Somewhere back in your very first diving course there will have been reference to air being compressible because it’s a gas, as opposed to water being non-compressible. When we start thinking about how molecules interact, it’s helpful to go back to school science. In solids the molecules are packed tightly together, they can vibrate but they aren’t free to move around much. In liquids the molecules can move but stay interacting with each other and in gases the molecules fly around freely bouncing off the walls of whatever contains them.
Thinking about gas molecules as random whizzing around bumping into other molecules and their container, it’s easy to understand why if we push more molecules into the container the number of collisions with the container wall will increase and the pressure goes up. This kinetic model of gases assumes that the gas particles themselves are very small (they are) and there’s a lot of space between these gas particles. That leaves lots of extra space for us to jam more molecules in there and compress the gas together. Hence gases are compressible. This nicely explains Boyle’s Law (remember that?) but makes a number of assumptions which create a concept of an ‘Ideal gas’. Sadly, Ideal gases don’t exist and we have to deal with real gases. But at low pressures Boyle’s Law is fairly useful.
Liquids don’t behave in the same way because the molecules are already close together and interacting to a limited extent. Each molecule forms temporary associations with the other liquid molecules. If you heat up a solid to melt it into a liquid, you can measure the temperature increasing. As you get to the point where the solid it melts, the temperature will stabilise (even though you are still heating it). This is the point where the molecules are absorbing heat energy to give them the kinetic (movement) energy to move around. Once all the solid molecules have absorbed enough energy to melt into liquid, then you can see the temperature start to rise again as you carry on heating it. During this phase the molecules will move faster, but they still interact with each other. Keep heating and give them enough energy and they will manage to escape the interactions and form a gas.
Water is a bit of a strange liquid, because it’s molecules interact more than other liquids, and this gives it some strange properties. Water molecules form hydrogen bonds to other water molecules, and then they break these bonds and reform them with other nearby water molecules. Although these bonds are quite weak, there are lots of them. If it weren’t for hydrogen bonds then water wouldn’t be a liquid at all. When the other elements in the same family of the periodic table, like Sulfur and Tellurium, bind to hydrogen they form gases not liquids.
Water is most dense (the molecules are packed tighter together) at 4 degrees. At this temperature the hydrogen bonds are quite structured and pull the molecules tighter. As the temperature rises the bonds start to make and break more frequently and allow the water molecules to move a little further away from each other, so water becomes less dense. At temperatures below 4 degrees, hydrogen bonds don’t form so well and so as water cools to become a solid, it also becomes less dense. This explains why ice floats on liquid water (good news if you’re a polar bear) and why cold water sinks into the ocean (that’s thermoclines).
So, these little fairly weak hydrogen bonds have a lot of influence and it’s them we are fighting against when we try to compress water. They have already done the job of pulling the oxygen dihydride molecules far closer together than we’d expect from the other elements in their group. It’s because of them that water isn’t a gas at room temperature. And once we’ve got as far as forming a liquid, there isn’t much compression left to achieve. At 4km down in the ocean, water has a measurable compressibility of just 1.8%. It’s not quite true to say water isn’t compressible, it’s just not very compressible and for the depths that we will visit we can probably ignore the marginally increased density.
Resistance to compression for any substance can be described by the bulk modulus value. This is a measure of how much pressure must be applied to reduce the volume by 1%. For solids, these values are predictably very high, eg diamond is 443,000,000,000 Pascals and steel is 160,000,000,000 Pa. For ice I would need 2,000,000,000 Pa to compress it by 1%. For liquids we would expect the values are lower and generally they are. Water bucks the trend though and needs 2,200,000,000 Pa. So, I actually need more pressure to compress water than I do to compress ice. Blooming pesky things those hydrogen bonds!
As divers we all know about the difference between diving in fresh and seawater in terms of needing an extra bit of lead because seawater is just a little bit denser than freshwater. Density is affected by 3 things, salinity, temperature and pressure. At extreme depths the density of seawater increases due to the high pressures. Given that we are recreational divers, the increase in density due to depth isn’t something we need to worry about. There are two common ways of dealing with measuring the density, we could use the density figures in grammes per cm3 or specific gravity. Specific gravity is a comparative scale, but it’s actually fairly easy to calculate actual density using the temperature and salinity for where we are diving (and there’s some great online software that allows you to plug in the relevant figures).
