SPGs on CCRs

[taimen]

Maybe I could read the manual, but does an AI Shearwater have a facility to complain obviously in the event of low gas pressure?

It flashes you "reserve pressure" and "critical pressure" warnings. You can set the reserve pressure limit. Critical pressure is either 21 bar or half of set reserve pressure.

A mode where it complained if it didn’t change might be nice, especially if it knew the solenoid was being activated, of course that would need an AI Petrel and soon they’d have built a Sentinel. I suppose a nerd could do it.

Something like that could be built into XCCR with its hard wired divecan pressure sensors. With JJ people use wireless transmitter with Shearwater secondary computers (Nerd or Perdix AI). JJs primary Petrel 2 doesn't read wireless transmitters and a secondary Perdix AI is not connected to solenoid.

Do the transmitters really burn power when the pressure is static?

Yes, the Pelagic transmitter (Shearwater, Aqualung, Oceanic and so on) does. It transmits with a constant interval (about 4 seconds) when pressurized. If it would transmit only when pressure is changing, you'd need another way to validate that the transmitter is alive and connected. It displays a value only if it is recent, otherwise it is interpreted as connection lost.

 

[Gareth J]

I haven't read through the whole thread.

I've been diving my Inspiration since the early 2000's.
I still have analogue contents gauges. They run down the velcro retainers on the inside of the over the shoulder counter lungs. (Not where AP normally position them).
I periodically check them, especially the O2, if I started with a used O2 cylinder. I spent May last year on an exped' where we had no access to O2. So I had 3 x 3litre cylinders for the week, so I was very conscious of my available O2 on the second and third days diving.

I blew a 3litre of DIL on a dive last year, a nasty surprise when I checked the gauge and realised how much gas had been lost (a leaking mask, zero vis, and high task loading - and I was stupid for not realising what was happening with all the mask clearing). It was the first time my open circuit buddies had ever seen me plug in my off board gas - it freaked them out so much that they aborted the dive. Good job it was only an inland practice dive.

I've dived with others who've had electronic pressure measurement on their CCR's. For some reason, which, I've never fully understood, they appear far more unreliable, compared with those diving electronic pressure measurement on OC. It may just be overload of information. Far too many aborted dives due to 'issues' and 'alarms'. The notable thing was when you got back on the boat the problem was always something stupid like a sender not keyed to the computer, or the gas was isolated. I know at least two who swapped back too hard wired signals.

I have conventional SPG's on the bailout bottles. Maybe I'm too conventional, or too old fashioned to change . Part of it just maybe that most of my diving the last two years has been shallow, i.e less than 50m.

 

[doctormike]

It may just be overload of information. Far too many aborted dives due to 'issues' and 'alarms'. The notable thing was when you got back on the boat the problem was always something stupid like a sender not keyed to the computer, or the gas was isolated. I know at least two who swapped back too hard wired signals.

This is another reason that I don't feel the need for sending tank pressure to my HUD or standalone - it adds complexity to the dive. If the pressure readings are on the main screen, that's two more numbers competing for screen real estate. If not, that's another process to manage (scrolling through menus), when glancing at a gauge is not so difficult to do.

I can see the desire to occasionally check your tank pressure, especially if you start with a partially filled onboard dil or O2 tank, or if the dive involved some unusual activity (heavy exertion, leaky mask, etc..). But I really don't need to be juggling digital inputs for anything other than PO2, depth and run time.

 

[KenGordon]

@KenGordon The Perdix and Teric do complain pretty obviously when your cylinder pressure drops below your configured reserve setting. And then again when it drops to some really low number (not sure exactly how they determine the "critical pressure" versus your Reserve that you configure). I see the messages every time I practice a valve shutdown drill.

The cylinder pressure is what turns the transmitter on. As far as I know, they stay on and transmit on their 5 - 7 second period continuously until tank pressure is removed.

Your idea about the transmitter shutting off when pressure is static seems like it could be problematic (e.g. no transmit means your computer can't read your tank pressure, even though it's static), plus more prone to have errors. The battery in the transmitter that is used by Shearwater, et al, lasts plenty long enough that I think that optimization would be counterproductive.

I agree that it would be nice if the computer alerted you if the cylinder pressure is not dropping. But, Shearwater has some other shortcomings in their implementation of AI. To make it all work like I would (and you might) want, I think it would require a large rewrite of the AI firmware code.

The computer can assume the pressure is the same as last time or assume you are dead or the transmitter has failed. I think that I is how Suunto does it. When you swap to deco gas the trace on the backgas goes away. The benefit is you don’t care about leaving your regs charged. I suppose it avoids the various issues with the broadcast interval requiring different versions of the transmitter.

 

[davehicks]

I am shocked there are people diving CCRs without some sort of pressure guage attached. I don't care how many hours of O2 you think your tanks hold, they all run empty eventually. If that happens overnight or in your car on the way to the dive, you might get a surprise.

