zondag 24 juni 2012

2012-06-24 10:04 hbos Shrimpexplosion!

Weather : Overcast / SW wind 4 bft / Rain  10.0 mm / 1007.0 hpa
Divers : Mark, Axel
Goal : Visibility check + Watersample.
Max depth : 13.0 mtrs 
Duration : 61 minutes
Air temp :  13.4 celsius
Water temp : 17.6 celsius minimum
Fish: 1 eal and 1 large perch. 1 pike, clouds of shrimp young and old
Plants : lucious growth of plant. Unfortunately by the reel a lot of sponges are dislodged and lying at bottom presumably done by divers holding the reel or swimming underneath it. 
Visibility  : 6.0 mtr at 3 mtr depth / 6.0 mtr at 6 mtr depth 
Yellow viz: 8.0 mtr at 3 mtr depth / 8.0 mtr at 6 mt depth (at Paviljoen)
Remarks : > 10 divers in water doing trainings
Watersample taken at 3 and 6 mtr depth for weekly analysis.

About visibility readings:
Measurepoint for Project Baseline is located at the right outer pile of the Paviljoen at 3 and 6 meter depth. We chose this location because regardless the amount of visiting divers the visibility is almost unaffected.
Light conditions are good with a dark backround.
This is why visibility readings seem to be optimistic as apposed to the general dive area.
When yellow visibility plate fades the contrast is completely gone, diver holding plate is not visible either.
Baseline visibility meter gives an output with more contrast so you can still see diver's black suit holding the meter.
The object is identifiable.
 
What's the visibility in a swimmingpool?
During a test in the swimmingpool  the baselinemeter gave max 12 mtr while we could see divers at the other end of the pool (25 meters). This was performed by two persons to get more input. The diver holding the meter was very clearly visible. The yellow plate disappeared after about 27 mtr (taken diagonally in the pool) At that point the diver wasn't also visible anymore. In this case the yellow plate gave a more realistic reading.

Today in outdoor conditions on a rainy day without sun in hbos.
-> Baseline visibility meter shone by measure diver him/herself gives 1,5 - 2 mtr less visibilty then yellow plate without lamp. 
-> Baseline visibility meter shone by buddy holding the meter gives the same reading as yellow plate without lamp.

So which reading is correct?
Internet says;
"Visibility is generally considered to be the distance at which an object underwater can be readily identified."

Basically when contrast has completely gone as done with yellow plate, an unknown object cannot be readily identified. Coming closer about 1,5 -2 mtrs to the object, it can be identified.
Dutch waters are usually green / yellow / orange coloured and it rarely has visibilities over 12 mtr.
For Dutch waters the Baseline meter meets the criteria better then the yellow plate given the fact you're vision is not impaired or if you gave corrected glasses in your divemask.
For clear waters (regardless the colour) with visibilities over 12 mtr the Baselinemeter is (to my opinion) not suitable.


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