Monthly Archives: July 2015

Sporasub SP1 Freediving Computer / Watch Review

The latest addition to my growing list of freediving kit is my new Sporasub SP1 freedive computer. This helps me with one or two of my frustrations which I am sure that I share with other new freedivers. How deep am I going? How long am I down for? How many dives have I done? One of the reasons I prefer freediving over scuba is the simplicity, so I really don’t want to clutter my body or my thought processes with anything more than i have to.

This is the perfect tool for me; as well as satisfying my curiosity, I con now benchmark one day’s diving against the next to see how my performance is changing over time.

Why did I go for the Sporasub SP1? Well, I started at the bottom of the price range (this cost £145 or $225), and worked upwards, luckily my criteria were satisfied by this one, which happens to be the cheapest on the market. It was also dropping in prices as the new Sporasub SP2 is available with wireless connection and heart rate monitor, and I wanted to snap one of these up before they stop making it.

Sporasub SP1 Time display mode

Sporasub SP1 Time display mode

What I wanted was simple:-

  1. Something that automatically kicked in to log dives without me having to remember to press any buttons.
  2. Something that allowed me to look at logs afterwards to see what I had done
  3. Something that logged surface time as well as dive time

What I didn’t want:-

  1. A scuba watch that also had a freedive function (watches with more than one button are confusing enough for me without having loads of extra functions I will never use).
  2. Something that relied on me having to press buttons before every dive (something else confusing that I can’t be bothered with).
  3. To spend double just to get extra functions that I won’t ever use unless I start training and competing at deep diving.
Sporasub SP1 Dive log Mode

Sporasub SP1 Dive log Mode, showing depth, water temp, and dive duration

What you get with the Sporasub SP1 freedive computer:-

  1. Dive mode automatically kicks in when the watch is submerged by 1.5m, you don’t have to do anything, it just starts logging straight away, even if the last time you looked it was just showing the time.
  2. When you get back to the surface it automatically starts to count surface time (you should be breathing up on the surface for at least as long as your last dive before you go down again)
  3. It logs dive time, dive depth accurate to 10 cm, water temperature.
  4. It saves these records for you to look at later, up to 199 of them.
  5. Depth warning, it will beep for 30 seconds when you reach a preset limit.
  6. Switch between sea and fresh water
  7. Other stuff you would normally get on a watch, time, date, stopwatch etc.

Things you might get on a more expensive computer that the Sporasub SP1 doesn’t have:-

  1. Wireless link to a heart rate monitor (like the Sporasub SP2)
  2. USB connection to download and analyse your dive stats and save them for ever more. (Sporasub SP2, Mares Smart Apnea, Oceanic Aeris F10)
  3. Time warning, multiple depth warnings, (Oceanic Aeris F10)
  4. hydration reminders, descent and ascent speeds (Mares Smart Apnea)

Summary – It works first time and every time, logging all the dives I do accurately (when compared to another borrowed dive computer strapped to my leash in a recent test dive).

I found the controls a little infuriating to start with when I tried to look at the dive starts whilst the screen was still displaying surface time. But this is more about my not remembering the instructions than design fault.

Who would I recommend this for? Any recreational non competitive freediver or spearo who wants an easy way to log their dives.

Who would I not recommend this for? More serious competition divers who want to really monitor things like ascent speeds, who need 10m warnings and want to monitor heart rates while diving. Those guys probably want to go for something more swanky, but they are going to pay in the region of £100 to £200 more for these functions.

Am I happy with the SP1? Yes. It looks cool too, though my wrists are so skinny I won’t be wearing it as a fashion accessory.

Boyle’s Law for the Shallow Freediver

One of the basic things we learn at school, and probably don’t think about again until we start to dive is Boyle’s Law – Simply put – As pressure doubles, volume halves (given constant temperature and some other stuff).

This is also something that is taught as part of any freediving course (or scuba), as it helps us understand the physiology, and the dangers, of gas expansion or compression in the context of diving.

So any freediver will have seen the table that illustrates that at 10m down the pressure has increased from 1 atmosphere to two and the volume in our lungs has halved. And when we get to 20m despite this being double the depth the pressure is only 3 atmospheres so it isn’t until you get to 30m where pressure will be 4 atmospheres, (double what it was at 10m) that the volume will be halved again, 1/4 of what it was at the surface.

Whilst this is a great help in illustrating the principle, the reality is the when we start diving we aren’t thinking in terms of 20 or 30m, the difficulties we need to overcome are all in the first 10m, and srangely there isn’t really a table anywhere on the internet that illustrates what happens in this band, until now!

I used an online Boyle’s Law calculator to work out what happens to our lung volume in the first 15m, and illustrate it, with a comparison to the deeper changes, for me and all to see forever more. Let’s hope it is correct.

So…..

