Scuba diving courses – Six things you were told that ‘just ain’t necessarily so’


Sometimes bad habits and half-truths find their way into all elements of our lives. Those small snippets of information that sound like they should work or you just assume that that’s the way things are done (have you ever put a spoon in an open bottle of champagne to stop it going flat in the fridge?)

Well scuba diving is just the same. It’s full a little quirks that have become either part of courses’ ‘folklore’ or have become a standard, de facto or otherwise. Well I have a list of a few that I know of – perhaps you have some of your own! I would love to hear them.

Drink water, not caffeinated drinks before and after a dive as dehydration is a leading cause of DCS

I can’t count the times that a well-meaning DM or instructor has offered me a drink of water right before or after a dive to ‘reduce my chances of DCS’. Whilst a cup of water is always welcome on a hot day, a single cup of anything is unlikely to affect my body chemistry enough to be the deciding factor in a bend. Maintaining a normal level of body fluid is something that needs to be an ongoing process that is difficult enough even when not diving. The best evidence is that severe dehydration is only one of a number of possible contributory factors to DCS and it may just cause symptoms to manifest themselves more acutely than they would otherwise. Monitoring the colour of your urine output (pale) is still the best guide to your pre-dive hydration and being mindful of all the factors you can influence to reduce your chance of a bend.

Don’t wear your mask on your forehead, it’s a sign of a panicking diver

James Sanderson Mask on head

No, a diver calling for help or splashing for attention or trying to stand on you is the sign of a panicking diver. A mask on the forehead is the sign of a diver making themselves comfortable at the surface if they so choose. I would be careful in a breaking swell as I don’t want to lose my prescription mask, but in still water after a cave dive for instance it can be a comfortable place to put my mask.


James Sanderson KnifeWear a knife on your calf

It’s essentially the worst place to hold a cutting device and can even become entangled in fishing line, causing the problem it’s there to solve. Somewhere around the middle of the body that can be accessed by either hand is best place to keep a small knife, line cutter or trauma sheers. It’s makes them easy to get at pretty much regardless of body position, be located by a buddy and cost a fraction of the cost of some ‘dive knives’ I see on peoples legs.

Always wear a snorkel

James Sanderson No snorkels

They have their place I admit it. That is in my kit bag.  OK so some agency instructors have to have one as part of their training standards, and you were probably sold one on your first course (can you see a connection there?) I see divers on boats where the snorkel is actually getting in the way and may cause a problem. If you need to surface swim, do it on your back. If you are at the surface at the end of a dive in a breaking swell keep your regulator in. Trust me, it’s OK to take it off now (see, no dive police).

Always surface with 50 bar in your tank

Why? What am I supposed to do with it? Does that tell me at what gas pressure I start back to the surface with? No of course not and that really  is the important pressure to know, when to START the ascent and head home (cave divers call it ‘turn pressure’). You need to know how much gas you AND a stressed buddy are going to need to make nice slow, safe ascent if you are sharing gas and it’s way more than you think (I will show you how to work that amount out in another blog).

Don’t go into deco

James Sanderson Decompression diving

Everybody who is diver and is reading this has made a deco dive. Think about it – by definition, if you are ascending and reducing the ambient pressure around you then you are starting to move the Nitrogen out of your body tissues; this is decompression. Ascending, off gassing, supersaturation and decompressing are all the same thing, it’s just that when diving within recreational ‘No decompression limits’ the amount of deco you have to do is less or the same as your ascent time without stopping. So the problem isn’t deco as such but there are three issues that a recreational diver does not have the proper training to manage (and are what technical training courses will start to cover). They are not knowing how long their ascent time needs to be based on their bottom time, at what depth they must make their stops, how long they have to be and most crucially if they have enough gas to perform that ascent. So unplanned deco is really our culprit… so get some training!

So these are my diving myths busted; there are dozens more. It comes down to being a thinking diver, constantly evaluating and questioning what is presented to you (as a ‘fact’ in diving is rarely every so).


