DIR SPG(s) config with isolation manifold?

[MikeS]

While looking at Halcyon Hose Kits I noticed that they have only one SPG hose. When diving doubles with an isolation manifold would it not make sense to have an SPG on both first stages so if the isolation manifold was closed you could still monitor gas supply? Per DIR is a single SPG used or one on each first stage, and why?

Thanks,
Mike

 

[Big-t-2538]

The idea behind an isolated manifold is that you have the gas available to you from both tanks at all times unless there's a neck-o-ring failure.

If the SPG is attached to either 1st stage, it reads the same...the first stages are not connected directly to the tanks. Thus in the event you lose the gas in one tank and shut down the isolator, you still have both first stages, an inflator, and an SPG....Edit, disregard this statement...it makes no sense.

I do believe though that you lose the SPG if your left post fails and is shut down.

 

[MHK]

While looking at Halcyon Hose Kits I noticed that they have only one SPG hose. When diving doubles with an isolation manifold would it not make sense to have an SPG on both first stages so if the isolation manifold was closed you could still monitor gas supply? Per DIR is a single SPG used or one on each first stage, and why?

Thanks,
Mike

Mike,

The best way to consider this subject is to bear in mind that under the holostic approach that DIR advocates your SPG is tantamount to a back up. Accordingly, your "primary" pressure gauge is in your head. You should be able to estimate within 200psi your pressure, you then confirm by checking your SPG. When using doubles, and dividing your dive into 5 minute segments, you'll know in the first 5 minutes if for some reason your isolator is closed. There is no need to add a failure point, or to spend additional money on a 2nd SPG, if a diver turns his brain on underwater..

Hope that helps..

Later

 

[Spectre]

If the SPG is attached to either 1st stage, it reads the same...the first stages are not connected directly to the tanks. Thus in the event you lose the gas in one tank and shut down the isolator, you still have both first stages, an inflator, and an SPG.

I do believe though that you lose the SPG if your left post fails.

This is incorrect. You lose your SPG if you have to shut down your left post. You lose your SPG if you have to isolate and it's the left tank that you were isolating due to failure. IF you isolated to keep your left tank from losing gas, then you'll still have your SPG, your backup reg, and your suit inflation. If you shut down your right post, and keep the isolator open, your SPG will read your tank contents.

However if you have a failure that requires the left post to be shutdown and/or the tanks to be isolated. It doesn't matter how much gas you have in your tanks... because you're out of there anyway. You either have enough to surface, or you don't, and you have your buddy to rely on.

The nice thing about having your SPG on the post that you aren't breathing primarily from; you can _easily_ notice if your isolator is closed, as the SPG won't change during your dive [since it's reading a tank that isn't being breathed off of].

 

[MikeS]

So putting aside the money issue for the moment, it seems that it comes down to a cost/benefit analysis of adding a second HP hose with an SPG. If I understand what you’re saying it’s that the benefit of adding it, mechanical redundancy and being able to monitor both sides of the isolation valve, is not worth the additional complication and failure point(s) introduced.

Mike

 

[Big-t-2538]

One word answer....Yes.

 

[MHK]

So putting aside the money issue for the moment, it seems that it comes down to a cost/benefit analysis of adding a second HP hose with an SPG. If I understand what you’re saying it’s that the benefit of adding it, mechanical redundancy and being able to monitor both sides of the isolation valve, is not worth the additional complication and failure point(s) introduced.

Mike

Mike,

Your comments are the smaller of the issues. The larger picture is that a diver should be taking 5 minute snap shots of their dive. In the course of those snap shots they should be estimating in their head how much gas they used, and how much they have left. Let's assume for arguments sake, and to make the math easy, that you normally use 100psi per minute. Assume you start your dive with 3000psi, at 5 minutes your tank should read 2500psi, at 10 minutes it should read 2000psi. If you are using doubles, and you of course checked your starting pressure before you got in the water, when you looked at your SPG at 5 minutes it should read 2500psi, if it doesn't then you can safely assume that you isolator is closed. Normally, we teach that you do a flow check as you start the dive, so the isolator should be caught then, but if for some weird reason you didn't catch it when you geared up, you didn't catch it during your flow check, you would then be able to catch it, when you checked your gauge every 5 minutes.

Certainly the added failure point is of one concern, the financial considerations are of less importance because if it's needed then money shouldn't be the issue, but the larger point is that it's not needed if you have a diver that turns his brain on during his diver as opposed to a diver that uses devices instead of paying attention..

Hope that helps..

Later

 

[MikeS]

Mike,
Your comments are the smaller of the issues. The larger picture is that a diver should be taking 5 minute snap shots of their dive. In the course of those snap shots they should be estimating in their head how much gas they used, and how much they have left.

I’m trying to get my gear squared away as much as possible prior to DIRF. So, I was only trying to tackle a small issue; where to connect the SPG(s). I didn’t mean to make light of your explanation, but was willing to except it “as is” at this point, in the context of where to connect the SPG(s).

But since it’s come up here are my thoughts:

My first thought was I’m diving for fun, why do I want to be obsessed with worrying about these snapshots every five minutes. But in thinking about it, I realized that I intuitively do this to some degree now. At any given time during a dive I have a running estimate of remaining air pressure and remaining NDL if for no other reason than to know how often to check the computer. So perhaps a more methodical process would not be a bad thing.

Mike

 

[Northeastwrecks]

You probably do check periodically. You glance, you read, you compare to where you should be and you adjust accordingly. It becomes a natural part of your diving and contributes to good situational awareness.

The people who scare me are the ones I've run into on vacation who dive until their computer starts to beep, then ascend.

 

[MaxBottomtime]

So putting aside the money issue for the moment, it seems that it comes down to a cost/benefit analysis of adding a second HP hose with an SPG. If I understand what you’re saying it’s that the benefit of adding it, mechanical redundancy and being able to monitor both sides of the isolation valve, is not worth the additional complication and failure point(s) introduced.

Mike

Not really. A second gauge will read exactly as the first one. Your gaining nothing and introducing more failure points, e.g. O-rings, hose and the gauge itself. If you ever had to shut down the left post and isolator (very unlikely) you should be aborting the dive at that point. If you were paying any attention at all to your pressure you should still have enough gas for you and your buddy to safely ascend plus a reserve. You don't need an spg to tell you that. It would be like adding another fuel gauge to your car, just in case the other one breaks. Unnecessary.