Jumping into a RB. Appreciate recommendations

[Bennno]

An example of how you can "outgrow" a mCCR..
A traditional mCCR which uses constant mass flow (CMF) principal with a fixed orifice typically has a depth limitation based on the IP of the first stage on the Oxygen regulator. Basically, regulators on oxygen side of a mCCR are modified to be fixed so that they do not compensate or adjust IP as depth increases.

You can change the spring and increase the max depth, no?

I've been wanting a breather for a while now but the only ones that go for 'relatively' cheap in Europe (at least as far as I know) are Classic Inspos but I really don't like that they have to be shipped to the UK for service, they seem to have design issues and they are not the best unit to take on a plane. At this point, I think, I would like a pre-owned KISS, I hardly ever see them on RW though... and the ones I've seen are never in Europe.

 

[macado]

You can change the spring and increase the max depth, no?

Yes but that would usually be bad idea or least not recommended by most manufacturers. On your first stage you can adjust the intermediate pressure (IP) by increasing or decreasing the spring tension. When you increase the IP you're also changing the flow rate of oxygen.

One would typically tweak this number in your rebreather class by checking flow rate with a flow meter and breathing from the unit while at rest or a slight workload while checking to see how quickly or slowly ppO2 increases. Based on this you can adjust the IP as necessary.

Typically they are adjusted as such that constant mass flow is injecting oxygen slightly below a diver's metabolic rate. This allows the diver to make small adjustments to ppO2 using oxygen addition

The downside is if you adjust the IP too high, you will increase flow rate which can result in O2 flowing higher than the diver's metabolic rate.

In short: Increasing or decreasing IP changes the depth restriction on your unit but does so at a cost. In a nut shell, increasing IP increases depth restriction and lowering IP will decrease it. Doing so however can result in delivering too much oxygen or too little oxygen. You don't want to do this unless you've tested the flow rate and made sure that it's tuned for your own metabolic rate.

---------- Post added October 19th, 2015 at 03:00 PM ----------

I've been wanting a breather for a while now but the only ones that go for 'relatively' cheap in Europe (at least as far as I know) are Classic Inspos but I really don't like that they have to be shipped to the UK for service, they seem to have design issues and they are not the best unit to take on a plane. At this point, I think, I would like a pre-owned KISS, I hardly ever see them on RW though... and the ones I've seen are never in Europe.

I would imagine in Europe you can get a rEvo II or III mCCR mini for a decent price used. I've seen some older rEvos go for 3.5k-4k USD (~3090-3500 euros) recently. The rEvo II is not CE approved but the rEvo III has CE approval if that matters to you. There is really nothing in a rEvo mCCR that ever has to be shipped to the factory for service. There is no head in the unit. Almost everything is user-replaceable. It's a beautiful unit for bringing on a plane. I bring mine as carry-on and put it in overhead compartment. I assume they would be popular on your side of the pond since they're made in Belgium but I've never dove in Europe yet.

I don't want to sound like a rEvo fanboy as I have many friends that dive different units. There are a lot of great units out there with pros/cons.

 

[Bennno]

CE doesn't matter to me, it's only imortant for when you wanna sell stuff commercially. I never even looked into rEvo and only looked on RW for units... I guess I should change that. Thanks

 

[kensuf]

Yes but that would usually be bad idea or least not recommended by most manufacturers. On your first stage you can adjust the intermediate pressure (IP) by increasing or decreasing the spring tension. When you increase the IP you're also changing the flow rate of oxygen.

One would typically tweak this number in your rebreather class by checking flow rate with a flow meter and breathing from the unit while at rest or a slight workload while checking to see how quickly or slowly ppO2 increases. Based on this you can adjust the IP as necessary.

Typically they are adjusted as such that constant mass flow is injecting oxygen slightly below a diver's metabolic rate. This allows the diver to make small adjustments to ppO2 using oxygen addition

The downside is if you adjust the IP too high, you will increase flow rate which can result in O2 flowing higher than the diver's metabolic rate.

In short: Increasing or decreasing IP changes the depth restriction on your unit but does so at a cost. In a nut shell, increasing IP increases depth restriction and lowering IP will decrease it. Doing so however can result in delivering too much oxygen or too little oxygen. You don't want to do this unless you've tested the flow rate and made sure that it's tuned for your own metabolic rate.

---------- Post added October 19th, 2015 at 03:00 PM ----------



I would imagine in Europe you can get a rEvo II or III mCCR mini for a decent price used. I've seen some older rEvos go for 3.5k-4k USD (~3090-3500 euros) recently. The rEvo II is not CE approved but the rEvo III has CE approval if that matters to you. There is really nothing in a rEvo mCCR that ever has to be shipped to the factory for service. There is no head in the unit. Almost everything is user-replaceable. It's a beautiful unit for bringing on a plane. I bring mine as carry-on and put it in overhead compartment. I assume they would be popular on your side of the pond since they're made in Belgium but I've never dove in Europe yet.