Blog courtesy of

James Sanderson

James Sanderson

James has spent nearly 10 years teaching and diving in some of the world’s most enviable and challenging dive locations. He is an active trimix and cave diver and now has nearly 2000 dives in such diverse locations such as Caribbean reefs, Fjords in the Arctic Circle, submerged volcanoes in Coral Sea and the caves of Florida, Mexico and France and wrecks of the UK's south coast. He now teaches exclusively as a GUE (Global Underwater Explorers) instructor for TecLife (

32 Replies to “Scuba diving courses – Six things you were told that ‘just ain’t necessarily so’”

  1. Jeroen van Meenen says:

    Nice article! Can you also say something around ‘reverse profile’ dives? Is that something that has been proven (not to do) really?

  2. Kristie Morgan ッ says:

    This is the biggest load of bullshit iv come across in a long time, by the sound of it you have been trained by instructors from the 50’s. I myself have been in the diving industry for 10years working as a instructor and I completely disagree with you.

    1. The main Reason for drinking water before a dive Is because the air in the tank is very dry, being hydrated helps reduce you getting a dry mouth after a dive. Not to reduce your risk of DCS

    2. “Wearing your mask on your forehead is fine” – disagree! It’s not a sign of a panicked diver, it’s a sign of a distressed diver – their completely different. A distressed diver will put their mask on their forehead therefore showing signs of being uncomfortable with their equipment
    For me personally, the most comfortable place for my mask is to stay on my face, because of my years of diving, wearing a mask has become second nature and it is the safest place for it to be. So I guess you get a bit stressed out after your cave dives?

    3. Diving with a knife…. Here’s a little tip for you, if you place your dive knife on the INSIDE of your calf then it won’t get entangled and it is also away from all your other equipment, I’m surprised you don’t know this if you do cave dives?

    4 and 5. So you don’t dive with a snorkel and you don’t feel you should leave your reserve of 50bar in your tank.. Well your fucked then if you come across and unexpected situation on the surface. Returning with 50bar in your tank is an industry standard, everybody practises this so just do it and stop being difficult.

    1. 1) Good that is perfectly fine and correct reason, I agree. Just tell that to the next person who offers you a cup of water to prevent DCS on a boat.
      2) No I like the feeling of fresh air on my face
      3) I stopped mix and cave diving with people who anything except their feet on bottom of their legs a while back. Why would I not want to wear a cutting device where I can reach it with both hands and my buddy can as well if I am face to face with him (you know, like in a cave)
      5) On a recreational dive I usually surface with much more than 50bar provided there has not been an OOG emergency. This is because I plan a gas reserve to take me and a stressed buddy sharing my gas from our max depth to the surface with a nice controlled ascent. I even build in time to sort a small problem out or shoot a bag before we start the ascent. I keep this amount in reserve and plan the dive around the rest. So most times I never touch it. The key is to know what gas pressure to START your ascent.

      1. Greg Hunter says:

        The mask one has always been kind of comical. I don’t know who started saying it’s a sign of panic but it’s certainly spread. Odd thing is most of us old timers remember every episode of Sea Hunt showing Lloyd Bridges with the mask on his forehead. He never looked panicked to me. I do tell my students not to put it on their foreheads though but for different reasons. First they might be using one of my masks and I don’t it getting knocked off and them losing it. Second if they’re going to go underwater after having it there oil or their hair or suntan lotion from their forehead can effectively remove the spit or defog or worse if the mask leaks the lotion can splash into their eyes while clearing it and really burn.

    2. Rich Walker says:

      “Just do it and stop being difficult”. Do you say that to your students when they ask difficult questions? Is 50bar the right amount in a 12L cylinder? Is it the right amount in a 7L cylinder? How about a 15?

    3. Nick says:

      You are an embarrassment to Kirra Dive.

    4. Gareth Burrows says:


      I wear my mask on my forehead after every dive when debfriefing the divers I coach and teach, yet rarely feel distressed about it. Wearing your mask on your forehead CAN be a sign of a distressed diver. The nonsense we teach people is that it IS a sign of a distressed diver. I promise I am very chilled out when diving. It’s usually the morons shouting at me not to put my mask on my forehead that appear to be distressed. Hence James’s title “aint necessarily so…”

      With regards to knives, allow me to give you a tip in the same vein as you gave James one. You use your knives with your hands, so if you can think of anywhere to store them that’s further away from the limb they need to be used by than on the calf I’d love to hear it. You can tell a diver thats been entangled. They have multiple cutting devices such as line cutters, and keep them within reach of both hands. There’s a reason why military divers carry their knives on their chest. they might need them.

