View Full Version : How Deep?

12-31-2011, 01:33 AM
This is a research question. I know I can probably look this up myself, but asking here is more fun and interesting.

How deep can you go before needing artificial light, like a flashlight? This is assuming a sunny day and clear saltwater.


12-31-2011, 02:24 AM
the midnight zone has no light which starts off at 3000ft i think. some light hits the twilight zone. thats where the lantern fish and squids are. that i remember was too deep to freedive by far.need a flashlight by about 600?

12-31-2011, 02:18 PM
In my story I'm dealing with depths down to about 300ft, so no need for lights except interiors and night time? Great!


Capt Nemo
01-01-2012, 02:56 PM
Actually, probably closer to 100' for needing a flashlight for the corners. For photography, you can get away with 400 on the surface without strobe, but by 10' you'll be up to 6-800+. It drops a lot faster than you think.

01-01-2012, 06:14 PM
YAY!thanks, nemo. i was hoping someone would have better info than me.

01-09-2012, 03:00 PM
Another research question. What effect would pressure have on water breathing Mers? I know the bends are caused by nitrogen bubbles forming in the blood stream. Can such a thing happen to water breathers? If not, what other dangers of great depths are there?

Again, I know I can look this up myself, but this is more fun.


Capt Nemo
01-09-2012, 10:58 PM
Water breathing mers would probably be OK to surface within the first 500-1000'. Anything deeper would probably have to decompress. But nitrogen probably wouldn't be the issue, it would be either CO2 or O2 causing the problem. Comming up from deeper depths they would need to spend about 12 hours minimum (probably more like 24) in the shallower depths before surfacing.

For air breathing mers, multiple deep dives would cause N2 decompression problems, as the air will compress in their lungs. About 7x 5 minute dives to 100' causes the DSAT algorithm in dive computers to say deco is needed, given standard freediving practices.

Air past 200' becomes toxic, as the partial pressure of O2 becomes greater than 1.6 atmospheres. At the surface, the PPO2 is at .2 atm, and increases by .2 every 33 feet of depth. So at a depth of 33 feet you would be breathing O2 at .4 atm. This is why deep scuba divers breathe anoxic mixtures past about 150 feet. They have to have less oxygen in in the breathing mix to try to keep things at around 1.25 max PPO2 on the bottom. A person breathing one of these mixes at the surface will suffocate.

01-10-2012, 12:05 PM
I think I understand, thanks. Most of my fictional Mers breathe either water or air, switching between gills and lungs (which deflate/collapse underwater).


01-19-2012, 03:42 PM
Here is the section dealing with the information you all helped with. I hope this rings true and there are no glaring errors.

