Prebreathing Survey

[Tassi Devil Diver]

I appreciate the explanation, and I don't mean to sound snarky, but that makes me love my JJ even more!

Doctormike, after reading through this topic it sounds like you would really benefit from an RMS type system in a rebreather.

Despite Uwxplorer's issues I have found the rEvo RMS scrubber prediction system to be reliable and of great benefit, it sure beats rule of thumb type systems, and recommendations from other divers. Whilst there have been issues with aspects of the hardware particularly the probes, this seem to have been sorted now.

I have had my rEvo for 6 years and approaching 500hrs on the unit, and regularly run the top canister down to a few minutes of cycle time on a second and/ or third dive, I have not experienced any precipitous drops in RCT as Uwexplorer describes. It is pleasing to see the sytem has been subject to independent scientific testing, and look forward to reading Dr Simon Mitchell's paper on the effectiveness of temp stick type systems when it is released.

 

[doctormike]

Doctormike, after reading through this topic it sounds like you would really benefit from an RMS type system in a rebreather.

Despite Uwxplorer's issues I have found the rEvo RMS scrubber prediction system to be reliable and of great benefit, it sure beats rule of thumb type systems, and recommendations from other divers. Whilst there have been issues with aspects of the hardware particularly the probes, this seem to have been sorted now.

I have had my rEvo for 6 years and approaching 500hrs on the unit, and regularly run the top canister down to a few minutes of cycle time on a second and/ or third dive, I have not experienced any precipitous drops in RCT as Uwexplorer describes. It is pleasing to see the sytem has been subject to independent scientific testing, and look forward to reading Dr Simon Mitchell's paper on the effectiveness of temp stick type systems when it is released.

Yeah, I was just kidding with that statement, I understand that many people have had positive experiences with the RMS system. It just seems sort of overly complex and apparently (from what I read here) prone to failure. For my purposes, since I don't do very long dives and 3 hours is generally enough for me for a weekend, I don't see the need to make the sort of risk analyses that are mentioned here. So no matter how reliable the RMS system is, it can't be more reliable than "pack the scrubber carefully, and dump the sorb after 3 hours", right?

 

[cathal]

I have found the rEvo RMS scrubber prediction system to be reliable and of great benefit, it sure beats rule of thumb type systems, and recommendations from other divers.

I have had my rEvo for 6 years and approaching 500hrs on the unit, and regularly run the top canister down to a few minutes of cycle time on a second and/ or third dive, I have not experienced any precipitous drops in RCT as Uwexplorer describes.

I have my rEvo RMS system since June 2013. Like you I experienced numerous RMS hardware failures with each failure having the exact same, well known, failure mode.
Unlike you I do experience precipitous drops regularly in RCT to the point where they are now predictable. The circumstances are as follows:
You pack two new scrubbers.
You do a dive or two in cold water (7degrees C) for about 60-90 minutes each time.
The RCT usually shows about 3hrs RCT left after the 2nd dive.
You take a break and 3-4 weeks later you build the unit for a dive.
The unit is now saying the RCT is about 90 mins.
You go for a short dive, 60 - 90 mins and after 15 minutes or so you drop down to about 37M (its a fresh water quarry)
And then it happens, my RCT drops to about 15 mins remaining.
So, I ascend immediately where water temps are 2-3 degrees warmer.
And then the RCT climbs back up to about 40-45 minutes remaining.
This all happens over the course of about 25mins from the start of the dive. I now don't trust the RMS once it drops below 1hr remaining and on any dive requiring deco. A new scrubber is packed regardless of what it says.

Rgds

Cathal

 

[paulraymaekers]

.........
You do a dive or two in cold water (7degrees C) for about 60-90 minutes each time.
The RCT usually shows about 3hrs RCT left after the 2nd dive.
You take a break and 3-4 weeks later you build the unit for a dive.
The unit is now saying the RCT is about 90 mins.

.......

Cathal, how can the RCT have changed when you have not dived the unit since the previous 3hrs left RCT?

if after your 2 cold dives it shows in the RMS log remaining RCT is 3 hours, this will not change in the 3/4 weeks you don't dive?

paul

 

[cathal]

Cathal, how can the RCT have changed when you have not dived the unit since the previous 3hrs left RCT?

if after your 2 cold dives it shows in the RMS log remaining RCT is 3 hours, this will not change in the 3/4 weeks you don't dive?

paul

Hi Paul
The short answer is I don't know, maybe the temp of the scrubber at the end of the last dive is more than the temp of the scrubber at the end of the 5 minute pre-breathe, I'm just guessing by now, I was rather hoping you could tell us . I try to keep my CCR skills maintained over the winter in a fresh water quarry over here. I jump in about twice a month, sometimes I miss a dive so a 4 week gap occurs. This is when I have observed this RMS behaviour occurring, a number of times over the last 5 years. The first time the RCT was 75 minutes at the start of the dive. It took us about 15 minutes to get to 37M where my buddy wanted to take photo's. Just after arriving there I noticed the rapid drop in RCT and we immediately ascended up to 17M where it came back to about 45 minutes. Since I stopped using the top scrubber once the RCT is down to one hour all is good. The scrubber is stored in a dry bag in an external room that has no heating.

Rgds

Cathal

 

[uwxplorer]

Cathal, how can the RCT have changed when you have not dived the unit since the previous 3hrs left RCT?

if after your 2 cold dives it shows in the RMS log remaining RCT is 3 hours, this will not change in the 3/4 weeks you don't dive?

paul

There is progress. You are now asking the questions we have been asking for years (albeit, in your case, with some undertone): how come the rMS is not working as advertised?

 

[cathal]

Cathal, how can the RCT have changed when you have not dived the unit since the previous 3hrs left RCT?

if after your 2 cold dives it shows in the RMS log remaining RCT is 3 hours, this will not change in the 3/4 weeks you don't dive?

paul

Hi Paul
As an addendum regarding the difference in the RCT on the last dive and the next dive, can I add that in my experience the RCT that is calculated on the new dive has never matched the RCT that was recorded on the last dive. I always assumed this was a product of either time in between dives or how I store my scrubbers to be honest.

Cathal

 

[paulraymaekers]

Cathal.... where are you looking to see the RCT after the last dive? I assume NOT on the main screen, but in the scrubber log page?
because the RCT in the scrubber log page does not change when you don't dive...

and looking at the main computer page of course is useless, you need the have the minimum recorded RCT during the previous dive to have info for your next dive...
paul

 

[cathal]

Cathal.... where are you looking to see the RCT after the last dive? I assume NOT on the main screen, but in the scrubber log page?
because the RCT in the scrubber log page does not change when you don't dive...

and looking at the main computer page of course is useless, you need the have the minimum recorded RCT during the previous dive to have info for your next dive...
paul

Hi Paul
I agree with your observations. What is of concern to me is the fact that regardless of the remaining RCT at the end of the old dive, its that the RCT at the start of the new dive reads usually 80 - 90 minutes at the end of the pre-breathe and then 20 minutes into the dive its now reading 15 minutes RCT and its always when its at depth i.e. it comes back a bit when you ascend. When this happens I immediately change the scrubber.

Cathal

 

[paulraymaekers]

Cathal, to understand what you experience, can you please clearly indicate:

You do a dive or two in cold water (7degrees C) for about 60-90 minutes each time.
The RCT usually shows about 3hrs RCT left after the 2nd dive.

Where do you read this 3 hrs RCT ?

You take a break and 3-4 weeks later you build the unit for a dive.
The unit is now saying the RCT is about 90 mins.

Where do you read this 90 min RCT?

paul