SCR Passive Vs Active

[Darren27]

I've been following the thread on SCR deco and was slightly confused by one post. It was mentioned that there were two types of SCR, Passive and Active. I have not spent too much time looking at SCR's so can anybody give me a quick Idiots guide to how Passive and Active SCRs differ.

Cheers

Darren

[keith_henson]

passive are like tha halcyon - so the gas is only injected into the loop when the bellow bottom out, After each breath there is a small amount of gas ejected from the loop.


Active flow is a constant flow of gas through a small injector port into the loop.


Anymore differences - i cant remember from my dive science course



Keith

[turbanator]

It's in the way the gas is added and personally I find it a bit counter intuitive.

Active is a constant flow through an oriface, like a Drager.

Passive is adding, usually automatically as the counter lung compresses, like a halcyon does.
Passive is a bit more complicated, but has some advantages in that it's harder to 'out-breathe' the mix, i.e. lower the fo2 too far.
The variation of the breathing mix is a lot lower.

hth

Paul

[MATTBIN]

http://crazy-rb-diver.com/html/what_...reather__.html

[NotDeadYet]

As I have a slightly mis-wired brain, I remember it as active systems are the one that do f*** all to get gas in the loop, passive systems have a mechanical system to add gas.

Advantages of active systems: they are simple
Disadvantages: they depend on a sonic orifice to add gas. Any contamination of the orifice and gas flow is no longer sonic and it starts to behave weird. They have no obvious failure modes. If the gas supply fails then you can breathe the loop until you die without ever noticing anything wrong. Even when running properly you can still overbreathe them and critically lower the loop ppO2, again with no warning.

Advantages of passive systems: very apparent failure modes, if something stops working provided you know what to look for then it gets noticed.
Disadvantages: I find the breathe not particularly pleasant. They are also a pain in terms of gas logistics, a gas can have quite a narrow useful depth band. Go outside these limits and you need to swap gases. Over about 30m you need to swap gases during the ascent or the ppO2 will drop terminally.

Just my own opinion, but I really don't see any advantages in going SCR over CCR. Sure you can argue there are no electronics but how hard is it to read 3 volt meters every few minutes? You don't need to steer a train but does that make it safer than a car? I've dived with a few people who used passive SCRs and were carrying as many bottles (if not more) than I was going OC so there is no logistical advantage either.

[Darren27]

Cheers all, I think that has cleared that one for me.

I'm not thinking of going SCR, but was just interested in the difference. If I make the switch to a rebreather it will most likley be for a KISS.

Once again, Thanks