How's the Viz?
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!
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!
August 4, 2013 @ 4:03 pm
Yep, you and Don both were. Yeah, we are lucky enuf to be sissies about conditions here. ;-)
August 4, 2013 @ 3:35 pm
Hey Mike! I am pretty sure I was in on that conversation 10 years ago! I usually decide on whether the dive will be worth it as I assess the conditions at the shoreline before I assemble my gear. If there are 4'-5' waves, or higher, as a Maui diver, I say Forget It! Fortunately that doesn't happen very often:) Certainly diving history and experience will always come into play when determining whether to go or not to go!
August 4, 2013 @ 3:04 pm
I came into your shop about 10 years ago and asked a similar question. Only my question wasn't about the method for figuring the distance, but rather how do you judge the end of your vision? Is it how far before you are unable to see your buddy at all? or just 'til you can't recognize them? Is it where you can't even make out the reef, or you just can tell if what kind of coral it is or see the small fish swimming it? There were 5 experienced divers in the shop at the time and they all gave me different answers. So, between not knowing how bad the vis is supposed to be at the end of your measurement, and how you actually measure it, it's no wonder every diver coming out of the water gives a different number. So at that point, I decided to only answer that question with a subjective term like, great, ok, or crappy. But that doesn't really work either because someone that's used to diving in California will say the water is calm and the vis is excellent on a day most Maui divers would rather skip. Typically, I really want to know how the vis is so I can decide which lens I want to take but since there seems to be no consistent answer to the question, I just jump in and hope for the best. 99% of the time, it's worth diving right?
August 4, 2013 @ 2:59 pm
Aloha Rick! With Flossie coming through we ended up with worse viz than your Missouri diving! But it is clearing up now:) Say Aloha to Shelly!
August 4, 2013 @ 2:47 pm
I love this blog because most of the time we are not out there diving with lines to help us measure vis. I never had any idea about distances underwater until I did my Divemaster map at Ulua beach with my buddy and a 100ft line marked off in increments of 25 feet. That's when I learned that I'd been calling 50 feet 25!
August 4, 2013 @ 1:37 pm
Reef Check California measures visibility by having one diver hold the end of a 100'/30m transect measuring tape while the buddy swims away. The first diver holds up a few fingers. When the second dive can no longer tell how many fingers there are, the tape tells the vis.
August 4, 2013 @ 1:02 pm
We just assume Maui viz is better than it is at the Lake of the Ozarks in Missouri, which means more than two arms length (anybody's arm, if you want to get technical...!).