      As for returning with 50 bar being an industry standard, wlel maybe the industry needs to wake up. Teach people to reserve the gas they need, not the “industry standard. reserving the same gas for a 5 metre bimble as a 30 metre dive is stupidity. We need to give people more credit and teach them like they are dumb as horse shit.

      Oh and if James has a problem at the surface I suppose he could always take his reg out and breathe in. Unless he’s overweighted he won’t be descending with empty cylinders.

  3. Alessandro Di Mase says:

    Dear James, I have a few comments:

    1) I disagree: Dehydration is surely not THE leading cause of DCS (the leading cause being the lack of respect of rules/procedures by the diver) but is one of the main contributing factors, and while one single cup of water won’t make the difference, why do you thing educating divers to keep drinking before and after dives is not a good policy? most of the times we dive in hot climates and diving leads to dehydration anyway.

    2) 50% agree: While it is not appropriate to teach divers to keep the mask on their forehead BECAUSE it is a sign of panic, it makes sense to teach them to hang them to their neck, instead, because you don’t want them (or your shop) to lose them, which is what will happen often when they are not expert and they keep it on their forehead after the dive. It is a good piece of advice worth 50 USD.

    3) 100% agree on knives

    4) Snorkel: 80% disagree: While it is better for a ted diver not to wear it and it is ok for an experienced diver do breath from the regulator, it is a matter of safety to teach inexperienced divers to use a snorkel at the surface: there is something called laryngospasm, that is the involuntary closure of the larynx due to the ingestion of a small quantity of water. Laryngospasm can lead to loss of consciousness and drowning. Most of the deadly accidents happen on the surface, inexperienced divers tend to remove their regulator and find it easier to breathe with a snorkel. Once again, what might be true for a very experienced diver is certainly not true for a beginner.

    5) 50 bars in the tank: 100% disagree. What to do with those 50 bars? first of all gauges are very imprecise, +/- 10 bars is a good estimate, then you need to take into account the minimum pressure needed for a regulator to operate properly, take 10 bars as a reasonable assumption and you are left with 30. Why do you need 30? suppose your reg free flows, that is more or less what you will consume during a normal ascend fro 20 meters with a free flowing regulator. It adds up to 50 doesn’t it?

    6) Deco 90% disagreement: we might agree about the fact that every duly planned dive has a deco profile built in because the ascent rate considered by the tables or by the algorithm of the computer is designed to ‘decompress’ the diver allowing a slow release of nitrogen. The fact is that we are not instructing the divers not to do Deco stops, we are instructing divers NOT TO PASS NDL LIMITS, allowing them to go directly to the surface from wherever they are. You say go and get training, but why should we push everyone to do dives where they have no direct access to the surface? While this is a good choice for some divers, given the physical, psychological preparation of most of the divers and their training level, I don’t think it would be a good idea!

    1. I think that some of my points are lost in translation.

      1) The complete pathology of DCS is unknown, we don’t know the exact role hydration plays but can assume (as do the USN) that normal hydration is good. I agree. But don’t tell people it is a ‘leading’ contributor like it’s a fact.

      2)Wear a mask where you want, but don’t tell people it is a sign of panic like its a fact.

      3) “Most of the deadly accidents happen on the surface,” please support this statement! How was a snorkel cited as a factor that would have changed the outcome?

      5) What pressure do you START the ascent with to get you and stressed buddy to the surface with 50Bar remaining?

      6) No I said don’t get into unplanned deco. If you want to exceed recreational NDL’s get training

      1. Alessandro Di Mase says:

        1) James, the etiology of DCS is far from being unknown, what is not certain is the individual’s response to each of the contributing factors, I agree that no factor should be indicated as THE leading factor, but you should be very careful when you say that you are debunking a myth, because divers should be educated to maintain a good hydration at all times. Besides the fact that maintaining your body hydrated is good per se, divers are subject to more dehydration for at least three reasons: first, as Kristie Morgan says, the air in the tank is dry and each time you breathe you subtract some water to your body, second, the increased pressure in the blood vessels stimulates diuresis as well as the loss of heath, third, in salt water you lose some water from your epithelium due to osmosis. The loss of water leads to blood thickening and blood thickening is for sure increasing your chances to get DCS. So it is good to educate drivers to drink liberally water and fruit juices before and after a dive, and to avoid caffeine contained in coffee or coke.

