up too much CO2 with all consequences thereafter. A scrubber passing through 0.5% CO2 is considered broken through.

There is not a scrubber I am aware of that works with breathing resistance to indicate over breathing.

I have two scrubbers with low WOB, breathing either of them is to me like breathing ordinarily at the surface. This WOB is the same at all depths and with all efforts I have been with them.

Basically you have to know how much time you have on a scrubber, how much you have used and keep a certain margin for unforeseen things.

As you know the amount of CO2 produced varies by person and work effort but if you keep control of your breathing pattern at all times, you can be fairly sure of scrubber duration.

 

[Randy43068]

Meng, how strong woud you consider that to be a limitation of the RB?

It seems obvious that breathing rate, thereby the need for increased CO2 removal, will increase under increased muscle loading, be that from swiming in more current or in an stressed situation. So how would one keep that under control?

 

[Cave Diver]

Yes, it is possible to overbreathe even a new scrubber. There are different scrubber designs, which have different efficiencies. In an axial cannister the gas flows from one to end to the other, while a radial flows gas from the inside out.

In a axial scrubber, when you start with a fresh pack, you have the entire height of the scrubber bed to absorb C02. As the scrubber is used up, you have less and less material that is available to absorb.

Think of it like a litter box. When the box is fresh, you have a lot of unused material. As it is used up, there is less to abosorb and all of a sudden you find yourself in deep ****.

Another thing to consider is the dwell time on the scrubber. The dwell time is the amount of time that it takes for your breath to pass through the scrubber. If you are under high workload and breathing hard or if the scrubber is partially used up, then there isn't sufficient dwell for the scrubber to absorb all the C02 and breakthrough occurs. This can happen even on a new scrubber, but is especially dangerous on a partially used, or a poorly packed scrubber.

Breakthrough basically means that C02 is being forced through the scrubber before it has a chance to be absorbed.

This is probably oversimplified a bit, but hope it gives you a better understanding.

 

[Randy43068]

makes sense to me.

 

[Cave Diver]

Meng, how strong woud you consider that to be a limitation of the RB?

It seems obvious that breathing rate, thereby the need for increased CO2 removal, will increase under increased muscle loading, be that from swiming in more current or in an stressed situation. So how would one keep that under control?

One thing to keep in mind with rebreathers is that slow is fast.

There is an abundance of gas available, so the best thing to do is slow down and work yourself through it. You don't have to force yourself in the current, you can take it slower and work yourself into the lee side of a wreck, hang on to the anchor line and slowly work yourself down/up, etc. Remember, gas is not your limiting factor.

If you do find yourself in a situation where you are working hard, the best thing to do is be aware of it and get your breathing under control as soon as possible.

Other options include flushing the loop to vent excess C02 or going to OC bailout, but its best to solve the problem on the loop if possible.

CCR is a tool. Like any tools there are right and wrong ways to use it. Being aware of its limitations and uses are some of the things you should learn from the instructor when you take the class.

 

[Randy43068]

Is there a downside to "flushing the loop?"

 

[Cave Diver]

Is there a downside to "flushing the loop?"

Rebreathers are incredibly efficient at utilizing the gas you have. When you flush the loop by adding diluent to it, you are using quite a bit of the onboard gas you have. Normally this onboard dil is used for things like inflating your wing. You use the 02 to maintain your PO2 at depth.

When you start burning through your dil, you're limiting one of the resources of your breather. As a fairly new Meg diver, if I was in a situation that was causing me to go to SCR mode, or doing dil flushes, I'd be heading for the surface as I did it, just to increase the comfort level.

 

[Randy43068]

Thank you for taking the time to respond to my questions. I think these contraptions are amazing but like things unknown such as this, they scare the stuff out of me.