Porth Yr Ogof Blog Post 25th March 2013

Fortunately, the weather was pretty much perfect for our Project Baseline dives today.

Well....when I say perfect, the underwater conditions were average, little bit of current, OK visibility and 4 degrees centigrade. Topside however, was a biting wind and had most of us moaning about it even before we started to get changed - despite bing wrapped up in cosy down jackets.

We figured it would be warmer in the water....

The objectives of todays PB dives were:

• Locate a good 'stake in the sand' photo point, which would be easily recognisable and easy to access, to make a continuing record of the cave. We thought that the top of the 'Pot' about 70m underwater from the Tradesmans Entrance.  
• Run visibility trials using a secchi disc and a variety of light sources, one by one and record the visibility readings. A tape measure would be used for accuracy.
• Patch up the line which had begun to wear through at a rub point at the top of the 'pot'.
• Enter through Top entrance in the afternoon and re-line Cobbets Loop, which leads to Upper Cave Water Chamber.
• Survey from 'Fourways' at a major toe off (iron railway shoe) to UCWC.
• Film/photograph any underwater fauna, specifically trout.

We managed to achieve all of the above despite blood curdling cold. I had expected the water temperature to be a chilly 6 degrees so we all wore drysuits. To our astonishment, it was actually 4 degrees centigrade!!

I was quite proud of myself in 3mm gloves!Dive 1 was to introduce Jarvist Frost to the cave and we dived upstream from Tradesmans, noting some litter along the way. Also noted were about 6 trout, most of them blind and white in colour.

I filmed these and took some screen shots. We arrived at the pot to realise that my failing memory had forgotten that I had already fixed the rub point just over a year ago, so it saved Jarvist a job at least.

He then had the unenviable task of holding up a large, floppy laminated secchi disc and the end of a tape measure, whilst jamming himself across the passage to stop the up-current from taking him away, while Rich and I measured the visibility using various torches, ranging from our helmet mounted 'Rude Nora' to a 10W HID.

Jarvist holds the secchi disc and tape measure

Not surprisingly, the helmet mounted lights, although powerful, reduced visibility significantly, whilst the HID seemed to cope best. Still, there was a metre variation between Rich and I using the same Rude Nora on the same setting. More work needed we think. Overall, there was a 2m variation in visibility between all of the lights. This is too wide a range for UK sumps where visibility typically ranges from an anecdotal zero to 5m at best. The exception being Wookey Hole and Peak Cavern, where visibility can exceed the range of your light (but only just!!)


Can underwater in Porth yr Ogof

We then swam a little upstream and found a nice looking bit of passage to do some 'arty farty' video to turn into screen grabs. My photography underwater is just not up to scratch, so we figured video grabs were the best thing to try for at this stage.

Once done, we rode the flow out of the cave in pretty poor vis and crawled unceremoniously out of Tradesmans, trying desperately not to add to the pile of cave diving kit which has been dropped and swept away by the current, never to be seen again!