        2) we said we agreed on that didn’t we?

        3) Every year we are given by our agency statistics about fatal accidents and training standards change in relationship with these statistics. Drowning at the surface is the #1 killer, the analysis of the accidents suggests that the failure to reach proper buoyancy leads to panic and panic leads to drowning. It is easier to solve the problem if you do not need to keep your head completely above water and it is easier for an entry level diver to breath from the snorkel. When you teach your students to keep their mask on, their regulator or snorkel on until they are safely back at the boat, you are removing some of the risk factors and improving their safety at sea. If you suggest that we teach to use the snorkel to sell more snorkels you are spreading the wrong message and you should be aware that you increase the risks for those who take you seriously

        5) One thing is the turning pressure and the other is the reserve pressure. Don’t you use the rule of the thirds in your cave diving? or a similar rule to determine the amount of air that you will not use during the dive and keep as a reserve for the unforeseen (you have already planned for foreseeable issues, haven’t you?). If not, please retrain, for your safety. I don’t think that any agency said that you should always reach the surface with 50bars, what we teach is that you should never plan and execute a dive that uses those 50 bars, and if you realize that you are approaching that limit, you should immediately agree with your buddy to end your dive making a normal ascent + safety stop. a recreational diver can always access directly the surface, and, in the worst case scenario is 4 minutes away from the surface, that is, if you count a SAC of 25 l/1′ (distressed diver) and a 12 lt tank, 25 bars, with no safety stop, that you can omit in case of an emergency,

        6) We agree on don’t get into untrained and unplanned deco, the disagreement is on the idea that everyone should get that training: it is not for everyone! I am not taking students into ted training if I believe that they lack either the psychological the physical or the diving capabilities needed. The right message is DO NOT PASS NDL if you are not trained (and equipped) to do so.

        Overall, I understand that you are frustrated by the superficiality of some professionals (when I train instructors and dive master I make sure that they know why they are teaching what they are teaching), but you should pay attention to the messages you are sending out when you qualify yourself as an experienced diver, because what most people will take away from your post is that they don’t need to drink water when diving, can throw away their snorkel, surface with as much air as they want and, why not, do some deco time if they are pleased with that. Communication is serious business and newspaper headlines are what people tend to remember, not the content of the articles.

        1. 1) “Although dehydration is commonly proposed as a risk factor for DCS in divers, there are no data to support this assertion in man. Animal studies are few and give contradictory results (Broome et al., 1995; Leni et al., 2001; Fahlman & Dromski, 2006; Skogland et al., 2008). We have recently shown that moderate dehydration and hypovolemia induced by a pre dive exercise might be involved in reducing post dive circulating bubbles (Blatteau et al., 2007) From the above finding we we hypothesize that moderate dehydration could be beneficial on bubble formation, while severe dehydration appears to increase the risk of DCS”

          So stay properly hydrated by monitoring your urine colour (straw by USN recommendations) And don’t believe that a single cup of water, coffee, soda or before or after a dive will impact your chance of DCS.

          3) I look forward to a link to a report or quote citing lack of snorkels or drowning at the surface as the #1 killer of divers.

          4) I use the principle of minimum gas. So in an ocean dive I always have enough gas for me and a buddy with a stressed SAC to perform a safe ascent. I deduct this from my total gas pressure and plan the dive the rest. I will put some detail on it via blog, or just google ‘GUE Minimum gas’

          1. Nick says:

            All divers in those studies were allowed to “drink water liberally”. There was no control group that did not hydrate. Guess why???

