Dive Depth Calculation

[Fouremco]

How are the published depths for dives calculated? Having spent a number of years on and around the Bay of Fundy, with 40' tides quite average in some areas, I'm wondering if the dive depths are based on high or low tide, an average or some other calculation. In the Bay of Fundy, a 60' dive could become a 100' or 20' dive, depending on the approach taken.

While I realize that there are few places in the world with tides even approaching those of the Bay of Fundy, I'm curious as to the approach taken.

 

[String]

Chart datum is usually the lowest astronomical derived tide. Therefore you HAVE to add on the tide height printed on tide tables to find the correct depth.

Example if a charted wreck is in 24m of water but you dive it at high tide with a 6m height off tables the actual depth will be 30m

 

[moneysavr]

Hey most dive boats have a depth unit, make sure you dive to the safe side,Inc water temp and work / task load !!round up not down=Add Feet'dive safe dont die!
Brad

 

[Rafael]

A good place to get the tides info is the weather channel website in the boat and beach section.

 

[donacheson]

How are the published depths for dives calculated? Having spent a number of years on and around the Bay of Fundy, with 40' tides quite average in some areas, I'm wondering if the dive depths are based on high or low tide, an average or some other calculation. In the Bay of Fundy, a 60' dive could become a 100' or 20' dive, depending on the approach taken.

While I realize that there are few places in the world with tides even approaching those of the Bay of Fundy, I'm curious as to the approach taken.

Dive tables, although expressed in depth (feet or meters), are actually based upon pressure at depth. Depth gauges carried by divers are actually pressure sensors whose output is converted to depth - 1 bar equals 10 meters or 33 feet, 2 bars equals 20 meters or 66 feet, etc.

Hence, the tide isn't important. It's how far beneath the surface, not how far above the bottom, that's critical.

 

[captndale]

How are the published depths for dives calculated? Having spent a number of years on and around the Bay of Fundy, with 40' tides quite average in some areas, I'm wondering if the dive depths are based on high or low tide, an average or some other calculation. In the Bay of Fundy, a 60' dive could become a 100' or 20' dive, depending on the approach taken.

While I realize that there are few places in the world with tides even approaching those of the Bay of Fundy, I'm curious as to the approach taken.

There is no uniform standard for measuring depths that used by those who publish dive site and shipwreck lists and books. It would be nice if they were all referenced to chart data and in common or at least consistant units of measure. Unfortunately, they are not. Sometimes measurements are taken from the vessel's fathometer which may or may not be calibrated. The fathometer's transducer may be located close to the surface or it may be located as much as several feet below the waterline. If depths are taken by divers from their depth gauges, they may be calibrated in either fresh water or sea water units, resulting in an automatic difference of 3%. The diver may have taken the reading while holding his meter on the bottom, or while swimming 6 feet above it. The published depth may even have been the result of second or third hand accounts or "estimates" recounted to the reporter. You should always take published depths as "ballpark" numbers until you have personally verified them.

 

[BlueDevil]

Hence, the tide isn't important. It's how far beneath the surface, not how far above the bottom, that's critical.

I think I must be missing something here. I go to my favourite dive site at low tide and it is 30m deep. I go back to my favourite dive site at high tide and it is 33m deep. How can you therefore say "the tide isn't important".

 

[String]

I dont follow that either. Depending on spring/neaps here the same dive site can have a 25ft (8m) depth difference in as little as 6 hours.

Tide directly affects the water depth at a given location.

There was a good point someone mentioned about information sources - diving publications may just have something like the depth measured when someone dived the site (which obviously ignores changes for tide) so can be inaccurate. A depth echo sounder on a boat is the same, did they remember to work out the % of depth the tide has and subtract that to give the proper figure etc ?

Marine charts and the depth contours and spot markings on there are all (or mostly all) factored to lowest astronomical mean low water) so at least if you read the depth of those you KNOW how to work out how deep the water will be at a specific time/date if used in conjunction with tables.

EDIT:- He mentioned "Dive tables" which i assume he means decompression tables which have absolutely nothing to do with this discussion which is where the confusion comes in.