SPGs on CCRs

[taimen]

What do people generally do between dives regarding turning off and purging? The risk being just turning off and then not turning on again. You put it back on, breath it for a while and it maintains 0.7 for long enough to fool you, you jump in the water, don’t look at the handset for a bit and go hypoxic swimming to the shot line.

With wireless transmitters it gets habitual to turn valves off and purge after diving to save transmitter batteries.
Predive, you open the valves and see that your transmitters activate, then cross check with SPG:s. During prebreathe and surface swim monitor that they hold pressure.
It doesn't protect if you don't look at handsets at all, but in my opinion having an electronic pressure sensor with alarm on O2 does add some margin against closed O2 valves on the surface. Most people don't routinely dig out the SPGs at the shot before descending, but they are more likely to check their computers.

 

[njdiverjoe]

SPGs front mounted under my counterlungs.

I've kept them for two reasons:
1) I check them as part of my pre-jump routine (after donning the unit).
2) When I get bored on deco checking how much O2/dil I used gives me something to do.

 

[doctormike]

The risk being just turning off and then not turning on again. You put it back on, breath it for a while and it maintains 0.7 for long enough to fool you, you jump in the water, don’t look at the handset for a bit and go hypoxic swimming to the shot line.

That's a predive procedure failure. Maybe 5 minutes isn't long enough to always detect a scrubber fault, but even 3 minutes should be enough to see a drop in PO2 if you have your O2 off, right? If your prebreathe lets you maintain 0.7 with no O2, I would think that it's not long enough...

 

[broncobowsher]

Dive my Revo stock, Over the shoulder gauges. I even mounted a small knife to the back of the Dil gauge. I am thinking of removing the gauges and putting transmitters on the end of some shorter hoses. No room to directly attach the transmitters. The only thing that has stopped me is the cost of a pair of transmitters. Already have the NERD2. I am still new to rebreathers, only had it for a little over a year. My plan for this is simply to clean up the hose collection up front. I've geared up and had to de-gear to pull a gauge out the was tucked in behind my back a couple of times.

I really don't want the digital gauge. I prefer the simplicity of an analog gauge that shows you are doing fine instead of a digital giving an exact amount and having to decide if that exact amount is correct. In another way of thinking of it, I don't want to know I have 12 gallons of fuel left in the gas tank of my car, I just want to know I have over a half tank and that is enough information. I see I have enough and the consumption isn't abnormal.

 

[rddvet]

Good point. The bottom line is that you need to always know your PO2. Unless your circuit board is frozen (which should be noticeable), if you run out of O2 your PO2 will drop. You should be aware of that long before you get close to hypoxia. If the O2 MAV doesn't do anything, then you are either out of O2, valve is off, or some sort of downstream problem in the first stage or feed line(s). If it's not the valve, then you either plug in offboard O2, bail out, or go to SCR mode, right?

Yep, all exactly how I learned. For my wife and I, the most important step pre-dive is the flow check and ensuring onboard gas is on. We have manual units with needle valves, so the oxygen needs to be off when not diving. I personally like that because it puts turning oxygen tank on a high priority. We watch each other turn on our gas at the gear bench, then we again watch each other do a flow check just like in backmount(our units have the onboard valves facing up). We watch each other reach back and confirm gas is on, flow check all regs, all mavs, and all inflation. We don't go under until that's done. We pretty much dive caves exclusively, so a little different than boat diving. Our plan for eventual (rare) boat diving would be similar. Only difference is we would drop in, stop at 10 or 20 feet and do a team regroup and flow check there.

 

[macado]

I'm trying to think of better wording for what I am trying to say here but I treat my SPGs like I teach my students in open circuit. I look to make sure the needles don't move when I add gas on both MAVs.

My pre-splash ritual involves me looking at both of my SPGs to confirm pressure and that they're on before jumping off a boat. I think this is part of a lot CCR predive jump lists. At least I know it's on the rEvo official jump list. Unfortunately merely breathing off a loop does not give you the same SPG feedback you get when breathing off a second stage. I make a habit of hitting the buttons on both my Dil and O2 MAVs while looking at the SPGs right before I splash. I also physically reach back and confirm my valves are on.

Another point was brought up in this thread about SPGs versus button gauges on bailouts. This could be an entire debate in itself. I had started a similar thread on a Facebook group and like everything on the Internet people started arguing.

For what it's worth I started slowly switching over a lot of my bottles to button gauges. My O2 and 50% have button gauges on them now. A couple of my bailout regulators also have button gauges. I don't have to deal with leaky spindles or hoses. Unless I am bailing out these bottles always start full and because I am planning the dive beforehand I typically have ballpark of how much deco gas I would need.

My deep bailout regulators still have SPGs because I do like to know what they have and I occasionally use these same regulators for sidemount so I kept the SPGs on there. Most of these regulators already had SPGs from open circuit diving and I see no need to replace them until they have issues.

 

[doctormike]

We watch each other turn on our gas at the gear bench, then we again watch each other do a flow check just like in backmount(our units have the onboard valves facing up). We watch each other reach back and confirm gas is on, flow check all regs, all mavs, and all inflation. We don't go under until that's done.

Yup, checklists are great, call and response checklists are even better!

 

[rjack321]

On my backmounted Meg I often do 2 dives. And there may be lots of ups and downs being with OC buddies. I like knowing how much dil and O2 I have left in real time. I might metabolically have "10hrs" of O2 but in reality its common for me to use 800psi of a 3L in a 75min dive with OC buddies. So I might finish the day with 12-1300psi in my 3L O2 and about the same in my dil (its dil and wing gas). Its kind of still funny to me that consumption of both is depth independent, 2x 180ft or 2x 80ft is almost the same.

For cave dives, especially in my sidewinder I "only" have 2L of O2 and often end up on sawtoothy caves, sometimes with low ppO2 bottom BO - both of which really suck up the O2. It's uncommon on the sidewinder but I put an spg on mine. It's on a milflex hose and attached to my o2 needle valve/mav.

My BOs all have antenna-ed spgs, I need a "handle" to grab and pull forward far enough past my shoulder to see. On my meg they have to get past the TOS counterlung. On the sidewinder just past my shoulder. I rarely check them, but I have had hose failures and it was nice to see how much I had lost and know should I feather or shut down and how much reserve I had and such information to give myself a barometer on where I stood.

 

[KenGordon]

With wireless transmitters it gets habitual to turn valves off and purge after diving to save transmitter batteries.
Predive, you open the valves and see that your transmitters activate, then cross check with SPG:s. During prebreathe and surface swim monitor that they hold pressure.
It doesn't protect if you don't look at handsets at all, but in my opinion having an electronic pressure sensor with alarm on O2 does add some margin against closed O2 valves on the surface. Most people don't routinely dig out the SPGs at the shot before descending, but they are more likely to check their computers.

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

Do the transmitters really burn power when the pressure is static? If I was building it it would only send if the pressure changed and would try hard to be completely asleep when the pressure was static. I think the Suunto one stop behave that way, but could easily be wrong.

 

[stuartv]

@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.