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Bubbles and Henry’s Law February 28, 2009

Posted by Chris Sullivan in Technical Diving.
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When my family moved from Australia to Canada in 1967, we noticed that a glass of water left out overnight would contain bubbles by morning. We didn’t notice this when we lived in Australia. My father, a chemical engineer,  mentioned to me once that he was talking to a colleague about this phenomenon, and told him to try leaving a glass of hot water out instead. Sure enough, no bubbles. He never explained to me why this happened, and I never asked, but much later I wondered why this occurred.

We learn about Henry’s law in diving. This states that at a given temperature, the amount of gas that dissolved in a liquid will be proportional to the partial pressure of each of the gases which surround the liquid when in equilibrium. Dissolved gas is what gives us various physiological problems in diving, including the decompression sickness, so it’s important to learn about it. The concept of equilibrium is important here – the gas doesn’t go into and out of solution right away. If it did, a beer would go flat as soon as it was opened, much to the disappointment of many divers I know. Similarly, it takes time for the body to expunge the excess nitrogen (and/or helium) from the bloodstream as the diver surfaces.

Temperature is important as well. For the bloodstream, it’s hopefully reasonably constant, but for beer and glasses of water, temperature plays an important role in the ability of the liquid to dissolve gas. Colder liquids can absorb more gas (which is one reason we like cold beer). The math behind this is considerably more complex than Henry’s law (which holds temperature constant). The Wikipedia reference above has an explanation of how it works, if you’re interested. Suffice to say that cold water holds more gas than warm.

So when we moved to Canada, the cold tap provided water that was considerably colder than room temperature, since it came from outside and the rooms had central heating. Cold water holds more gas than warm, so when we left the glass of water overnight some of the air came out of solution in the form of bubbles, many of which were still clinging to the sides of the glass when we got up in the morning. In Australia, with warmer outside temperatures and no central heating, this didn’t happen, as the water and air temperatures were much nearer to each other.

Hot water left overnight cools down, so air is absorbed into solution, and no bubbles appear. If that glass was put into the refrigerator for long enough, then brought back out into the room, bubbles would form as gas would be absorbed by the water while it was chilled.

Go ahead and try this at home!

Cozumel Day 3: Cedral Wall February 27, 2009

Posted by Chris Sullivan in Dive Log.
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Continuing with February 10, 2009 diving in Cozumel with Blue XT Sea Diving. This 50 minute dive down to a maximum of 78 feet was just another drift dive in Cozumel, with its abundance of sea life and coral. The water temperature was again about the lowest it gets in Cozumel, at 25C (77F), which is a few degrees more than the warmest I see in Canada.

I’ll let the pictures speak for themselves…

Black Grouper

Black Grouper

This Black Grouper was almost as long as me, and was lazily holding station by swimming against the current. I drifted by about 6 feet away.

French Angelfish

French Angelfish

Angelfish are really common in Cozumel, so this isn’t one of those rare finds, but nice to see and my wife likes them.

cozday3a-juv-spotted-drumJuvenile Spotted Drumfish seemed to be everywhere on this trip, as were juvenile fish of many other species. I suspect that February is prime hatching time in the reefs (and February dives in other parts of the Caribbean and elsewhere have proved similar). I think we found at least one Drumfish every day we were diving, and eventually I stopped photographing them.

With my little Pentax Optio 5Si camera, now quite obsolete (the current model is 1/2 the price as has 12 mpix, but naturally doesn’t fit my underwater housing), has a significant delay in between when you press the shutter button and when the picture is taken – almost a second. So it’s really hard to get good photos of Drumfish as they never sit still, changing direction constantly.

Some day I’d like to get a really good underwater camera, but I’m a bit afraid of the expense, as there’s the price of the camera, then the housing, then the flash units, and so forth. I could probably by a rebreather for the same amount of money, or a high definition underwater camera. I’m going to keep at it with what I’ve got now until I’ve finished all my diving courses and see where my interests lie at that time.