          2. Alessandro Di Mase says:


            1) I googled the text of your post to find the study, and I was not able to find it. What I found is a study that contradicts the one you posted:


            Objective:To investigate whether prehydration 90 min prior to a dive could decrease bubble formation, and to evaluate the consequent adjustments in plasma volume (PV), water balance and plasma surface tension (ST). Methods:Eight military divers participated in a crossover trial of pre-dive hydration using saline-glucose beverage (protocol 1) and a control dive with no prehydration (protocol 2). Drink volume was 1300 ml (Osmolality = 324 mOsml-1) and drinking time was 50-60 min. The diving protocol consisted of an open-sea field air dive at 30 msw depth for 30 min followed by a 9 min stop at 3 msw. Haemodynamic parameters, body weight measurements, urine volume and blood samples were taken before/after fluid intake and after the dive. Decompression bubbles were examined by a precordial pulsed Doppler.Results:Bubble activity was significantly lower for protocol 1 than for protocol 2. PV increased after fluid ingestion by 3,5 % and returned toward baseline after diving for protocol 1, whereas it decreased by 2,2 % after diving for protocol 2. Differences in post-dive PV between the 2 conditions were highly significant. Body weight loss before/after diving and post-dive urine volume after diving were significant in both protocols but the relative decline in weight remained lower for protocol 1 than for protocol 2, with reduction of negative water balance due to higher fluid retention. There were no differences in ST after fluid intake and after diving for the 2 protocols. Conclusion: Pre-dive oral hydration decreases circulatory bubbles, thus offering a relatively easy means of reducing DCS risk. The prehydration condition allowed to attenuate dehydration and prevent hypovolemia induced by the diving session. Hydration and diving did not change plasma surface tension in this study.


            This study has a control group,and although it is based on a small sample, and would require more data sets, it is based on humans and actual dives and not animals and deco chambers. What is important here is that it confirms what knowledge of physiology and logic should suggest: diving leads to loss of water and body weight, loss of water leads to hypovelemia, and if you have even a basic knowledge of fluid dynamics you will know that gas exchange in a fluid is affected negatively by increase in the fluid density and by the surface tension. Whilst the latter is not affected by pre-dive hydration, the first is (look at the chart in the study if you don’t believe it).

            Another interesting fact found in the study is that the cardiac load is decreased both by pre and post dive hydration, this is another finding that corroborates what logic and fluid dynamics knowledge are suggesting: it is harder to pump a denser fluid in a pipe, there is an increase of pressure in the system and you will need more energy for the pump (heart).

            Yes, you can check your hydration with the urine color method, but you must be aware that some drugs, supplements or foods can change the color. But why should you wait to see signs of de-hydration to hydrate? this is illogical and dangerous. The USN/Army method that you are using also suggest to keep a constant hydration to avoid dehydration (find the instructions here

            Conclusion: we all agree that hydration is not the leading cause of DCS, the leading cause being probably the diver’s behavior, but you need to keep hydrated and hydration helps to prevent bubbles, so be careful in giving out messages that contradict a healthy behavior!

            3) This is not what I said: I said that #1 Killer is drowning on the surface, and that protecting airways is one of the good surface habits that can avoid drowning on the surface, good surface habits include not removing the mask and breathing either from a snorkel or a regulator until you are safely floating. I don’t care what is the technique used, as long as airways are protected. If you have a better technique, fine, but encourage beginner divers to protect their airways, always!

            4) I know the GUE principle of minimum gas, now try to apply it to a recreational dive from 40m on a 12l or 15l, what do you find? roughly 50-60 bars depending on your considerations on the ascent profile. By using the 50 bar rule you give a recreational diver a simple tool to cover the worst case scenario in their dives. Asking recreational divers to calculate their reserve gas introduces potential errors that can be avoided by the simple 50 bar rule. Lack of air is #2 Killer, so why giving out a message that could lead to dangerous practices?

          3. data says:

            About the snorkel, I have two comments:
            First off: In my (very limited) experience, it is much more likely that someone inexperienced will breath in water when using a snorkel than when being at the surface in normal “swimming” mode.

            Secondly, if I am following your argument correctly, no matter if one is true or not: how likely is it for anyone to experience a laryngospasm when not actively drowning? I think a healthy person is unlikely to drown because of a laryngospasm, and the causal relationship is rather the inverse, but I am happy to be convinced of the opposite.