After a few minutes, they cleared us off of the foc'sle so they could drop anchor. We gathered on the well deck, where we were given a safety talk and headlamps. All of the Mers were welcome to go over the side. There were also four non-Mer divers, three from the Aquarium and Professor Williams. Ben, since he was not scuba-qualified, would have to stay behind. When the bipeds were ready, we all went in.
The S.S. Sutherland Express was sitting upright in about forty fathoms of water. Much of the superstructure, anything that could come loose, had been cut away before she was scuttled. The vessel herself should be perfectly safe for even surface dwellers to dive in. Also, having been on the bottom for less than a year, she was remarkably, even eerily clean. She looked like she was riding a sea of sand while we flew down to her. The amount and variety of sea life we could see on the way down was stunning. There were corals and sponges, rays and fishes of all sizes and colors. I could identify only a very few of them; which were good to eat, which to leave alone. In some cases, which were animal and which plant.
Tara stopped just above the gaping hole where the freighter's funnel had been and motioned for us to wait with her there. The biped divers were so slow. I took her hand.
Tara! This is so beautiful! I thought to her, But so strange. I'm totally bewildered!
She nodded, It'll never grow old, no matter how many times you see it! And we're looking at it with our own eyes, not through a porthole or mask! Worth a tail, don't you think?
Tail! My God, it's worth a life!
Isn't that what we all lost, Hannah, our old lives?
I had to agree with her.
When the divers, with their silly tanks and bubbles caught up, we went down inside and switched on our headlamps. Two of the Aquarium divers carried powerful lights. The third held a camera as well. We swam down the funnel-shaft into the boiler room. It was empty, the huge oil-fired boilers had been salvaged. A shoal of bright yellow fish, no larger than the palm of my hand flitted past me in a panic. Skye picked up a spiny lobster, grinned and rubbed her belly. She's always been partial to crustaceans. I wagged a finger at her and shook my head. I wanted to wait for Tara to say it was OK. The young Mermaid shrugged and released the lobster.
We swam back to the engine-room, it was also empty. The trough where the huge propeller-shaft spun was partly full of sand. Tara placed a hand on my shoulder and pointed upward. There was a catwalk and a large hatch.
That leads to the crew's quarters and offices, where most of the scientific instruments and such will go. We're gonna try to put in a dry-room too; a place where we can talk.
Tara next led us forward to the cargo hold, the largest space on board. It was impressive, larger than a highschool gym. Nets had been stretched over the open hatches far above our heads. More nets blocked off the rest of the ship. Tara made a biting motion and pointed at them. I swam up to her.
That's to keep large predators out. This is where we'll be sleeping tonight!
Tara looked at her watch then held up a small school bell and rang it. This was the signal for everyone to gather around Alex and me, so we could communicate. The divers were told to head for the surface, the rest of us had about an hour before dark to explore and forage for food. When the bell rang again, we were supposed to enter the hold and close the nets. Meanwhile, we were all to move about in groups of three or more, no one should swim alone. They also warned us to watch carefully while eating, the remnants are likely to attract sharks. All these precautions were kinda taking the bloom off the rose for me, but I knew they were necessary. We were in the wild ocean now, not a suburban lake. There was more to worry about than the occasional fat, lazy gator.
We all stuck rather close to Tara, she knew what was safe to eat. We spotted only a few small sharks who seemed respectful of the large school or pod of big predators that we were. If worse came to worse, of course, there was always Alex and his lightsaber, which he wore hooked to a belt around his waist. I swam next to him.
Will that thing work down here? I asked him.
I don't see why not. It's been in deep water before without ruining it.
Have you ever turned it on in water before?
No. You want me to test it?
We stopped. I grabbed Jackie as she went by, to keep the three swimmer rule. We waited for the others to go round the hull, out of sight, then Alex switched on the saber. It was incredibly bright in the dimness. It was also rather loud, making a bacon frying sound as little steam bubbles popped and crackled around it.
Jackie nodded and gave us thumbs up. Alex switched off and we started after the others, only to see them ahead, staring at us. The noise must have brought them back. Tara grinned and held her hand in front of her mouth as though she were giggling.
Just seeing if it worked. thought Alex sheepishly as we rejoined them.
http://mernetwork.com/wiki/images/thumb/2/29/Separator_f.png/200px-Separator_f.png (http://mernetwork.com/wiki/File:Separator_f.png)

As the sun went down, the bell was rung again. We swam into the hold where we counted tails. all present. There was a camp-out feel to the whole thing, only there was no fire and no singing of Kumbaya (or anything else). The inability to talk proved rather annoying. In the lake one could simply make a hand-motion and surface right away, here it took several minutes to get up-top. Alex and I were kept busy, flitting about as Mers waved their arms to get our attention. Altogether, however, it was a fun evening. One by one, the headlamps were switched off as people settled down.
Skye shyly told me that she wanted a night light. It was so much darker here than the bottom of the lake. We took Alex's lamp and set it in a corner and left it on. This gave the whole hold a faint glow. Edna volunteered to curl-up with her. Soon the young Mermaid was sound asleep. Mers floated here and there about the hold, singly, by couples or small groups; tails and fins moving gently or not at all. It looked alien and peaceful at the same time. Finally, I settled against Alex and drifted into pleasant dreams.


05-23-2020, 05:44 PM
Alveric, very nice story. I discovered your post 2012 only today (2020). Since you asked about deep diving, here is a recent link
showing ladies wearing mermaid suits going scuba diving. A couple of dives are in the 150' depth range! You see it is quite bright down there, still, at least in clear caribbean or south pacific waters. Colors are very blue, of course, down there.