Like Angelfish, Squirrelfish are a dime a dozen in Cozumel.  My wife really likes this picture so I’m including it. In my book, any picture where the fish is more or less pointing towards the camera instead of away from it is a good one.

Cozumel Splendid Toadfish

Cozumel Splendid Toadfish

The toadfish is a cool looking creature that makes a low sound you can often hear without being able to locate the source. They hide under ledges so I’m often upside down when taking the picture, which, along with the delay in the shutter and surround current means that I have difficulty framing the picture properly. To date, I’ve never taken a decent picture of one. On my last dive of the trip, I had one lined up perfectly when my battery died.

Buoyancy: Salt Water vs. Fresh Water February 26, 2009

Posted by Chris Sullivan in Training.
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Apparently this is a great source of confusion – or perhaps my previous writings about the subject have attracted many searches, and so biases my view of what divers are interested in. Anyway, I’m going to attempt to write something clear on the subject. Here are some of the searches on the topic that have found their way to my blog:

bouyancy of sea water
bouyancy salt
buoyancy salt water vs fresh scuba
buoyancy calculations salt water
buoyancy difference between salt water a
buoyancy fresh salt
buoyancy of salt & freshwater
buoyancy of sea water calculators
salt kg required for tank foot
salt water buoyancy calculation
salt water fresh water buoyancy
which is more buoyant fresh or salt water
why is bouancy more in salt water than fresh
why salt water is more buoyant than fresh
sea water density bar msw
why is salt water more buoyant
is salt or fresh water more buoyant
buoyancy salt water fresh water

Obviously people want to know. The answer is fundamentally simple – objects (including divers) are more buoyant in salt water than in fresh water because salt water is denser than fresh water. Why is salt water denser than fresh? Because salt is denser than water and so if you add it to water the resulting solution is denser than pure water. By how much? The salinity (saltiness) of the ocean varies, but the generally accepted average amount is 2.5%. So  salt water weighs 2.5% more than the same volume (a gallon or litre, for example) of fresh water.

Buoyancy is an upward force equal to the weight of water displaced by the object. A cubic foot of fresh water weighs 62.4 pounds, so an object with a volume of 1 cubic foot would experience 62.4 pounds of upward force due to buoyancy when immersed in fresh water. Gravity will exert an opposite (downward) force equal to the object’s weight, so if the object weighs less than 62.4 pounds it will float. If the object weighs more than 62.4 pounds it will sink. If the object weighs exactly 62.4 pounds it will be neutrally buoyant, and will stay where it is unless pushed by something (current, turbulence, a diver, etc.).

In salt water, that same 1 cubic foot will displace 64 pounds, because that’s what a cubic foot of sea water (which you recall is heavier by 2.5 percent) weighs. So there is 1.6 pounds more buoyancy in salt water than in fresh. That means if an object with a volume of 1 cubic foot weighs 63 pounds it will float in salt water and sink in fresh water. So objects in salt water are more buoyant than objects in fresh water because salt water is denser than fresh.

Note that it is incorrect to say that salt water is more buoyant than fresh water. Objects in salt water are more buoyant than objects in fresh water. The buoyant force is exerted on an object, not the water itself.

Hope that clears things up!

P.S. The density of salt is 2.16 grams per cubic centimetre vs. the maximum density of fresh water at 1 gram per cubic centimetre.  Sea water is about 3.5% salt by weight. A kilogram of sea water will have 35 grams of salt and 965 grams of fresh water (I’m ignoring the stuff that’s in sea water which isn’t water and salt). The volume of the water will be 965 cubic centimetres, while the volume of the salt will be 35 grams divided by the density of 2.16 which is 16.2 cc. The total volume of the components of this kilogram of sea water is 981.2 cc, versus a kilogram of fresh water at 1000 cc, so the combined density is almost 2% more with the salt than with fresh. This is a little less than the 2.5% real world difference – what’s going on?