            While at the topic of drowning, I have to add one point to the discussion about the panicked diver. “No, a diver calling for help or splashing for attention […] is the sign of a panicking diver.” – If a diver is drowning, he will do none of those two, because he has better things to do with his air and hands. I know that James most likely did not mean it like this, but I encourage everyone to read up on the behavior of drowning victims, and the problems of detecting it.

          4. Alessandro Di Mase says:


            We obviously train people how to breathe from a snorkel and to control their airways in presence of water in the snorkel. What you want to avoid is an unexpected ingestion of water in the airways. In my experience most of the students are unable to use correctly a snorkel before the training.

            When water enters the airways of a conscious person, the victim will try to cough up the water or swallow it, involuntarily inhaling more water. Upon water entering the airways, both conscious and unconscious persons experience laryngospasm, that is the larynx or the vocal cords in the throat constrict and seal the air tube. This prevents water from entering the lungs. Because of this laryngospasm, water enters the stomach in the initial phase of drowning and very little water enters the lungs. Unfortunately, this can interfere with air entering the lungs, too, and may lead to hypoxia and unconsciousness. This is a process that can begin with a surprisingly low quantity of water, such as a spoonful. This is why we want to keep the airways protected, at all times. Being on the surface with unprotected airways does not necessarily mean that you will drown, the chances are on your side that you will not in 1.000+ dives, but this is just a matter of risk management, we are professionals and we want to remove the inherent risks from recreational diving. It is required by our ethics and we are responsible for the well being of our clients.

            Your comment about drowning persons behavior is very good: a person who is drowning cannot call for help (because of the above) and generally goes unnoticed, even in a crowded pool. In general a panicking diver is not yet a drowning diver, drowning can occur in a later phase when the panicked diver turns into an exhausted diver incapable of establishing a positive buoyancy and protecting airways.

          5. Phew, OK here we go again. I will simplify my article for you.
            1) Maintain a normal fluid balance, it’s part of a holistic risk management of DCS. If you hear someone offering you a single cup of water to mitigate the risk of DCS they are poorly informed.

            You think I am saying that proper pre dive hydration is not important, when it clearly is. I am saying that a single cup of water makes no difference because it’s not enough.

            2) I look forward to the link for this accident analysis. Even as a PADI instructor I would not wear one and did not encourage their use after training.

            4) You clearly do not understand minimum gas or GUE protocols. I ask you again to arrive at surface with 50 bar when do I start an ascent assuming both divers have 12’s. By the way for a 40m dive I would be diving twins with 21/35 and a deco gas. Gas planning is not hazardous, encouraging 40m air dives on a single tank is.

          6. Nick says:

            But it is not one glass of water. You say yourself one glass before or after a dive. So DM’s offer you a single glass of water before or after a dive to reduce your risk. On a standard double dive day then that would be 4 glasses of water they offer you over 3 or so hours, on a liveaboard it could be double figures. Someone participating in a diving day should consume around 15 glasses of water in the day which works out at say 1 per hour while they are awake. So in fact one single glass of water before and after the dive is important to maintaining hydration and thereby reducing your risk of DCS.
            One question also… You come back on the boat and start to develop possible DCS symptoms. The O2 provider tells you obviously to lie down and puts you on oxygen. Protocol then says to give you fluids (usually anyone half smart will have you on fluids while the O2 is getting sorted) but he says to you there is only one glass of water left on board. Do you drink it? No? You’re a fool. Yes? You’re a hypocrite.

          7. Of course I maintain a healthy level of hydration before and after diving, that’s what I teach. I don’t assume that being dehydrated and drinking a single cup of water will really help. That was the point.

          8. Alessandro Di Mase says:

            1) Now your message is almost acceptable.
            2) It is proprietary information. You can access it on the PADI Pros site, if you have paid your membership.
            4) thank you for incensing me, be reassured, GUE minimum gas requirement is fairly easier than the thermo-dynamics courses I took for getting my engineering degree at university, even if I am a poor PADI Course Director I can get the meaning of it, the point is that what you are saying about arriving at surface with 50 bars is not a requirement. The requirement is not to plan the use of those 50 bars. Can you see the difference?

