Spg, AI, both

[Scuba fryd]

At the risk of derailing the thread, how many use 2 transmitters while diving doubles with a manifold? My tech instructor insisted I get a second transmitter, which I did. I can see how it might be necessary if I had to shut down a valve and then balance decompression obligations vs having anything to breathe. But I’m not doing deco, so figured I would head straight for the surface in the case of a valve shutdown, and I wouldn’t be at the bottom in the first place without a good reserve, so not sure how that second transmitter is actually useful.

I my opinion I would just end the dive like you normally would on the other hand if I dive dobbels I have space for a spg so I would just have it on since it doesn't take up much space and if my buddy gets confused it's easy to just show the spg

 

[Scuba fryd]

Transmitter tucks under the rubber hose retention strap on the inside (next to me). Lollipop hangs free, but the hoses are stiff enough so it lays next to the tank. I've had several transmission dropouts, mostly related to swimming with my hands streamlined back holding onto the lower boltsnaps. Bringing the Perdix back in front restores comms within 20 seconds or so. Gauge is very nice to have if I donate a tank.

Hmm I have seen somebody talk about this couldn't make the reading wrong have you noticed anything ?

 

[Scuba fryd]

It's silly to use both and defies logic. Some divers love to clip every piece of equipment they own on to themselves. Even instructors.

Next time ask him why he recommends both.

Ho doesn't recommend he says I have to or I can't get my self reliant not 2 transmitters not 2 spgs one of each because it's my transmitter runs out I have my spg and if I break my computer I don't know how much air I have left and when to switch tanks. Probably getting an other teacher then because you can't really have bots on a sm set up with out it being in the way but he doesn't get that.

 

[Edward3c]

I have both. When my last spg started miss reading I considered not replacing it. However, there are times when, for one reason or another, I've forgotten to attach the transmitter, or as happened twice this year forgot to strap the computer on (I have a second computer attached to my BCD) and it wasn’t worth the hassle of going back to my car to get it. Might be old age catching up with me, so I replaced it.

 

[Belzelbub]

Ho doesn't recommend he says I have to or I can't get my self reliant not 2 transmitters not 2 spgs one of each because it's my transmitter runs out I have my spg and if I break my computer I don't know how much air I have left and when to switch tanks. Probably getting an other teacher then because you can't really have bots on a sm set up with out it being in the way but he doesn't get that.

Yeah, maybe look into a different teacher. His arguments don’t really make sense.

If your computer dies, and you don’t have a backup, your dive is over. Subsequent dives, too.

A transmitter failure would be mostly equivalent to an SPG failure. With a big exception. The SPG failure may go unnoticed for a while, and is likely to report higher than actual.

Instructor seems stuck in old ways and unwilling to adapt to something new.

 

[stuartv]

Have AI or have an SPG. Having both is the worst option.

It's funny how basically everyone agrees that it's fine to have just one SPG and no AI. But, some people will say it's not okay to have AI and no SPG. Essentially, that is hypocritical.

Yes, AI can fail. But, so can an SPG. Your procedure following a failure is the same, either way.

These days, when talking about a PPS or Shearwater transmitter, I don't see any evidence that SPGs are more reliable than AI. So, if diving with only an SPG is okay, then diving with only (PPS or Shearwater) AI should be okay, too.

Personally, I have used AI since the first dive after completing my OW cert. I have used AI on every dive. I have only used an actual SPG on a relatively few number of dives (for my primary gas - I do use SPGs on deco and bailout cylinders). And despite the much less usage, I've seen/had more failures from an SPG than from AI. I have never yet (in 800-ish dives) had to miss a dive or end a dive early because of an AI failure.

Once you have AI, adding an SPG just adds something like 4 O-rings that are each a potential source of failure (and gas loss) plus a HP hose that is also a potential source of failure and gas loss. All that for no real benefit.

Adding an SPG also makes it where you can put your rig together and check your SPG to verify your cylinder is full - forgetting your computer or not verifying that your computer and AI is working. I used to have a button gauge on my single tank reg set. I took it off so that it forces me to get out my computer to verify cylinder pressure before a dive. That, in turn, ensures that I don't forget my computer (e.g. leave it in the truck when my boat heads out) and that my AI does not have a dead battery or some other failure.

 

[OMyMyOHellYes]

Have AI or have an SPG. Having both is the worst option.

It's funny how basically everyone agrees that it's fine to have just one SPG and no AI. But, some people will say it's not okay to have AI and no SPG. Essentially, that is hypocritical.

Yes, AI can fail. But, so can an SPG. Your procedure following a failure is the same, either way.

New procedure: have both AI and SPG, but add a J valve to the mix.

Just in case ... that way you can continue the dive in the event of both AI and SPG go INOP.

 

[gmerick]

Hmm I have seen somebody talk about this couldn't make the reading wrong have you noticed anything ?

When comms restore, the Perdix shows what looks like good readings on both tanks. When I import the dives to SubSurface, sometimes there is discontinuities in the pressure graph; sometimes it shows up as an additional cylinder.

 

[stuartv]

New procedure: have both AI and SPG, but add a J valve to the mix.

Just in case ... that way you can continue the dive in the event of both AI and SPG go INOP.

My dad started diving around 1959-ish. He has teased me about having any SPG at all. And for having a BCD.

Back when he was first diving, he dived doubles with no SPG and no BCD. The procedures were, one, get your weighting exactly right and control your buoyancy by your breathing. And, two, dive with one tank in the twinset shut off. When it got hard to breathe, you open the tank and let them equalize, then shut if off again. You now know that you are at "half". When it gets hard to breathe again, open and equalize again, and now know that you are at "one quarter".

I always chuckle a bit on the inside when somebody comes along and wants to knock a new, better way of doing things because that's not how they did it "back in the day". Like knocking using only AI... I hear them knock AI and insist on an SPG and in my head I hear them telling me I don't need an SPG at all - or a BCD. Right... Sure thing, old timer. LOL!

 

[stuartv]

When comms restore, the Perdix shows what looks like good readings on both tanks. When I import the dives to SubSurface, sometimes there is discontinuities in the pressure graph; sometimes it shows up as an additional cylinder.

The transmitter ID doesn't change. If you didn't do a gas switch on the computer (e.g. change the computer from Air to Nitrox32), then it should not show an additional cylinder. That sounds like a bug to report to the Subsurface developers.

If you did do a gas switch on the computer, then I'm not sure what Subsurface "should" do.