Spg backup to transmitter

[doctormike]

I don’t believe in the need for a backup to a transmitter (or a backup to an SPG). The parts required to add another backup (splitter, hose, swivel, SPG) is another four o-rings, the two on the swivel of which cause an annoyance and have a higher failure rate than a transmitter. To quote your post, you and I have happily dived for decade(s) safely with only one form of pressure monitoring on a set of doubles. The SPG or transmitter are “already” the backup to your gas calculations and planning. There shouldn’t be more than a couple hundred psi swing in either direction relative to what you’re expecting.

Yup! That all makes sense...

So then assuming that you have two accurate forms of tank pressure measuring, no need to thumb a dive if one of them fails.

 

[L13]

If you put the button on the other HP port there are no additional o-rings/splitters/or hoses (you do add the slight risk of the SPG mechanism itself failing) so all those arguments are bogus, and the button is almost as easy to see as an SPG on a short hose (unless you do the lollipops, but those are a bad idea for other reasons). It should be noted that buttons come in various sizes, from about .5" to 1" or more in diameter, the ones I use are ~1". It would be difficult for me to read a .5" one, and I would not consider it adequate.

My opinions:
1) From a gas leak perspective I think Transmitter + Button is safer than SPG on Hose, but not quit as safe as Transmitter only.

2) From a gas display perspective, all three displays types are adequate (in sidemount, with a 1" button in the top HP port) when they are working. Transmitter is best, Button is worst.

3) Some say SPG on hose is more reliable, some say Transmitter is more reliable, some say Button is unreliable. Transmitter + Button is more reliable than Transmitter only and might be more reliable than SPG on hose.

The compromise of Transmitter + Button has already paid off for me once, when I was able to complete an (open water) dive with no loss of pressure indication despite a loss of Transmitter.

 

[grantctobin]

and the button is almost as easy to see as an SPG on a short hose (unless you do the lollipops, but those are a bad idea for other reasons). It should be noted that buttons come in various sizes, from about .5" to 1" or more in diameter, the ones I use are ~1". It would be difficult for me to read a .5" one, and I would not consider it adequate.

2) From a gas display perspective, all three displays types are adequate (in sidemount, with a 1" button in the top HP port) when they are working. Transmitter is best, Button is worst.

You’re suggesting that you’re happy with using a button SPG to calculate turn pressure in an overhead? That’s how this reads

 

[tursiops]

I use a button SPG on my single backmount reg, not as redundancy to the transmitter, but so I can see whether the tank I am setting up on is full or used, without having to fire up a dive computer. I do not use buttons or SPGs on my SM regs, just a transmitter, even for cave diving. If a transmitter fails, it is dive over.

 

[L13]

You’re suggesting that you’re happy with using a button SPG to calculate turn pressure in an overhead? That’s how this reads

No, I am saying I am happy using a button SPG for all SPG functions in open water while diving sidemount with the transmitter working on the other tank. I would be ok with a button on each side mount tank in open water. BUT, I wouldn't want to push Murphy if both transmitters failed, I would surrender, thumb the dive, and hope I made it to the surface before he got really pissed.

I don't have enough experience in overhead environments, so would defer to my instructor until I did in those environments.

 

[Nemrod]

If you think transmitters to be more reliable than SPGs then just have a tx. If you think an SPG to be more reliable than a tx then just have a SPG. I am not sure why complicating the situation with multiple redundancies and additional points of failure (that rarely fail) makes sense if you are going to end a dive if either fails. Even a burst SPG hose or a tx O-ring failure is hardly an immediate emergency.

Some button gages are plenty big to read and I have never had one fail, not sure the problem with a button. Those tiny ones perhaps.

Sometimes I wonder if folks just dive to collect data to display on their computers screens or if they still dive for adventure?

 

[doctormike]

If you think transmitters to be more reliable than SPGs then just have a tx. If you think an SPG to be more reliable than a tx then just have a SPG. I am not sure why complicating the situation with multiple redundancies and additional points of failure (that rarely fail) makes sense if you are going to end a dive if either fails. Even a burst SPG hose or a tx O-ring failure is hardly an immediate emergency.

Right. I don't understand the need for tank pressure measurement redundancy. I think that it comes from the days when transmitters were unreliable, but people still used them because they liked having the data on their wrists and they liked logging their gas consumption.

Now that TX are pretty reliable, I don't see the point. Plus, if you have two data sources, what do you do when they differ? I guess just follow the lower one? But then you have lost your redundancy.

Some button gages are plenty big to read and I have never had one fail, not sure the problem with a button. Those tiny ones perhaps.

I might not feel comfortable using some cheap off-brand tiny knockoff button gague. But if you wanted to use something like that and wanted a more reliable solution, just use a regular SPG on a 6" hose. Yeah, it's a bit more money, but much more reliable and more flexible in terms of mounting for optimal visualization.

 

[tursiops]

Right. I don't understand the need for tank pressure measurement redundancy. I think that it comes from the days when transmitters were unreliable, but people still used them because they liked having the data on their wrists and they liked logging their gas consumption.

Now that TX are pretty reliable, I don't see the point. Plus, if you have two data sources, what do you do when they differ? I guess just follow the lower one? But then you have lost your redundancy.



I might not feel comfortable using some cheap off-brand tiny knockoff button gague. But if you wanted to use something like that and wanted a more reliable solution, just use a regular SPG on a 6" hose. Yeah, it's a bit more money, but much more reliable and more flexible in terms of mounting for optimal visualization.

I've never had a (MH8A) transmitter fail. I've thrown away several SPGs. How should I interpret this information?

 

[lostsheep]

Someone, somewhere, has the mean-time-between-failure data to compare SPGs to transmitters but we are never gonna see it so have some anecdotal evidence; I think I've seen more high pressure spool failures than every other oring failure combined.

And yet I am still diving SPGs, maybe I'll start a go fund me

 

[doctormike]

I've never had a (MH8A) transmitter fail. I've thrown away several SPGs. How should I interpret this information?

Well, assuming that your study design is good, your cohorts are matched and your N is big enough, you might have reached statistical significance and concluded that a TX is more reliable than an SPG. Or it could just be anecdotal.

But assuming that you are accepting this conclusion as valid, then you should just dive with a TX on each gas source and be done with it.