My chemistry knowledge runs out at this point but my guess is that when salt dissolves in water, the salt molecules pack a little bit closer together with the water molecules than they do with each other, making the volume a bit smaller than the sum of the volumes of the salt and the water separately. If I find out the real answer I’ll post it here, unless someone beats me to it.

Buoyancy of Concrete February 25, 2009

Posted by Chris Sullivan in Training.
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Concrete isn’t usually considered to be buoyant, although hulls of boats have been made from it. A block of concrete is negatively buoyant, meaning that it will sink. Chicago gangsters in the 20’s used this property of concrete to construct cement overshoes, that would be fitted to a (soon to be) dead person to make the body negatively buoyant for underwater disposal.

The density of concrete varies with its exact composition, but averages around 2,400 kg per cubic meter, or 150lbs per cubic foot. So a cube of concrete 1 foot on each side weighs 150lbs on dry land. We say that this concrete block displaces 1 cubic foot of water. The same volume of sea water weighs 64 pounds, which is the upward buoyant force that is applied to this concrete block when it sits on the bottom of the ocean. The apparent weight of this block will be 86 pounds, which its actual weight of 150lbs (downward force) minus its buoyancy of 64 pounds (upward force). In fresh water, the apparent weight would be a little more, 87.6 pounds, because a cubic foot of fresh water weighs only 62.4 pounds so provides less upward force.

This kind of question is commonly found in the divemaster and instructor exam, although the weight and volume of the object are both given, so you don’t have to worry about density. They’re usually written something like “a concrete block weighing 150 pounds with a displacement of one cubic foot is submerged in sea water, what is the minimum amount of lift required to raise it?”. From the previous paragraph it’s hopefully apparent that the answer is 86 pounds.

Sometimes these questions, posed identically, are about outboard motors. It might be hard to estimate the displacement of an outboard motor as it is a highly irregular shape, which is why the exam questions helpfully provide you with the number. The old story about Archimedes’ problem of measuring the volume of a crown posed the same problem, which he solved by measuring the amount of liquid displaced by the object, and thus its volume. With a large enough graduated cylinder, you could dunk a crown (or an outboard motor) and measure how much the water rises in the cylinder which would be an accurate measure of its volume. Archimedes realized this after overflowing a public bath after getting into it, or so the legend goes.

Concrete blocks on the other hand are often made in nice regular shapes. One familiar example is a pyramidal frustrum, which is simply a pyramid with the top sliced off. A pyramidal frustrum with a square base and a square top is a common shape for buoy anchors. If you call the length of one side of the base b, the length of one side of the top t, and the height of the block h, then the displacement will be ((b+t)/2)2 x h. If you make a block that is 2 feet square at the base and 1 foot square at the top and 18 inches high, then using this formula the volume will be 1.5 x1.5×1.5 which equals 3.375 (3 and 3/8) cubic feet. The block will weigh just over 560 pounds, and displace 216 pounds of sea water, so it will appear to weigh 344 pounds under water. Of course, you’ll want to add an iron ring bolt to the top on which to attach a line, and that will add a bit of weight.

If you understand all of the above, you’ll have no trouble with this section of the divemaster or instructor exams.

Cozumel Day 3: Dalila Wall February 24, 2009

Posted by Chris Sullivan in Dive Log.
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Today (February 10, 2009) we did two more local reefs, Dalila Wall and Cedral Wall. The first was spectacular with marine life, the second not so much. Within the first two minutes we were at our maximum depth – for me this was 105 feet as I was trying to get closer to a Nurse Shark I’d spotted, but it went deeper and against the strong current so I returned to our planned depth of 90 feet.


There were 6 divers (Steve, Debbie,Tom, Stephanie & Jennifer) on the boat Shamu from Blue XT Sea Diving, along with Capitan Mago & Divemaster Pedro. A first for me on the dive was to see a group of 4 juvenile Spotted Drumfish. I’d never seen more than 2 previously. Most of the time I’d seen them in pairs. My cheapo camera has a significant and annoying delay after pressing the shutter release before it takes the picture, so I couldn’t get a good shot of the four together, although I got one that I can use if I need to prove my claim. Out of the 6 shots I took one came out nicely which you see here.cozd3-turtle-queen-angelfish

We also encountered a couple of Turtles. They’re so used to divers that they often just ignore us, especially if they’re eating something. This one had a Queen Angelfish hanging around picking up the scraps that the Turtle was missing as it fed.