          9. The plan is not to have an out of gas emergency. I repeat my point that I don’t think you quite understand. We will serve divers better by teaching them what pressure they need to start an ascent not what pressure to end the ascent at. A normal ascent would use about a third of minimum gas.

          10. Alessandro Di Mase says:

            James, where on hearth did you find out a recommendation that you should always surface with 50 bars? the recommendation is to start your ascent BEFORE you reach 50 bars. And this is just for recreational diving. If someone teaches 50 bars on surface, it certainly does not make any sense.

  4. Nick says:

    To all divers reading this article…Nowhere in James’ list of qualifications is there a mention of a medical degree, familiarity with fluid dynamics, experience as a physicist or mention of expertise in the pathophysiology of inert gas bubbles at extrapulmonic sites and as such I would advise seeking out the medical literature or expert opinions regarding dehydration. Fahlman et al (of the Navy Medical Research Centre) 2006, concluded that “Hydration status at the time of decompression significantly influences the incidence and time to onset of DCS” which has been confirmed by others since in testing on a variety of animals. It is also known that hydrated blood has a greater surface tension reducing potential bubble expansion and enhances inert gas removal. I would also be careful taking physiological advice from some one who groups decompression, ascending, off gassing and supersaturation as “all the same thing”.

    1. GLOC says:

      Nick, the point James was making was that “a single cup of water just before you get in the water” is unlikely to prevent DCS. Whole body dehydration will not be resolved by a single cup. Therefore you need to be properly hydrated which needs to be undertaken over a period of time BEFORE the dive.

      1. Nick says:

        Who said prevent? James wrote “reduce my chances…” A single cup of water won’t prevent DCS, no, but it will reduce your chances. By how much is a case by case basis. Wearing a seatbelt does not prevent you having a car accident but it will reduce the chances of you being killed in it.

        1. data says:

          Nick: He is somewhat agreeing with you anyway. He was just trying to get people to drink more water, and not to assume that a few sips are enough of hydration to really decrease your risk of DCI.

    2. Rich Walker says:


      I have a PhD in fluid dynamics – cardiovascular fluid dynamics to be precise. It is extremely unlikely that one cup of water immediately before a dive will reduce your chances of DCI. It certainly won’t significantly change your hydration status.

      1. Nick says:

        Thanks Rich, whilst the risks concerning dehydration are poorly defined many studies have concluded the same thing as Gempp et al 2008, which was ” Predive oral hydration decreases circulatory bubbles, thus offering a relatively easy means of reducing decompression sickness risk”. As you are an expert in this field I would be interested in any studies you have done that refute this, but it seems this is the general consensus of experts and the reason why H20 is the equal first part of field treatment (and a big part of emergency treatment) as it improves circulation and perfusion. Any extra fluids will help this.

        1. Rich Walker says:

          I can’t find the reference to the Gempp study, so would appreciate a link, Nick.

          You do need to remember that bubbles do not equal DCI. Bubbling is a normal part of any ascent. Low bubbles do not mean a lower risk, in the same way that high bubble scores do not imply DCI is imminent.

          1. Nick says:


            Sorry it is only an abstract, I no longer have access to the journal. Yes I am aware re bubbles. I participated in studies in the US and Reunion 13 conferences and was most surprised by the results.

        2. The objective was to encourage the average diver not think there is some magic bullet or rabbits foot in a single glass of water. My assertion was “So stay properly hydrated by monitoring your urine colour (straw by USN recommendations) And don’t believe that a single cup of water, coffee, soda or before or after a dive will impact your chance of DCS” It’s about managing the variables that we can and mitigating those we can’t. I don’t need a medical degree to understand and communicate that.

          1. Nick says:


            Subject: Re: New comment posted on Scuba diving courses – Six things you were told that ‘just ain’t necessarily so’

  5. Paul Mason says:

    Kirstie, do you know the reasoning behind the 50 bar advice? If I have my knife on the inside of my left leg and my right arm is entangled how will I retrieve it easily? I find having it on my waistband makes it accessible at all times with either hand.

    I am impressed however that your name is similar to Kirby Morgan, was this a deciding factor in your career choice?

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