I don't want to sound like a rEvo fanboy as I have many friends that dive different units. There are a lot of great units out there with pros/cons.

I'm sure you're aware that you can get different size orifices and that will help the flow problems. You can even get a 0.0025, which allows you to keep your 0.8l at a pretty high intermediate pressure.

Constant mass flow

 

[macado]

I'm sure you're aware that you can get different size orifices and that will help the flow problems. You can even get a 0.0025, which allows you to keep your 0.8l at a pretty high intermediate pressure.

Constant mass flow

Yes sorry. I should have mentioned that but I kept it simple. I "think" storage rEvo orifice is .0035 on a mCCR but I've heard others are between .0025 - .0035 for most stock rebreathers?

I've never bothered looking up changing mine out as it supplies enough flow for the dives I'm currently doing.

 

[Superlyte27]

Why? There seem to be very few pre-owned MCCRs on the market. Isn't that because people are quite happy with them? In what way do people outgrow them?

I guess it depends on where you are at. Most of my friends who were diving mccr's have since sold or are trying to sell them for electronic units.

---------- Post added October 20th, 2015 at 11:58 AM ----------

You can change the spring and increase the max depth, no?

Let's say that your mccr is set to deliver 1lpm of O2, which is perfect for you during the working portion of a dive. That's how much your orifice is delivering every minute the oxygen is turned on. But, you increase spring pressure so you can have oxygen delivered during deeper dives. The problem with that is, now it's delivering too much oxygen during the shallow portions of the dive and during deco. Instead of you getting 1.0. You're getting more. So your PPO2 is constantly climbing, necessitating dil flushes to drop the PPO2, which now means you're blowing through DIL.

While I'll agree there are some things you can do to overcome the limitations of mccr, theres no perfect answer with MCCR in my opinion for deep dives.

---------- Post added October 20th, 2015 at 12:11 PM ----------

...starting off with a mCCR is a good choice. You learn to fly the unit manually without a solenoid simply by the virtue of not having one to begin with mCCRs are arguably simple, easy to repair and understand. In theory, learning on mCCR gives you discipline for when **** hits the fan and you are on a eCCR.

I would have agreed with this 5 years ago, when I started off with mccr. I dove an mccr rEvo for a couple of years. Then a buddy of mine lent me his rEvo hccr rms and I realized that task loaded deep dives while on a scooter with several bottles was SOOOOOO much easier if the solenoid behaved in a parachute mode. For example: Manually, I would be driving a 1.1 PPO2, but if the task at hand necessitated some greater priority like weaving the scooter through small tunnels, or hanging bottles on the line and my PPO2 started to drop from the increased workload, the solenoid in parachute mode would inject oxygen at 1.0 PPO2.

Another insight came when I co-taught a Trimix class in Eagle's Nest. A student froze solid for 6 minutes upon ascent. I had to bring him to the surface. So, that meant dumping his wing, his drysuit. My wing, my drysuit, venting my lungs, maintaining my PPO2, etc. etc. etc. Of course this would have been a LOT easier with an electronic unit.

Today, while I appreciate my time with mccr, I wish I had just bought right the first time. I'm fortunate in that I didn't take a loss on my first rEvo to buy my buddy's rEvo HCCR RMS. But most people aren't that lucky. Just buy right the first time and be done with it. Just because it's an electronic unit, doesn't mean you have to use the electronics. You can set the computer to only fire the solenoid if it drops way low. I know some people who run their eccr Revo's and hammerhead's completely manual and only use the solenoid in parachute mode.

---------- Post added October 20th, 2015 at 12:12 PM ----------

 

[abscuba]

You can find most major units in troupe I see used ones for sale all the time. You should be considering what you will be doing in 3 years and not so much just the next year. By the time you purchase a unit, get training, get some dives on the unit to get comfortable, then get the unit set up exactly the way you like you be at a year. So start with the end goal in mind. There are many great units out there and there are only a couple of major brands that I would hesitate to dive with. I personally use a rEvo and I love it, I got it used from Superlyte27. It is a simple unit very easy to work on and maintain have not had an issue getting parts even here in Hawaii. I would do your research on units and if you can try a few on try dives. Talk to people who own units you are considering to help decided which way you are going to go. SCRs have their place, but if after a year you are planning to go deeper it might be better to just go for the CCR.

 

[Superlyte27]

abscuba, when are you coming to Florida to go cave diving?

 

[abscuba]

Hoping with in the year, have been wanting to get into it. Will be leaving Hawaii next year then will be heading that way to wander down some caves.

 

[Bennno]

... I see used ones for sale all the time.

Where? I see Megs and rEvos for 6k and up. Most of which are located in the US.