Pedro found some Conch shells in a heap and moved them to reveal this Octopus, the first I’ve seen underwater. Other Angelfish, more Nurse Sharks, some large Grouper, Barracuda and various Box fish were also encountered.


The marvelous winter weather continued at  Cozumel, with the day hot with some scattered cloud. Underwater it was about 25C (78F), with a reasonably strong current.

Back to Reality February 22, 2009

Posted by Chris Sullivan in Miscellany.
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I’m now at home, looking out my picture window at my snow-covered back yard, yearning for Cozumel while steeling myself for the 4:45 alarm clock tomorrow morning. Next stop is Pensacola in late April, to dive the Oriskany. Over the next several days I’ll continue to write about the diving in Cozumel. I managed to get 8 two-tanks dives in, while only diving one location twice with the help of Christi at Blue XT Sea Diving and her guides.

The Fish of Cozumel February 20, 2009

Posted by Chris Sullivan in Ecology.
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During our stay at the Occidental Grand in Cozumel, there were several days I took off from diving, and on each one I went snorkeling in front of the hotel at least once. While I’d done this the last time I was there, and had been amazed the large school of fish (Big Eyed Jacks) that were hanging around at the furthest point in the swimming area, this time I undertook to inventory all the species of animal life to be found beneath the surface. I was surprised at the number of species that I encountered, and managed to photograph many of them. Here’s a list of the ones I can name:

  1. Spiny Lobster
  2. Spotted Moray
  3. Southern Stingray
  4. Yellow Stingray
  5. Horse-Eye Jacks
  6. Bluestriped Grunt
  7. French Grunt
  8. Parrot Fish
  9. Peacock Flounder
  10. Arrow Crab Spider Shrimp
  11. Smooth Trunk Fish
  12. Spotted Trunk Fish
  13. Great Barracuda
  14. Sergeant Major
  15. Needlefish
  16. Sand Diver
  17. Trumpet Fish
  18. Blue Parrotfish
  19. Juvenile Angelfish (Probably French)
  20. Blue Tang
  21. Nassau Grouper
  22. Banded Butterflyfish
  23. Bluehead Wrasse
  24. French Angelfish
  25. Honeycomb Cowfish
  26. Rock Beauty Angelfish
  27. Yellowtail Snapper
  28. Ocean Triggerfish
  29. Squid of some kind, almost transparent
  30. Various Sea Anemones

The Southern Stingray swam right in front of me about 30 feet from the beach as I was heading out of the water. Many of the others were hiding under ledges created by discarded concrete slabs or by the concrete anchors for the buoys that marked out the limit of the protected swimming area. Most of the photos were taken from under water while I was holding my breath. Not being equipped with weights I had to kick to stay under water which made the photography more difficult.

The big school of Jacks was still there, under the watchful eye of a Barracuda which appeared to treat it as its private buffet. The school changed places once in a while but never went very far.

If I’ve got any wrong please correct me.

Occidental Grand Cozumel February 19, 2009

Posted by Chris Sullivan in Miscellany.
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This is our third visit to this hotel, this time for two weeks. As usual I’m using Blue XT Sea Diving run by Christi Courtney who pick me up at the dock each day that I’ve gone diving, except for 1 day when we dove the reefs north of San Miguel. That time I took a cab up to La Caleta Marina, where their boats are moored.

Being quite far south of Marina, and even further south of San Miguel, the resort is isolated from most activity on the island. I’m quite happy to hang out at the resort, which is all-inclusive, with my only getaways being the diving expeditions. It does though present a problem for the dive operators who have to restrict themselves to the southern reefs when picking up divers down here. Likewise for those who like to shop in town, experience the night life, and so forth would have more of a logistical problem here.