Probably one of the biggest risks in CCR diving is jumping in with O2 not turned on. Not making a ritual of seeing that guage pressurize and ensuring you have enough for your run time is a serious mistake.

 

[rddvet]

I blew a 3litre of DIL on a dive last year, a nasty surprise when I checked the gauge and realised how much gas had been lost (a leaking mask, zero vis, and high task loading - and I was stupid for not realising what was happening with all the mask clearing).

Why not just drive off of offboard gas all the time?

I am shocked there are people diving CCRs without some sort of pressure guage attached. I don't care how many hours of O2 you think your tanks hold, they all run empty eventually. If that happens overnight or in your car on the way to the dive, you might get a surprise.

Probably one of the biggest risks in CCR diving is jumping in with O2 not turned on. Not making a ritual of seeing that guage pressurize and ensuring you have enough for your run time is a serious mistake.

I felt it was unsafe before I went into my class. During and after my class I realized it wasn't an issue at all. The point my instructor made was that if you're jumping off a boat without your O2 on, then you are complacent and shouldn't be on ccr. More importantly if you jump off of a boat with a breathable p02 and your O2 shut off and don't figure it out, you've got bigger problems. I no longer think it's dangerous at all and really don't see the need for spgs. I feel there's a ton of complacency in ccr diving and I'm very aware of my own complacency. I make sure I follow all of the checks I have to guide myself through setup to splash dilligently and the same way every time and don't really worry about missing turning my oxygen on.

 

[doctormike]

I am shocked there are people diving CCRs without some sort of pressure guage attached. I don't care how many hours of O2 you think your tanks hold, they all run empty eventually. If that happens overnight or in your car on the way to the dive, you might get a surprise.

Probably one of the biggest risks in CCR diving is jumping in with O2 not turned on. Not making a ritual of seeing that guage pressurize and ensuring you have enough for your run time is a serious mistake.

While some people dive rebreathers without SPGs to monitor tank pressure during a dive, I don't think that anyone here is suggesting never testing tank pressure at all. Of course you need to know how full your tanks are when planning a dive....

 

[davehicks]

I felt it was unsafe before I went into my class. During and after my class I realized it wasn't an issue at all. The point my instructor made was that if you're jumping off a boat without your O2 on, then you are complacent and shouldn't be on ccr. More importantly if you jump off of a boat with a breathable p02 and your O2 shut off and don't figure it out, you've got bigger problems. I no longer think it's dangerous at all and really don't see the need for spgs. I feel there's a ton of complacency in ccr diving and I'm very aware of my own complacency. I make sure I follow all of the checks I have to guide myself through setup to splash dilligently and the same way every time and don't really worry about missing turning my oxygen on.

All I can say is that you need to be aware of other points of view than your instructors. What you relayed is the sort of arrogant macho BS that gives tecdivers a bad name.

Seriously, this is just bad logic. Just because you have O2 flowing now and your PO2 is good now, does not mean you have more than 100 PSI in the tank. If you jump off the boat without complete awareness of your gas supply then you may end up with "a bigger problem."

 

[stuartv]

While some people dive rebreathers without SPGs to monitor tank pressure during a dive, I don't think that anyone here is suggesting never testing tank pressure at all. Of course you need to know how full your tanks are when planning a dive....

I don't think that was his real point. It seems to me that his point boils down to this: If you check your cylinder pressure, mount your cylinders on your CCR immediately, then go through all your checklists and, essentially, get in the water more or less right away, you could be okay with no pressure gauge.

But, if you have any time where the cylinders are just sitting, connected to the unit or not, do you KNOW that you didn't have an issue resulting in a leakdown?

No SPG (mechanical or electronic) seems particularly problematic if you're diving from a boat. And especially if you're doing more than one dive. How do you know you didn't have a slow leak during the boat ride out? Or during the SI? I don't know about anybody else, but I try to make a point of doing zero assembly/disassembly to my unit when I'm out on a boat. Including not disconnecting the cylinders.

 

[davehicks]

While some people dive rebreathers without SPGs to monitor tank pressure during a dive, I don't think that anyone here is suggesting never testing tank pressure at all. Of course you need to know how full your tanks are when planning a dive....

Several people report that feel safe with no pressure guage on their rig at all. If you don't have a pressure guage attached to your rig All_The_Time, then you are not going to check it before every dive.

I have found my O2 bottle bone dry more than one time when setting up for a dive. I didn't turn if off after the previous dive (constant flow MCCR) or it got jostled, or something just went wrong. You may not be aware of what your pressure was at the end of the day's diving (10 hours after all, good for a few days!) and misjudge your starting pressure the next morning.

I don't check my guages in the water most of the time, as I check it before the dive. But if I am on the third dive of the day and starting with under 1000 PSI in the tank you bet I check it half way though the dive.

It should be obvious that you are putting yourself and everyone around you at risk with such complacency. Is there a single CCR manufacturer that suggests running without guages?