Archive for the ‘equipment’ Category.

Dive #126-130: PADI Divemaster Course Part 3

Dive # Date Time Down Dive Time (mins) Max Depth (m) Average Depth (m) Water Temp. (C) Objective -- Notes
126 2013-12-04 12:35:00 23 7.3 4.5 22.0 Test HOG Backplate and Wing --
127 2013-12-04 14:00:00 22 6.4 5.2 20.0 Test Halcyon Infinity Backplate and Wing -- Did dive solo, just around Plunge wharf.
128 2013-12-07 12:32:00 41 11.3 7.3 19.0 Work on dive site map of Clifton Gardens -- Deployed a DSMB to mark the position of the big anchor (the one nearest the Clifton Gardens Wharf), so that bearings could be taken from various positions on the land. Measured the distance, using a long tape, from the anchor to the big buoy in Chowder bay.
129 2013-12-07 15:17:00 18 10.0 5.1 20.0 Work on dive site map of Clifton Gardens -- Took (surface) bearing from big buoy to SMB deployed on previous dive (290 degrees). Measured distance from the same SMB to the 4th pole of the Clifton Gardens wharf (26 m). Retrieved SMB.
130 2013-12-15 12:48:00 32 12.5 8.2 20.0 Try out new Halcyon Infinity BCs --

Accumulated time underwater: 3 days, 13 hours and 26 minutes

Buying a strobe

I’ve taken all my underwater photos with a Sony DSC-W15 and a Sony MPK-WA housing. I was thinking of upgrading this pretty old camera, but after some research I’ve realised that a strobe is a really important part of an underwater camera set up, and that a new strobe would make a much bigger difference than a new camera (for a similar amount of money).

A strobe is a slave flash, needed because the camera’s built-in flash is usually not powerful enough to light up a dull submarine scene once you go deeper than about 10 metres. More light allows the camera to be used at faster shutter speeds and lower ISO settings, resulting in sharper images with less noise. A basic underwater strobe set up involves a frame on which both the camera housing and the strobe are mounted, with the strobe on an arm which sticks out to the side so that it lights the scene from an angle. This helps reduce back-scatter, which refers to the reflection of light from the strobe back into the lens from suspended particles in the lens’ field of view, causing specks of brightness to appear in the photo. In determining the compatibility of a strobe with a particular camera and housing combination, there seem to be two key issues: the way the strobe is triggered to flash at the correct time (i.e. when the shutter is open) and the way the brightness of the strobe is controlled.

There are a number of different flash synchronization mechanisms. Some strobes use an electrical connection and are triggered by the camera much like a normal flash. Another method is to attach a fibre-optic cable to the underwater housing in such a position that the light from the in-built flash of the camera goes into the cable and triggers the slave strobe optically, via a light sensor on the strobe. The speed of the electronics is sufficiently great that the slave strobe will flash before the shutter of the camera has closed. Lastly, some strobes use the red-eye reduction feature of the in-built flash of the camera to time the flash of the strobe. You tell the strobe that your camera does, say, two or three pre-flashes, and it counts them and uses the timing to trigger its light. It seems for the last two of these three methods, some strobes do not need a fibre-optic cable, but use sudden changes in the ambient light of the scene (caused by the in-built flash of the camera) as a trigger.

The brightness of the strobe can also be controlled in a number of ways. The simplest is to manually adjust a knob on the strobe itself. This seems to be quite satisfactory much of the time. Some strobes have a light meter which allows them to automatically adjust the brightness to the lighting conditions. The most advanced brightness-control method is using TTL (through the lens) technology. This allows the camera to send information to the strobe about the amount of light coming in the lens. The brightness of the strobe can then be automatically adjusted for the aperture and shutter speed settings of the camera, as well as for the lighting conditions.

Having a strobe that works with your particular camera and housing is the first requirement. Then there are a number of variables to consider. The maximum power (indicated by the Guide number), the temperature of the light (how white or yellow it is), the firing angle (how wide the light spreads out from the strobe) and the recharge time (which determines how quickly a second photo can be taken after the first) seem to be the more important ones. Other variables include the weight (under and over water) and the dimensions.

There’s also the price, of course, which limits me to a choice of only a few strobes from the manufacturers that come more highly recommended on various internet forums. These include the Ikelite AF-35 and the Inon S-2000, for instance, as well as the Sea&sea YS27. Much pushing toward the Inon and Sea&sea products here, here and here. To see the differences between these, there’s a strobe comparison tool at digitaldiver.net.

Where to buy strobes? Well in Australia, there’s Sea Optics and Digital Diver. One of the diveoz links above also contains a reference Under Water Digital Camera, a Japanese store that ships internationally.

Authorized Mares Service Centres in Sydney

Here’s one in Paramatta: http://www.scubawarehouse.com.au/ . Mares has a list of all of the NSW Mares dealers, but I think only the ones with the ‘Mares Lab’ icon are proper service places. There’s another place in Beverley Hills. Haven’t found any in or around the city though.

New dive gear bag

I just got a Cressi Gorilla from Adreno Scuba centre. I chose it because it’s waterproof, so I can put wet dive gear in a car without worrying about the car, and because it has a stopper to let water out. This means you can wash your gear in the bag and drain before proper drying. Should be handy for maintaining dive gear in a small unit. Looking forward to trying it out next weekend.

Calculating how much air a tank will hold

Pressure units: 1 MPa (mega Pascal = 10^6 Pa) = 10 bar = 145.03 psi

Multiply tank volume by working pressure divided by atmospheric pressure to get the amount of air a tank will hold when full. This is simplest in bar because atmospheric pressure is approximately 1 bar, so no division is necessary.

The tank volume may be given in litres, kilograms of water or cubic feet. Since the density of water is 1 g/cm^3, the number of kg is equal to the number of litres. When the volume is given in cubic feet, it indicates the amount of air in the tank at working pressure (rather than atmospheric pressure), so to convert cubic feet to litres, multiply by 28.3 l/cu (1 cubic foot = 28.3 litres) and divide by the working pressure in bar. For example, a 65 cu tank with a working pressure of 230 bar, holds 8 litres, since 65 x 28.3 / 230 = 8. A 12.2 litre tank with a working pressure of 230 bar, holds 12.2 x 230 / 28.3 = 99.1 cubic feet.

Very interesting and informative discussion on scuba tank markings on the diveoz forums, here.