There are two restaurants on the resort plus the buffet. One restaurant has a Mediterranean style, and is OK. The other restaurant, the Sonora Grill, has more western fare and is not bad, but neither are anywhere near world class. However, it met our expectations. In Toronto you could spend a full day’s room and board here on a great meal out. In the end, other than for a change of pace, we prefer the buffet. There is tons of variety and a huge amount of fruit and salad, which we favour in our diets, so it keeps us coming back here.

My  main complaint is the telecommunications. So-called high speed Wifi is available for the outrageous price of US $15/hr, $25/day, or $100 for 5 days. It is much slower than my not-very-good high speed service at home. Telecommunications on the whole southern half of the island was down for most of yesterday. On a positive not, Hotspot International, the provider, was quite helpful in providing me with an extension to my 24 hour access to compensate for the downtime,

The telephone service from the hotel (which was also down yesterday) is USD $1.50 for each 3 minutes for a local call. There is no direct dial so all calls go through the hotel operator. I don’t like this nickel and diming from a so-called 4.5 star resort, which is, after all, advertised as all-inclusive.

Having said that, I wouldn’t hesitate to come back here, but we’ve decided that we’re going to try somewhere else next year. Maybe Los Cabos.

Cozumel Day 1: Palancar Horseshoe & Dalila Reef February 10, 2009

Posted by Chris Sullivan in Dive Log.
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My first day of diving for this trip was February 8th, 2009. Our boat Shamu was waiting at the dock when I got down to the beach at 8:35, with Capitan Mago and our Divemaster Arturo from Blue XT Sea Diving. They had told me 8:30 to 8:45 and were right on time. We headed directly south to the first dive site called Palancar Horseshoe. It was a fine day, cloudless at 8:30 but with a few clouds over the shoreline by the time we started the dive.

We were in the water at 9:04, initially heading west, before turning north which is the general direction you go when Diving in Cozumel. I presume this change in direction is where the name Horseshoe comes from. The dive was fairly dull, although we saw a sea turtle, the sting ray depicted below, and some large grouper. Toward the end of the dive we passed the memorial to Martin, who owned a local dive shop but passed away a few years ago. It’s in a sandy stretch where I also saw the stingray.


I got as deep as 92 feet but spend most of the first part of the dive at 80 feet, then went progressively shallower to take advantage of the multilevel profile. I was under for 50 minutes in all, and the water was 78F (25C) which is several degrees cooler than my November trips. I was also a bit underweighted with 10 lbs. This was the weight I’d used in 2007 but it didn’t work as well this time, and I found myself having to swim downward on to maintain my safety stop depth. On Dive #2 I went to 12 lbs which was much better.

Between dives we stopped on the beach for about and hour and a half, during which I walked back into the scrub for a bit and came across a bird they told me was called a Pavo Real (in English, literally a Royal Turkey, but usually called a Peacock). I heard him before I saw him, as he slowly walked away from me. About 20 minutes later I went back with my camera and he was sitting down again, and before he got up and walked away I took a couple of pictures. There were also a pair of iguanas sunning themselves nearby but I didn’t try to get a picture of them, as there are plenty running around the grounds of the hotel.

cozumel-20080208-pavo-realEventually the others returned from the nearby hotel where they were having a Coke, and we headed off to our next dive at Dalila Reef. This dive, a full hour, also looked a little desolate, although I put my camera on its macro setting and looked for some of the small stuff.

From watching the TV series, The Last Frontier, with John Stonemen, I’d learned about one of his favourite reef inhabitants, the Damsel Fish. This little fellow, perhaps 2-3″ long, defends his section of the reef against all other fish, although for the most part he shied away from me. He was swimming in and out so much it was hard to get a good picture, especially with the delay between pressing the shutter release and the taking of the picture, but after a few tries I got something I thought was reasonable.

cozumel-damsel-fishHe didn’t want to have too much to do with me, but whenever another fish came by, he’d chase it away.

