On the "added pressure" when swimming or moving your arms underwater (dynamic pressure)
TL,DR: Dynamic pressure is practically negligible. Use the normal stated WR as a benchmark and forget about adding extra because you move your arms veeery fast (unless you are Flash or Quicksilver).
(Warning, very long post, but also quite interesting) original source here:
http://forums.watchuseek.com/f17/water-resistance-myth-vs-reality-159142.html#/topics/159142?page=1
What About Dynamic Pressure?
This perhaps is the biggest myth and urban legend of all. It was mostly created by the watch industry and then spread trough watch retailers and watch brochures and manuals then also trough word of mouth of course. It was not so widespread in the 80's but by the late 90's and early 2000 it had spread alot.
Why they spread that myth to begin with probably have to do with weakening the warranty terms and the fact that they want to encourage people to be carefull with their watches.But also primarly due to marketing reasons that enables them to charge a extra premium for higher rated watches.
Anyway the myth is about movement in water. Apparently as you move around in the water especially your arms an extra pressure gets applied to the watch and the deeper in the water you go the higher this extra presssure will get due to the movement. I have read statements that these movements can add several Bars/ATM/PSI of pressure to a watch. Therefor it is not safe to take your watch anywhere near its stated deepth rating. I have read statements that you should not go any deeper then maximum 30 meter(100') with a 100 m rated watch or 60-70 meter (200-230') with a 200 m rated watch. 30-50 m rated watches should not be submersed at all.
Whats the major problem here?
First let me state that I was a firm believer of this "Dynamic Pressure" myth up untill just a few months ago. This whole journey into this topic for me was the presentation of the Suunto Core on Suuntos website prior to its actual release. I thought it looked fantastic and almost imediatly feel in love with it especially in combination with its seemingly superior feature set compared to the competion. Prior to the Core I pretty muched ruled out Suunto out of the realm of my interest frame when it came to ABC watches, not because of their active functions which in fact many times where even better then for instance the PathFinders/ProTrek of the Casio line up, but because I saw them as fragile crap due to their poor 30 M water resistance rating(the Observer with 100 m rating was an exception but did not appeal to me for other reasons). But when the Core was presented on Suuntos website it seem to have it all including a 100 meter rating.
But when it was closer to the Core's release I suddenly noticed that the specs had changed on Suuntos website now it was suddenly rated to 30 M just like its predeccesor the Vector so I was very dissapointed and descided to mail Suunto to get it clarified. Apparently they did a misstake before and that 30 m was the correct fact. But by that time I had already worked up enough interest for the model so that I contemplated buying it anyway, all that I demanded from it was that I could do some surface swimming with it then I would be happy. So I mailed Suunto and asked again if i could use it when swimming. And the answer was yes!
This confused me because what I knew prior to this told me that 30 m and 50 m watches should not be used for submersive water activitys such as swimming, what confused me even further was the fact that the Core came equipped with specially designed stinger buttons called UW(Under water buttons by Suunto) also the demo on the website showed it was also equipped with a cool depth gauge down to 10 meters which further suggested this was a watch that could be used under water.
At first this lead me to believe that Suunto since it is a special company that has a very scientific image that pride themself with preciscion instruments for professionals was more honest with their rating and therefor took Dynamic pressure into account for their rating and gave their watches a more honest rating then the rest for the watch industry so therefor a 30 M Suunto was the equvivalent to a 100 Meter watch from other manufacturers.
But this turned out to be a wrong assumption, I later found out that they just test their watches according to the standard ISO 2281 just like the rest of them.
So because I was a believer of the dynamic pressure theory all I was left with was a big mystery, how come a 30 m rated watch was seemingly adapted for underwater use?
This mystery lead me to seek out the answer and learn more about water resistance and the effects of dynamic pressure specifically. I did found some important pieces to the puzzle here in this forum from older forum posts but also from Wikipedia and also some Swedish scientists that I had enquired about this mather.
What I did found out shattered the dynamic pressure theory to pieces. It simply was nothing more then a lie turned into an urban legend and myth.
Apparently pressure can only be applied to an object as the result of added mass/weight that is applied to the object(in this case added depth with an increased weight of the water pillar above you) in question, or as a result of expansion or due to electro magnetism, another possible source of pressure is some external forcing preventing expansion or inversion. Another source is gravitational pull due to accceleration or decceleration but that reason is somewhat tied in to reason number one the one about added mass.
So what kind of forces can a swimmer/diver apply to his/hers watch? first we have the depth ofcourse. If we use a watch similar in size to a Raysman. Lets say we are at 100 meter depth. The size of the watch is about 5cmX5cm thats 25cm2 in surface area. 1m is 100 cm so 100m is 10.000cm 10.000X25= 250.000 Cm2 of water above the watch that is pressuring against it. The weight of that water is 1000.000/250.000= 250Kg(550 ibs) of pressure against the watch at that deptht. This is known as the hydrostatic pressure.
The diver(staying at the same depth) can only change that pressure against his watch in 2 ways either by moving his arms up or down but the maximum reach of ones arms is very limited usually not more then perhaps 120cm(4') or so.... that is only a change in pressure of 0.12 bars or 3Kg(6.7ibs) of pressure, very little difference not much more then 1% compared to the rest of the pressure at that given depth.
Second way to increase pressure at the watch is trough speed/acceleration. Either by swimming or by moving our arms up and down. The maximum speed we can move our arms in free air is often not more then 3-6 feet per second and it moves even slower under water. And when it comes to Swim speed even an Olympic swimmer usually cant swim any faster then 6-7 feet per second. If we add the maximum output of that we get up to 10 feet or 3 meters of acceleration per second which is the equvivalant of about 10Km/h or 6.25 mph. That aint very much force/pressure in water. Someone smart here at the forum(CycloneFever) calculated this and I quote:
"Without repeating all the calculations here (they involve denominators and the greek alphabet and are PITA to type out), at a depth of 330ft(100 m) and moving your arm at 3 ft/sec, the dynamic pressure is in the order of magnitude of 0.14 feet of head or 0.04% of the depth. Even assuming you could move your arm at 20 ft/sec (14 mph!) the dynamic pressure is only about 6.2 feet of additional depth