Salinity (how much salt in g is dissolved in 1kg of water) varies quite a bit around the world. To understand the factors affecting salinity you would need to look at how much freshwater run off enters the area. Freshwater is less salty and will have the effect of partially diluting the salt concentration. Local climate will influence how much water evaporates from the sea; in hotter conditions the concentration of salt rises. Lastly, we need to consider how much current circulates the water. One of the most saline seas is the Red Sea, with little freshwater, high temperature and confined circulation. Salinity levels at the northern end can be as high as 4.0% (much higher than the world average of 3.5%). By contrast, in the Baltic Sea, especially around the Gulf of Finland and the Gulf of Bothnia, salinity can be as low as 0.8%. There’s lots of freshwater running in, limited circulation of saltier water through the narrow channel to the North Sea and the lower temperatures reduce evaporation. This sea water is so not salty that it could be safely drunk in a survival situation. In the UK we actually see a little variation between the west coast (3.5%) and the east coast (3.4%). Don’t try drinking this stuff for survival!
So salt levels can vary, and as they do, so will the density of the water that you need to displace. How often have you heard someone in a dive resort tell you that you might need a bit more lead here because the seawater is a bit more salty? This got me thinking, how much more salty would it need to be for me to notice,or are there other explanations?
Let’s start with the UK. The density of seawater on the Atlantic side at average temperature (assume 12oC) is 1.0267 g cm-3 whereas on the North Sea side, the density will be 1.0258 g cm-3. I know from experience that I haven’t noticed the difference. I do know that wearing twin 7’s in fresh water (1.000 g cm-3) means I don’t need any additional lead, but exactly the same setup in Manx seawater takes 4kg of extra lead. So, for each extra 0.0067g cm-3 I need another 1kg of lead to get my head under water. Taking this logic forward means that in the salty Northern Red Sea at the coldest part of the year (24o), when the density is 1.0275 g cm-3 I’ll be needing another 1kg to sink, which seems reasonable. But, and it’s a really big but, I don’t wear the same neoprene drysuit, Fourth Element undersuit and base layers in the Red Sea as I do in the UK. If I did, I’d need the extra lead and treatment for heatstroke!
The Red Sea is a bit of a salinity extreme; for most of the world the average salinity applies, which makes it pretty much the same as the UK. I think it is much more likely that other factors come into play when we head off to other waters. Every time I drag my 5mm wetsuit out I struggle to remember just how much weight I had last time, and of course my suit will be compressing with use and losing buoyancy from new anyway. I err on the side of caution and take an extra lump of lead for the checkout dive. Let’s face it we would all rather be under the water having to control a little extra air in our BCDs than bobbing around like a cork on the surface while the dive master forces a smile and swims over to us with another block.
Most of us will use cylinders from the dive centre and there can be a huge variation in the weight of steel cylinders. A 12l cylinder can weight between 13 and 15 kg, and a 15l can vary from 16 to 19kg. Unless your dive centre has cylinders of different weights, they may not even appreciate that they have ‘lighter’ 12s. The situation is worse still if the cylinders are aluminium and are neutral in the water…then you’re really going to need some lead round your waist! Your confusion at needing more lead is easily explained by blaming it on the ‘salty water’ but perhaps there are more factors at play?
Once upon a time my Editor and I went diving together. It was a few weeks after he had penned an opinion that back entry dry suits were an integral part of the buddy relationship. It was, he opined, important to trust your dive buddy to close the ridiculously expensive brass zip without trapping your undersuit or that annoying flappy bit of neoprene stuck in the back of several suits. Relying on your buddy to ensure the zip was closed all the way, contributed to the mutual support aim of buddy diving. As we stood kitting up for our dive, I happily fastened my front-entry plastic zip with the minimum of fuss and decided to tackle Simon about his ill thought-out piece.
I have a front entry suit because I like being responsible for myself…or more precisely I don’t always trust my buddies, especially if my buddy is a trainee or new to dry suit diving. I lack the ability to rotate my neck like a barn owl to check that everything is OK behind me. It only takes one trainee, who earnestly assures you that the zip is closed when in fact it’s half an inch open, to make you reconsider. When that cold rush of sea water starts running down your shoulder, you know that this is one mistake you won’t be making again!
But how do you get the dive manager or boat crew to double check your zip without offending your buddy? Surreptitiously sidle over to the crew as you leave harbour, keep your voice low so it can barely be heard above the engines (and definitely not by your buddy) and assume some wistful position that doesn’t look like you’re hugging a large imaginary tree? And of course all the while you must try not to offend your buddy and generate “trust issues” because at the very first time you are supposed to rely on their assistance you bailed and found another source of help.
So for me a front entry suit solves all of these problems. If my zip isn’t closed properly, then that’s my fault and my soggy right leg. For anyone thinking of getting a suit with a plastic dry zip, they are fabulous but never ignore the need for silicon greasing the stop end, even between dives if you’ve peeled out of your suit. But it’s my responsibility and I’m good with that.
