Random question about a beeping computer in surge

[DukeAMO]

I have a random question. I was on the manta ray night dive a few weeks ago, off of Kona. The goal during this dive is to stay near the bottom, basically in one position, to watch the mantas. There was pretty significant surge, so once every couple of minutes we would have to kick or grab a boulder to avoid getting pushed around. One of the divers near me had a computer that would beep (three fast beeps) pretty much every time the surge picked up. Her depth wasn't changing. What do you think it was beeping about? I forgot to ask her when we got back to the boat.

 

[RandomGuy1]

Just an uneducated guess, but I would say ascent rate alarm may have been going off.

 

[DukeAMO]

Yeah, it sounded similar to an ascent alarm, but she was maintaining her depth right on the bottom. Would it be possible for surge to somehow "trick" the computer into thinking the depth was changing? It must have gone off 10 times!

 

[fiziks]

Surge is caused by waves passing overhead at the surface. With a wave high enough, the ascent warning will go off as the trough behind the wave passes. You may not think your depth is changing, but the amount of water above you is, and that is what triggers the ascent alarm. I haven't been to Kona, so I have no idea what the wave action is like or what the depth is, but for example, you might be at 24 feet under the crest of the wave, but only 20 or 21 feet under the trough behind the wave. A three to four foot change in a second will trigger the ascent warning on a lot of computers.

 

[Steve_C]

Could depend on where the computer is. If it is on your wrist, then your body can be still and your wrist move through several feet. I know that going up the anchor line I used to go slow but would sometimes move my hand up about 3 feet quickly from down to up. My computer light would blink. I now do that slowly also to avoid ticking it off for the next dive.

 

[clownfishsydney]

Fiziks has it exactly right. This can happen even on much deeper dives if there is a large enough surface swell. Uwatec computers are particularly bad.

 

[hsinhai]

As the computer is sensing water pressure to calculate depth, any movement of the wrist, or a large movement of water can cause a change in the pressure that is being read by the device, this would then incorrectly be ready by the device as ascending or descending depending on the direction of movement relative to the sensor. If the change of pressure dropping is too great, then it would alarm.

 

[DukeAMO]

Thanks! I learned something new today.