Follow the maker’s instructions when maintaining O-rings, but bear in mind that applying grease is by no means always necessary
Why do some manufacturers recommend greasing O-rings while others recommend that you do not

Some O-rings (and flat gaskets) are simply compressed into place when you seal up whatever it is you need to make watertight. Others are piston O-rings that need to move as the enclosure is tightened.
A good example of this can be found in most torches or lamps that must be broken open to get at the batteries. You screw down the sealing part onto the O-ring, which must be able to move if it is not to distort, with disastrous consequences.
Lightly grease these O-rings to provide lubrication.
It doesnt matter if you grease (with the appropriate silicone grease) other O-rings, but manufacturers often suggest that you do not, because the grease can allow foreign bodies such as stray hairs or grit to stick, which affects the integrity of the seal.
The silicone grease does nothing towards making this seal - although its worth pointing out that sometimes the act of greasing an O-ring with a fingertip can in itself reveal defects or grit. But if you are simply closing something like a camera-housing lid, it may be that you dont need grease.
The best advice is to follow the manufacturers instructions. If they do recommend greasing, be sure to use the grease they suggest, because O-rings, such as the blue ones in Sea & Sea cameras, are made of various materials, and some greases are not appropriate.

Point-and-shoot, or point-and-wait
I dive with a Nikonos V but want to upgrade to a digital camera. The choice, I thought, would be easy - but I was wrong! Can you explain the practical differences between a digital compact (eg Sea & Sea DX8000) and a digital SLR like the Nikon D70s Ive read the specs, but what does it all mean in the water
Ariel Northway

Digital compacts are a good way to start in underwater photography because they are inexpensive compared to a full DSLR with housing.
However, with a compact there will be a significant time delay between pressing the release and grabbing the image, as it switches from writing the image to the LCD (used as
a viewfinder) to writing it to the memory. It will be even slower when macro focusing as well.
There will be a limited choice of file type. Some expensive compacts can record RAW files but use a lot of file-writing time, during which the camera will not be usable for another shot.
The jpeg files achieved are of a quality good enough for personal pictures, websites and emailing to friends, and a compact offers the advantage of a range of lens angles (zoom to macro) continuously available during the dive.
A digital SLR is significantly more expensive by the time you have bought a lens and housing for it.
You will probably also want a flash, to get good colour at depth. Think of spending at least £3000 - most photographers spend a lot more than that.
There is virtually no time delay in getting the shot, but you do have to commit in advance to the lens you want to use, wide-angle or macro, before sealing it in its housing.
You can usually shoot RAW files. Kurt Amsler, the Swiss underwater photographer, once said, If you dont shoot RAW, youre missing the point of digital photography.
DSLRs have a big buffer for multiple shots, so file-writing time is less of a problem.
So-called bridge cameras are advanced compacts that bridge the gap between cheap compacts and the cheapest DSLR. If you have been getting good shots with the Nikonos,
I imagine that you would be happy only with the results from a DSLR.

Feel, not quantity
Having spent a day being confused by a local dive-shop owner, I would like your opinion. I have a Scubapro MK16 /R390 (diaphragm) regulator set-up but I was told that, being a chunky chap, I might get better air supply from a piston first stage. Is this just marketing talk, or would it actually give me any benefits
Stoney Steve

A lot of information still in circulation has become outdated. When we started testing regulators, we found that quite a few were inadequate performers past 18m. All that has now changed.
Even Scubapro, once the loyal proponent of the piston-type first stage, now offers a very high performing diaphragm-type regulator (the MK17). Take a look at our regulator comparisons archived on Piston or diaphragm, they are generally all good performers. Today, its more about the feel of the regulators delivery of air than the quantity.

Its not the cold, its the dirt
I am looking to buy my first set of regulators. I will be diving mainly in the UK, so they need to be suitable for coldwater use. I want a regulator that is fairly small. Do I need to get the optional coldwater kit to use one successfully in cold water If so why, for example, is the Mares Proton Ice so named if I have to add extras to use it in cold water

An environmental protection kit stops the action of a regulator being interfered with by detritus that may be in dirty or polluted water. Some claim that it stops ice crystals that may form from doing the same.
We believe that these kits are often sold as coldwater as a marketing strategy. I am sure you will find that the Proton Ice will perform perfectly well in the coldest clean water without it. However, do bear in mind that the coldest water encountered in Britain is usually at inland freshwater sites, and these might not be as free of sediment, mud and pollutants as you would like.

A side-exhaust regulator can be rigged to come over either shoulder, but the user has to take appropriate steps to avoid
Side-exhaust pros and cons
What are the advantages and disadvantages of regulators with side-exhausts over standard demand valves

A side-exhaust regulator can be put in the mouth either way up, so can be rigged to come over either shoulder. The exhaust bubbles pass up one side of the face - as is the case with any regulator unless the user is perfectly horizontal.
However, when the exhaust valve opens to let out the gas, some water will inevitably make its way back in.
If the exhaust valve is lower than the mouthpiece, the water will be expelled with the next breath, but if you turn your head so that the exhaust valve is higher, you will experience a wet inhalation.
Side-exhaust regulator users learn not to drop their head right-ear-down when the exhaust port is positioned on the left, just as conventional regulator users avoid inverting their second stage. It is something users get used to, but probably explains the popularity of the conventional layout.

Semi-drys in the UK
Ive bought a secondhand 7mm semi-dry suit and Id love to do some UK diving, but Im worried that Ill freeze. I have been told a good-fitting semi is as warm as a drysuit until youre out of the water but I dont have much natural insulation. Was my local dive shop just trying to sell me a surplus suit, or is there any truth to this claim

A well-fitting semi-dry or even a wetsuit can be as warm as, or even warmer than, a drysuit in the water. It is more about heat loss after a dive.
The cooling effect of evaporation in a biting wind can be extreme. Long RIB rides after diving can be awful. Towelling off in these conditions can be very uncomfortable too, and attempting to get back into a cold wet semi-dry suit for a second dive is only for those inured to suffering.
That said, I used to take two semi-dry suits with me on club hard boat dives, so that I always had a warm dry one to get into.