Front entry suits frequently have two zips, the dry one and a cover zip, and this can cause endless problems too. I took my eye off the ball one day whilst doing a dry suit introduction in the pool. I will accept some of the blame, but we had just done a session at the dive centre trying on suits, and the concept of a dry zip and a cover zip had been discussed as we established that this particular suit was a good fit. I am to blame for thinking that our discussion would be remembered barely an hour later when we kitted up on poolside. When I turned to look at my two eager divers, they had closed their zips and were ready for the stride entry. Yes, the cover zip was closed. No, the dry zip wasn’t. Yes, the suit filled with water (luckily the warm pool version). No, the diver couldn’t climb up the pool ladder unaided. The phrase “I seem to be getting a little wet” was a total understatement on her part. Once dekitted, we laid the unfortunate lady down and rolled her around on the pool surround to empty the water. To give her credit she laughed nearly as much as we did and gamely carried on the orientation session. Five years on she is still diving, in a front entry suit, which she knows has two zips and one of them is very important.
Sadly she’s not the only one who’s been caught out in this way. Even some quite experienced visiting divers have missed the ‘hard to do up’ brass zip and relied on the ‘easy to do up’ cover zip in one of our rental suits. A cold shot of Irish Sea water down the leg is a salutary lesson in the need to familiarise yourself with hired equipment. So for anyone who read, noticed and remembered Simon’s treatise on the importance of back zipped suits for buddy trust and diving, maybe I was wrong to criticise him and perhaps divers with front entry zips could do with their buddy’s assistance, just sometimes.
For a while just before Christmas a couple of years ago, the common description of the Isle of Man as ’80,000 people clinging to a rock in the Irish Sea’ couldn’t have seemed more real. Our only winter capable ferry managed to ingest some lobster pots cunningly set in the entrance to the harbour, destroying one of the bow thrusters essential for manoeuvring into the tight confines of her berths both here and in the UK. A period of windy and stormy conditions meant that several sailings were cancelled, the schedule went to the wall and running the Dive Centre during this time was a challenge. Customer orders couldn’t get through so there were a few ‘I ordered it in plenty of time, but it didn’t arrive yet’ apologies to some of the local divers on Christmas morning. But in general the poor weather meant there wasn’t much diving going on anyway however the security of supplies should actually be a worry for all divers. We see it as a challenge to arrange expeditions to remote locations, carrying with us tonnes of equipment, fuel and supplies to dive in some of the lesser known spots. The planning takes weeks if not months, and part of the satisfaction is diving knowing that you have overcome all the obstacles. But just think what your diving would be like if that was the level of organisation that you had to run to all the time.
It’s no secret that the current economic climate is hitting leisure industries hard, and that includes dive equipment manufacturers, retail operations, dive training schools and charter businesses. The whole sector has seen a contraction in spending, and those who depend on it for their living are working harder and longer than ever to keep going. I spent 2 hours last weekend advising a diver on kit, discussing the relative merits of different options, measuring him for a dry suit and painstakingly working out a competitive price for the choices. As I slaved over the pricelist and a calculator I saw the dreaded smart phone in his hand and watched in horror as he scanned the barcode and searched for an online price. Despite the time that I spent, the various configurations that I’d rigged for him to try and the detailed knowledge I provided, he ordered online whilst standing just outside the shop!
It’s a free market and of course he can choose where to buy from. Some of the big dealers can get much better trade rates than the small guys and rely on high volume of sales to make their money. However, a word of warning; if the kit was supplied by us, we would happily set it up, take him for an orientation to his new kit in the pool followed by a weight-check dive. Any warranty issues would rest squarely with us and if we couldn’t sort it out in our workshop, we would happily lend him replacement kit whilst we dealt with the returns process. These are all little things that we would do as we recognise that the commitment to buying dive kit is a big step, and a little guidance along the way can make a huge difference. How many new divers buy exactly what someone in their club, usually the loudest person in the bar, told them to get? And how many change their kit within two years?
With heavy hearts now we have had to introduce an admin charge to deal with warranty issues for which although we are a dealer but did not supply the item. If we didn’t make even a small amount of profit selling it, we can’t justify paying the postage to send it off to be repaired. And if the owner of new kit wants to try it out, then we will be charging him for the pool or open water sessions. So how much does the odd £20 that he saved really matter? Obviously to him that mattered a lot, but perhaps now he’s seen the value to the service we provide he’ll reconsider? The good news is that no-one has yet invented a way to download air via a USB cable, so we’ll be seeing him back in the store real soon. Have a think about the security of your supply chain for diving. Local Dive Stores have overheads and may cost a little more, but they will be around to supply your air, provide repairs at short notice, hire kit, give advice and support your purchases. Your diving activities would be infinitely more difficult if they disappeared.
Michelle has been scuba diving for over 20 years. Drawing on her science background she tackles some bits of marine science. and sometimes has a sideways glance at the people and events that she encounters in the diving world.