Double tank SPG question

[tbone1004]

How does it add a failure point? While a transmitter might fail to transmit, it is no more of a failure point with regard to air loss than the plug with o-ring that it replaces.

by far the piece most prone to failure on a regulator set is the HP hose, same reason you shouldn't use a short hose on your AI transmitter. The spools are dynamic o-rings under high pressure and are prone to leaking all the time, the hose itself is prone to leaking, etc. The transmitters are more than reliable enough to not even consider putting an analog gauge on there. Keep it on the boat in your regulator bag, but no reason to bring it in the water.

 

[Compressor]

A good debate. For new divers reading this thread, it should be obvious that there is more than one way to do things and people are passionate about it.

 

[TheCoolBeans]

SPG on LHpost serves 2 purposes…. It shows you what pressure you have and it also lets you know if your manifold is open/closed.

10mins into your dive you check your gauge and it’s not moved? Can only mean your isolation valve is shut.

Also, in your original post you mentioned shutting the left post down and also the isolation valve too?

I’ve never known a left post to just fail. 99% of the time it will be a free flow on the reg you’re breathing on that you will need to shutdown, another reason why we put our primary on the RH side but I digress.

The only reason you ever need to isolate is if the manifold itself fails (or if a tank ruptures or even the valve itself).

So yes a leftpost failure leading to a shutdown would render the SPG out of action.

But you’ve gas planned so you already know you have enough gas for you AND your buddy if needed to surface from the deepest / furthest part of your dive with the safest margin for error.

You did a gasplan for this right?

 

[L13]

A plug with an o-ring is just that. A transmitter and SPG require that o-ring, plus a hose (with connections on each end) plus a spool.

I've never seen a HP hose fail at the first stage connection. I've often seen HP spools fail.

I've been using transmitters for years, and never have used a hose or spool to connect it to the regulator.

 

[azstinger11]

The more i read, the more it seems i'm weird that I cross check my AI and SPG throughout a dive lol

 

[DAJ]

Your not weird, you just do it the way you like. I do the same. Transmitter right first stage/ SPG left first stage. Perdix AI on the left arm in dive mode and another on the right arm in gauge mode both linked to the one transmitter.

 

[Nick_Radov]

In a back mount double tank setup with a manifold, the SPG usually is on the left-hand post, I believe. And you breathe the tanks down simultaneously. Should the right-hand valve fail, you close that valve and the manifold and you can breathe the left-hand tank down while you end the dive and ascend. You can keep monitoring that tank pressure, since the SPG is on that first stage.

But what if the left-hand tank, first stage or valve fails? You close the manifold and the failed left-hand side and breathe from the right-hand tank while ascending. So you cannot monitor your pressure and have to rely on the last known pressure and an immediate end of the dive. Is that it? No redundancy on the SPG then? Or am I missing something?

You wouldn't normally close the isolator valve just because of a first-stage regulator failure. That valve is only there to protect against a tank neck O-ring failure or certain unlikely failures in the manifold itself.

If a failure occurs and you aren't certain which side is leaking, then you might temporarily close the isolator during the troubleshooting process in order to limit gas loss. But then after you (or your buddy) verify which side has failed and shut down that post you should open the isolator again to regain access to the gas in both tanks. Before moving out, do a flow check to verify that you know which valves are open or closed and which regs are working.

While it's always a good idea to train for and practice dealing with gas leaks, the actual risk of a sudden major leak from back mount doubles in the middle of a dive is very low. I'm not the most experienced diver here but I've spent a lot of time underwater with doubles for over 20 years and I've never seen it happen. (I'm not saying it's impossible or that you should ignore this risk, just pointing out that it's highly unlikely in practice.)

Better to prevent leaks in the first place. The most likely causes are first stage IP creep due to lack of maintenance or a crack in a LP hose due to letting it go too long without replacement. Buy an IP gauge and check your regs occasionally to verify that they are in spec and not creeping. Check your hoses for any signs of cracking and replace them proactively every few years (or more often if heavily used). If a reg is going to fail at all, it's most likely to occur when you first pressurize it before getting in the water.

I have heard stories of cave divers damaging regs or manifolds by smacking into the ceiling. Don't know much about that but apparently it can happen?

 

[Tracy]

I had one blow apart a ways back in the cave a couple months ago. I was trying out using them for O2 monitoring on CCR.

Just saying. It can happen. I can break anything.

 

[Wibble]

Shutting down one of the two valves does not shut down the corresponding tank, You still have all the air, but just one reg and no SPG.
The case in which you loose one tank is when you are forced to close the separation manifold. Which in my experience I have never seen happening (and in fact I have no separation manifold because it is substantially useless, it is just another failure point, and the risk of having it closed by mistake is serious).

The argument for a manifold with valve between the two cylinders is to protect your remaining gas in the case of not knowing which side the gas is leaking out from.

i.e. Boom! Gas all around you. Shutdown the manifold first, then you've saved 50%** of the gas. Then proceed to shut down each post and work out where the leak is coming from.

Other people go for the right post with the longhose first -- as this is "more likely" to be leaking -- then move to the manifold, then the left post (after turning the right post back on)..


** yes, it'll be slightly less than 50% as gas is leaking as you shut it down...

 

[Wibble]

I've been using transmitters for years, and never have used a hose or spool to connect it to the regulator.

Transmitters make great handles to lift the cylinders on to the boat... Hence many people add a short hose to have the transmitter out of the way of people grabbing your equipment.