Boyles Law for Freedivers

Boyles Law for Freedivers

The reduction for each stage shown in the right hand column illustrate how the rate of change drops rapidly with depth.

From this table we can make a few observations….

  • If you duck dive and put your lungs where your fins were (say 2.5m), the air in your lungs has already reduced by 20% and you have experienced the quickest build up of pressure than at any other time  in your dive.
  • We all know that by diving to 10m our lung capacity is halved, but of that half, very little is lost between 7.5m and 10m, most of it is before that, (7% between 7.5 and 10m, compared with 20% between the surface and 2.5m.
  • You only have to get down to the 12.5 – 15m zone and the differences are relatively small.
  • The volume of air lost between the surface and 2.5m is roughly the same as that which is lost between 20m and 70m.

I have found that with the practice I have had, equalising comes much more easily over these first couple of metres than it used to. Without thinking I do a frenzel manouvre as I turn to dive which seems to free things up in the first metre, then grab my nose after I have started to straighten out and drop, this means after my fins have submerged and I have made the first kick, which puts me down at 3m, there isn’t a build up of pressure.

A boat dive, and an interesting refresher

So yesterday I joined a boat dive. FreediveUK were taking out a couple of AIDA 2* students and they sometimes take previous students out to get a bit more practice. There were two main reasons why I joined the boat trip; firstly to get some depth experience. It is difficult to find deeper water shore diving where I live so since my course none of the dives I have done have been more than about 10m. Secondly I was sold on the great visibility they are getting down in Cornwall at the moment. I haven’t experienced more than about 5m since I qualified and I was looking forward to being wowed out by the clarity.

Unfortunately only one of these was achieved, I got some extra depth but the weather was atrocious and the viz was some of the worst I have seen (do you see poor viz, or not see it?), in fact it couldn’t have been worse, at the surface you could barely see your hand at arm’s length, beyond about 12m was darkness!

This trip was also an opportunity to get used to my new freediving watch, the Sporasub SP1, but more of that later.

The second site that the knowledgeable dive boat skipper (Mark from Atlantic Scuba) found was just calm enough, and deep enough to drop the rope to 16m. As soon as we got into the water and I saw how poor the viz was I had quite a deja vu for my own 2* course, over a year ago in that cold dark quarry. The first few free immersions down to about 10m were fine, but when it came to clipping on the leash and diving down the rope I got all tangled again and faced the embarrassment of not being able to achieve what the other two guys were about to do.

It is funny how a few extra things to think about make it so much more difficult. Where previously I have been building confidence with every dive, suddenly I felt like I was back to square one, a few seconds aligning myself with an almost invisible rope, trying to pinch my nose with the hand to which the leash was attached, and suddenly I hadn’t equalised properly. It all started feeling a bit alien and before you know it I had turned at 8m, a depth that last week I was doing without even thinking about it.

Luckily while they were going through their drills with the instructor I had a practice doing some variable weight dives with the dive-master. This is simply holding on to a heavy weight whilst he controls the descent of rope over the first critical couple of meters, then he lets it drop. The prospect of descending at rate over which you have almost no control is slightly daunting the first time, but as soon as you feel any discomfort you can just let go, the weight speeds to the bottom and you pull yourself up on the rope as usual. The advantage of this practice is that it removes all factors from the equation apart from equalisation. You don’t have to move, fin or worry about anything else. It came surprisingly easy and was a real confidence booster, especially at depth, when it suddenly became so dark you couldn’t see any contrast in the light at all, and the only way of knowing where was up was the direction of the rope.

My watch told me that I was mostly dropping the weight around 12 or 13m, I think this was due to slightly painful sinuses, and proved I needed to maybe descend more slowly. I’m not sure if this kind of descent is suitable for very new beginners, if you don’t know how well you equalise, or aren’t aware of the warning signs then you should probably be careful.

As soon as I got back on the other rope I finned straight down to 16.4m (couldn’t see the plate at all!) and came slowly back up all nice and comfy, no problems, relaxed descent, and much more relaxed in the darkness.

The things I learn’t from this trip were:-

1/ I have to admit I’m really not comfortable yet at depth in very low viz. My lungs give me all the time in the world, and in clear water I am happy to stop and hang around for 10 or 15 seconds to equalise, get my bearings, align to a rope, (or just look at fishes), then carry on. But as it gets darker I will turn at the slightest hint of a problem, I’ll overcome that and yesterday was a confidence booster.

2/ The first dive or two of the day beyond about 8m result in an immediate urge to breathe, my body subconsciously thinking I need to breathe because my lung capacity has quickly halved (nearly) these contractions, (just like when you do breath hold practice) interfere with equalisation and cause confusion. That feeling goes away after the first couple of dives and this helps with keeping calm and no longer interferes with equalisation.