Flashlight in plastic bag?

[spt29970]

I wouldn't waste time fiddling with plastic bags for dive lights, but amazingly enough some people actually put their digital cameras in them ( ewa-marine*:*Home* ). I once saw a video from someone who put their cellphone camera in a ziplock bag! I guess you call this thinking outside the box.

 

[LeadTurn_SD]

Agree with previous posts.... now for some history.

In the 1970's, many of us dove with dive lights that were nothing more than regular, cheap long D-cell flashlights slipped inside a rubber sheath which was secured to a plexiglass less cover with a hose clamp. Sort of looked like a flashlight in a yellow condom

Worked down past 60 feet or so if I recall..... so I'm not so sure about the "crush depth" stuff unless you are going deep.

Best wishes.

 

[ClayJar]

(On the other hand, if you also wanted to take a stuffed animal down for a good squeeze, it may be worthwhile. :biggrin

And why exactly would somebody want to do this? Isn't it something that 13 year old boys would want to do with their little sister's stuffed animals?

Nobody ever said you had to have a reason for everything. Some of my best culinary exploits have been the result of following random whims. The same can be said of many things. As long as you stay out of danger zones, just having fun is a great thing.

In the kitchen, you stay clear of cross-contamination, make sure you cook raw meats to prescribed temperatures, and keep things from burning. In diving, you watch your depths, times, ascent rates, buddy communication, and so on. As long as you're within the relevant envelope, what's the harm in trying even pointless things just because? It can be quite fun. (Doing a two and a half hour dive playing cards and board games on a platform 17 feet down certainly had no good reason, but take it from me, we had a blast. )

Of course, if you simply *must* have a reason for everything, you could always use a set of photos of a progressively more crushed teddy bear to illustrate squeeze to students. They're more likely to remember a very uncomfortable teddy bear than they are to remember a bag or bucket of air. :biggrin:


On the original topic, by the way, I do have one little warning. There are two ways a sealed container (like the concept flashlight) can fail. The usual way is for water to find a way in, gradually filling the container with water. The other way is for the pressure to get greater and greater until the container catastrophically implodes (with nothing gradual about it). A light probably does not have enough air volume inside to cause a particularly bad implosion, and it's most likely to leak (by a seal or cracking the lens or what have you) instead of imploding. (One of our tech-diving instructors was talking about a dive where he came across an unbroken light bulb. He thought it was cool... until it imploded right next to his hand. Apparently, it rather stung. ) Anyway, if it does pop, don't panic. :grin:

Does it? I always thought that 3 atmospheres of pressure against a sealed body is the same whether it's water or air. If you take a non-waterproofed flashlight and throw it in a non-rigid bag, as the bag compresses, won't you be subjecting the sealed batteries and the sealed bulb to increased air pressure even if the bag is not physiclaly pressing against the hull? My bet is that you'd have an imploded bulb and more seriously, some leaky/explody batteries.

The batteries can be considered incompressible. Unless you're using a sealed lead acid battery with an enclosed gas space, there's nothing to compress. (Neither the electrodes nor the electrolytes are gases, and any evolved hydrogen or is trivial and can be ignored, assuming the cell isn't vented to make it a completely moot point.) As for the bulb, such a small, thick glass bulb should easily handle the pressure. I'll have to see if I can scrounge one up to bring down on this weekend's dives, just for an empirical anecdote.

Of course, since we're talking about an LED flashlight, there is no glass bulb anyway, and I would not want to be diving at depths that could crush a solid-state LED into oblivion.

Nonsense.

Sorry buster, the internal pressure in the bag would be the same as ambient pressure... basic Boyles Law in play. If something would crush at 10 metres outside the flexible container, it'll crush inside. Density of the gas increases in direct relationship to ambient pressure. Volume of gas showing an inverse relationship. Basic physics illustrating a basic chemical law.

There's no need to get all snippy, especially as you are an instructor talking in front of new divers. Also, you've missed my entire point. (For the record, I'm only a divemaster, but I do have a bachelor's degree in chemical engineering. By the way, Boyle's Law (and all the other special cases of the ideal gas law) are really *physical* laws, not chemical laws, but hey. Anyway, just saying that I might not *quite* be worthy of a "Sorry buster" this time -- when I am, you are free to let me have it. :biggrin

The key is that the flashlight is not effectively sealed. If you sealed it in a tight-fitting plastic bag, it would then be effectively sealed, with the internal pressure at whatever it was when you sealed it (and the external pressure being whatever ambient is). Let's say that the pressure of 10 meters of water would crush it. Now, if you sealed it in a larger plastic bag with, say, five flashlight-volumes of air, when you get to 10 meters of water, the pressure on the outside of the flashlight would be the same as what had crushed the previous sacrificial flashlight in our thought experiment, but since the flashlight inside the bag is not effectively sealed, the pressure inside would be the very same (giving zero gauge pressure measured across the walls of the flashlight).