Soon enough, fighting the current in order to take photos became tiresome, and I continued on to other things.

The next little thing I saw was an Arrow Crab Spider Shrimp. The most remarkable thing about the photograph is the splashes of red and orange that were revealed by the flash. The background looked dull in natural light.

One unusual thing about the dive was that the current wasn’t strong, and was mostly flowing from North to South, which is opposite the usual direction.  The dive lasted just over an hour, and right at the end we saw some large fish, including a huge midnight blue parrot fish gnawing away at the reef.


Cozumel Day 2: Palancar Bricks & Paso del Cedral February 9, 2009

Posted by Chris Sullivan in Dive Log.
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There’s no day 1 post yet, but today is day 2 so it’s fresher in my mind. The boat arrived almost exactly at 8:30. They’re getting prompter, probably because the four people I’m diving with, Tom, Debbie, Stephanie and the other guy whose name I forgot don’t make them wait at the harbour. We’ve been diving the reefs near the Occidental Grand as usual, including Palancar Bricks and Paseo del Cedral. The latter ends just a little north of the hotel, and the boat ride back is about 5 minutes.

I have no pictures for today as the camera wouldn’t turn on. I don’t know if that’s because it was damp (which it didn’t seem to be), or if I installed the battery incorrectly, or if the battery discharged in the charger because it wasn’t plugged in. A disappointment nonetheless. The camera is working fine now and I’ll investigate the battery position a little later. I didn’t think that it could be installed improperly.

The second problem I had was that I forgot my computer. That was a little more serious. I had my watched so the procedure is to revert to tables in this case. So, dear readers, that’s what I advise you to do. Naturally I didn’t do this myself, I was simply a little more conservative on depth than the others, trying not to be deeper at any time,  and did a longer safety stop. On the first dive, I did 2 minutes at 20 and 3 at 15, and on the second 3 minutes at 20 and 5 at 10. That should have taken care of things, but it’s rough guesswork all the same.

On the first dive to Palancar Bricks I had a lot of trouble with my right ear. The usual procedure of ascending a few feet didn’t work, so I just maintained a depth a few feet above the pain threshold for a couple of minutes and let it work itself out. Finally it opened up and I was able to clear and descend with the others, eventually to 96 feet.

The current was moving northward swiftly today, although not faster than I could swim. Both dives, but especially the second one on Paso del Cedral were on a sandy bottom with coral outcroppings. There were numerous swim throughs on both dives and the team seemed pretty good at getting through them without hitting the coral more than once or twice. It can be pretty hard to do in the current and takes some concentration and contortionism to avoid striking the coral with a fin.

One of the benefits of my ear problem was that I saw a nice big sea turtle at a depth of only 20 or 30 feet nestled in the corals. The others were too deep to see him at that point. He even turned obligingly toward me as I whipped out my camera. That’s when I discovered it didn’t work, which was a shame as it would have been a good shot in the brighter shallow water. There were a few other interesting sea creatures to be seen, including a large lobster resident in one of the swim-throughs which was surprisingly out in the open and not looking happy to see us.

The second dive on Paso del Cedral was in a swifter current than the first. There are less swim throughs and more individual stands of coral. It’s fun to observe the way the current flows around the coral, and if you are getting ahead of the group you can just exhale as you pass one and drop down behind it sheltered from the current. There were many Barracuda on this dive, just hanging about lazily as they usually do. We also saw some of the giant Blue Parrot fish that frequent the area, some massive Grouper, and I caught a glimpse of a huge green moray, that swam out of sight as we passed overhead.

Our surface interval was at the same place a yesterday, a beach with some ruined construction of I don’t know what, right next to a resort hotel. The other divers elected to go over to the resort and have a (non-alcoholic) drink, while I stayed by myself among the ruins, filled out my log book, and communed with nature. Nature today was offering an Iguana sunning itself nearby.

All in all a very efficient diving day, and I was back on the hotel beach by 12:15.