What is the first question everyone asks when you finish a scuba dive? “What is the visibility!?!” Recently, one of our regular Scuba Instructor-type customers, Dave, came into the dive shop and, of course, I asked him, “What was the viz?” He said it was about 25 feet at the end of the first reef at Ulua Beach.
Typically, I ask this question of different divers throughout the day and I usually don’t think about how they’ve determined their response. However, on this particular morning I was inspired to ask Dave how he measured visibility.
Dave said he imagines the body length of a diver and mentally figures how many divers between him and the farthest that he can see. Now I wonder what particular diver Dave imagines: is it a tall person, or short? Does he include the fins (and what size)? Is the diver kicking with their legs stretched out horizontally, or is the diver bicycling vertically?
I started thinking of the different ways we can measure viz, and this is what I came up with:
1. As I mentioned above, Dave likes to use body lengths.
2. Our repair technician, Kelly, suggested using the length of a known object such as the wreck of the 70 foot long St. Anthony. That’s helpful when you’re at the St. Anthony, know how long it is and can see one end from the other.
3. Arm lengths: Start by touching the bottom, then fully extending your other arm and touching with that hand, then extending the first arm and touching the bottom, etc. The idea is that you are using the distance between your outstretched arms as a measurement over and over until you reach your subject. As current and surge do not factor into this type of measurement, this method can be quite accurate.
4. Kick cycles: In the Advanced Open Water navigation dive, we place 100 feet of line on the bottom and then kick at a normal pace along the length. We count every other kick, which equals a full kick cycle by each foot. At the end of the line, we know how many kick cycles it takes us to travel the distance of 100 feet. Mine is approximately 33 kick cycles, depending on current and surge.
5. Timed swim: In the same exercise as above, you or your buddy times your swim along the 100 foot line which gives you an estimate of how long it takes you to swim 100 feet. My time is a little over a minute, again depending on conditions. For both of these measurements we go both directions and take the average; this compensates for any current, or other water movement.
6. Use a measuring device such as a marked line: You can have your buddy remain stationary while holding one end of a line marked off at every ten feet. You can then swim with the line until your buddy is just at the edge of your vision, then read where the measurement is on the line.
7. I once read that you can use a 12 inch square of black rubber and measure the distance from where you can see it to where you can’t. You could use a measured and marked line between you and your buddy as a signaling device for a nearly exact measurement.
8. Finally, you can use the method we usually use, “WAG” (wild ass guess)!
While doing a little research on this topic, I also came across an excellent article in Dive Training Magazine discussing measuring vis and outlining even more methods – you should check it out!
Let me know if you have any other viz-measuring techniques. Like any good diver, I am always learning…
If you would like to learn more about some these methods of measuring distance underwater, ask us at Maui Dreams about the PADI Underwater Navigation Specialty or the PADI Search & Recovery Specialty.
Here’s to great dives with great viz!
Aloha, Teri
p.s. For the latest viz reports, you can always check our home page; we update the weather section daily…often based on YOUR reports!
