Equipment My first out of air situation

[scubadada]

If you look at a typical dive profile, like the one below, despite variation in the gas consumption rate (purple line), the tank pressure curve is relatively smooth with no acute changes (pink line). The line always slopes downward with ongoing gas consumption.

View attachment 914127

The tank pressure curve supplied by @ispasov looks very different. There are 2 smaller blips at about 6 and 11 minutes with a decrease in pressure that recovered back to baseline. The 3rd decrease at about 35 min is quite dramatic. At about 40 min the pressure recovers to baseline, or nearly so, but falls again to zero. There is one more increase in pressure at around 50 min. It easily seems most likely that this represents mechanical obstruction of the dip tube and/or first stage filter. Perhaps the OP can inquire with the operator to find out what inspection of the tank, valve, and regulator revealed.

View attachment 914129

Even in a dive like this, with a very significant increase in gas consumption due to exertion mid-dive (purple line), the tank pressure line shows no abrupt change, just a steeper slope during the exertion (pink line). And, just to be clear, the tank pressure curve is never going to increase during the dive (except for Avelo )

 

[JMarc]

Even in a dive like this, with a very significant increase in gas consumption due to exertion mid-dive (purple line), the tank pressure line shows no abrupt change, just a steeper slope during the exertion (pink line). And, just to be clear, the tank pressure curve is never going to increase during the dive (except for Avelo )

I've got a very steep slope on the pressure line the first time I used a DPV: the orientation of my octopus was such that I've left a lot of bubbles behind me.

 

[wdeutsch]

I would expect a transmitter failure to be all or nothing, whether because of a dead battery, loss of synch, or whatever. It continuing to work while providing a plausible but incorrect reading sounds more like the behavior of a mechanical device, such as an SPG. It would certainly be worthwhile for the OP to check it against another gauge, electronic or mechanical, before his next trip.

I'd kinda expect the same thing, except I own a transmitter that reads several hundred PSI high, consistently. I only use it to watch for leaks/freeflows on my pony tank, so don't need accurate readings. But, if something like that developed in OP's transmitter, it could lead to thinking everything was OK until the transmitter was reporting gas left when there really wasn't.

 

[tursiops]

I'd kinda expect the same thing, except I own a transmitter that reads several hundred PSI high, consistently. I only use it to watch for leaks/freeflows on my pony tank, so don't need accurate readings. But, if something like that developed in OP's transmitter, it could lead to thinking everything was OK until the transmitter was reporting gas left when there really wasn't.

What brand transmitter do you have? I've never heard of this on the PPS MH8A type transmitters.

 

[wdeutsch]

What brand transmitter do you have? I've never heard of this on the PPS MH8A type transmitters.

Aqualung MH8A. I even took it apart to see if there were any calibration screws I could reset, but no such luck.

 

[tursiops]

Aqualung MH8A. I even took it apart to see if there were any calibration screws I could reset, but no such luck.

Interesting. And how did you determine it reads high by a "few hundred PSI" ?

 

[wdeutsch]

Interesting. And how did you determine it reads high by a "few hundred PSI" ?

Comparison to several other gauges that all agree with one another - both SPG, compressor output gauge, and another M8HA transmitter. I suppose it *could* be the one that's right, but then my oceanic M8HA transmitter would be reading several hundred pounds low. So, same issue.

 

[tursiops]

Comparison to several other gauges that all agree with one another - both SPG, compressor output gauge, and another M8HA transmitter. I suppose it *could* be the one that's right, but then my oceanic M8HA transmitter would be reading several hundred pounds low. So, same issue.

Thanks. Trying to understand. It is an offset of a few hundred PSI, so reads (say) 350 when it is actually 150?

 

[wdeutsch]

Thanks. Trying to understand. It is an offset of a few hundred PSI, so reads (say) 350 when it is actually 150?

I'd love any insight on it as well. I've surmised that it's an electronics issue of some sort, having to do with the calibration algorithm, but would be interetsted to hear your thoughts.

It reads about 500-600 PSI high on a full tank (AL80) and seems to keep roughly that ratio as the tank pressure drops - as though the issue is with the slope in a y=mx+b type relationship. But, it's not *exactly* that. There looks to be some amount of positive offset at low pressures. Of course, that's by comparison to other gauges, so the offset could be from those having non-linear responses too. I don't have a gauge that has sufficient accuracy and traceable calibration to compare against, unfortunately.

To be fully transparent, I haven't spent a lot of time trying to characterize it. As soon as I realized it was off that much and wasn't able to be recalibrated, I relegated it to pony use and haven't investigated more. If you're interested, I'm open to experiments. A little light science is always fun!

 

[tursiops]

I'd love any insight on it as well. I've surmised that it's an electronics issue of some sort, having to do with the calibration algorithm, but would be interetsted to hear your thoughts.

It reads about 500-600 PSI high on a full tank (AL80) and seems to keep roughly that ratio as the tank pressure drops - as though the issue is with the slope in a y=mx+b type relationship. But, it's not *exactly* that. There looks to be some amount of positive offset at low pressures. Of course, that's by comparison to other gauges, so the offset could be from those having non-linear responses too. I don't have a gauge that has sufficient accuracy and traceable calibration to compare against, unfortunately.

To be fully transparent, I haven't spent a lot of time trying to characterize it. As soon as I realized it was off that much and wasn't able to be recalibrated, I relegated it to pony use and haven't investigated more. If you're interested, I'm open to experiments. A little light science is always fun!

Hmm, I DO appreciate the y=mx+b comment...that is my native language. And the problem seems to be mostly m.
If it were mine, I'd first try getting an RMA from Huish before trying to fiddle the electronics:

"All returns, service, warranty, and exchanges require a return merchandise authorization (RMA). To submit a request for an RMA, please send the following information to TAC@huishoutdoors.com:​

​

1. Dealer/customer number (if available)
2. Product information and specifications (i.e. part number, model, size, color, serial number, etc.)
3. Reason for return"

Good luck! Always nice to have a trustworthy spare, instead of something you'd only use on a pony.