With the flashlight inside the bag not effectively sealed from the air in the bag, you would have to descend to such a depth as all the air in the bag had been compressed into the air space inside the flashlight. Descending an additional 10 meters would then be sufficient to crush that flashlight, as no more air would be present to migrate into the body of the flashlight to continue equalizing the pressure.

 

[elan]

Nonsense.

Sorry buster, the internal pressure in the bag would be the same as ambient pressure... basic Boyles Law in play. If something would crush at 10 metres outside the flexible container, it'll crush inside. Density of the gas increases in direct relationship to ambient pressure. Volume of gas showing an inverse relationship. Basic physics illustrating a basic chemical law. Density is just another way to describe pressure... Basic physics all on its own.

Original Poster... buy a divelight.

If the light is not sealed which I doubt, he has a good point if the sealed parts like bulb and possible batteries, can withstand that pressure.

In other words if one has extra air inside the bag the pressure inside and outside the light can equalize, in case the bag does not have the air it will not equalize until anything collapses and the volume is reduced.

 

[spt29970]

I think that people are underestimating how strong materials are. I find it unlikely that the flashlight would crush, unless it were truly cheap. It is more likely that the bag would wrinkle as the pressure increased and would eventually tear, causing a flood.

Along similar lines, I had a friend that put a laser pointer inside an old Q40 flashlight body. Of course this is a rigid enclosure.

 

[elan]

Even if the light can withstand the pressure I think all the risks are not worth it. With so many things you need to take care of adding another variable such as this light in the "condom" which can break anytime is just plain stupid

 

[ClayJar]

I think that people are underestimating how strong materials are. I find it unlikely that the flashlight would crush, unless it were truly cheap. It is more likely that the bag would wrinkle as the pressure increased and would eventually tear, causing a flood.

The only suspect part I see in decent flashlights is the front lens. Replacing that (or supplementing it, as in the case of LeadTurn_SD's description of the old days) can go far. (Look at the do-it-yourself can lights with modified Maglite heads, for example.)

The rest of the flashlight body is usually cylindrical, too, which is an excellent shape for a pressure hull.

With so many things you need to take care of adding another variable[...] is just plain stupid

To paraphrase House, "Everything fails." (Every light I've had flood or otherwise die on me has been a "nice" dive light. My current favorite is a cheap piece of junk I found on the bottom under the bow of a wreck. Go figure. :biggrin

 

[DBailey]

I would concur with the general consensus that there are plenty of dive lights out there even for a small budget, but the geek in me can't help but comment on diving with a "bagged light". (Consider this the long-winded musing-on-a-Friday version.)

<snipped out all of the juicy stuff>

(If you do dive with a bagged light [using a small dive light as backup, I hope], please take pictures. I can't help but think it would be quite entertaining. )

What if you canned the light like my grandmother used to can tomatos? Obviously, you'd need to turn the light on first.

 

[ClayJar]

What if you canned the light like my grandmother used to can tomatos? Obviously, you'd need to turn the light on first.

Actually, since canning creates a partial vacuum inside the mason jars, they should be decent as long as you don't go too deep. The jars are often much thicker than other commercial glass containers. On the other hand, I'm not sure I'd appreciate glass on the boat, and I know it's prohibited on many beaches, which would mean it's useless for shore diving.

On the other hand, if you used blanks from plastic bottle making (such as the "Baby Soda Bottles" from Steve Spangler Science), those should work really nicely, I'd imagine. Drop a small flashlight inside, cap it off, and enjoy your dive. (I've seen them used for commercially-available emergency dye markers, actually.) You could even have a pushbutton on/off switch that you "press" by striking the end of the tube against something (your other palm, for example).

Still not as useful as an inexpensive dive light. Also, I wouldn't balk at going to LeisurePro or a grey-market discount retailer to buy a cheap but real dive light. A warranty on a really cheap light would be like the warranty on a pair of scissors I recently bought at a fabric store. They had a lifetime warranty. If they ever broke, just mail them and $1 for processing, and the company would send you a brand new pair. The catch? The scissors only cost $0.99! (On the other hand, for a $1k can light, I'd like a warranty, please. )

 

[Teamcasa]

You could always fill the flashlight with mineral oil. It won't collapse but it may not be very bright either.