Perdix AI compass

Please register or login

Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

Benefits of registering include

  • Ability to post and comment on topics and discussions.
  • A Free photo gallery to share your dive photos with the world.
  • You can make this box go away

Joining is quick and easy. Log in or Register now!

I would suggest that for most diving needs, declination is not really required as sighting the compass will generally be enough (given the relatively short distances involved).

This may of course change as distances increase (such as using a breather or scooter diving).

Have to say I love the Perdix compass - the fact that it shows the delta to the bearing/reciprocal, is a boon for navigation as, so long as you can work out the division, you can easily work the course out for any shape (90deg for square, 120 deg for triangle etc).
 
I'm quite proficient with compass, on land or in the air, and used classic compass underwater, but digital compass on Shearwater is awesome. I have digital, tilt compensated compass on my ,now secondary DC, Matrix, but it's class below of SW's one.
 
I know what you are saying and don't completely disagree, but consider this true scenario. An instructor leading a class with me tagging behind surface swam out to a buoy below which was a platform. The class descended, did some skills and then swam around. The instructor then wanted to head back to shore, which was in a westerly direction. I'm following and wondering why the instructor is going south. At some point he sees that something is messed up and looks at me. I point to the west, but now we really need to go northwest to reach shore. Problem: the instructor has a Perdix and had changed the battery without re-calibrating the compass. Moral of the story: that compass can be off quite a bit if not calibrated, and sometimes it is important to have accuracy.
That instructor had two problems. He didn't calibrate his compass to the new battery and he didn't calculate himself to the shore before he entered the water. Either should have gotten him back in the right direction.
 
Declination is different from calibration.
You calibrate the compass to account for the magnetic interference the battery causes to the magnetic field. Dry batteries have an iron core. Compasses don't like iron. I have to calibrate my Perdix every time I change the battery. You declinate the compass to compensate for the difference in true north VS magnetic north. Here in North Carolina where I dive the declination is -10 degrees. I never have to change that unless I go somewhere else to dive (or have to reset the computer to defaults).

It really doesn't matter if you are just shooting an azimuth. If the boat is anchored at 240 degrees true and your compass is reading incorrectly 226 degrees magnetic because you haven't set declination, it isn't going to matter so long as you swim 226 degrees following your compass to get back to the boat. You'll find the boat because your azimuth was incorrect the same amount as your lubber line is. Both point to magnetic north, not true north. Where you have a "problem" is if one of your coords is magnetic and the other is true. So if I shoot an azimuth to the dock 300 yards away with my declinated compass and tell my buddy my azimuth, and his compass isn't declination, he's gonna be 10 degrees off if he's swimming by his compass trying to find the dock with my coords I gave him.

If I'm looking for a BUOY on a nav chart and the chart says that buy is 240 degrees from my location (nav charts use true north), then I'm gonna miss it by 10 degrees if my compass isn't declinatied. Most "real" nav charts contain the declination for the area on the map in the compass rose printed on the chart.

For what is worth, declination (Minute of Angle) is an angular measurement. If you're swimming 10 miles, 10 degrees might matter. If you're swimming 100 yards on a dive, that 10 degrees isn't enough to make much of a difference. 10 degrees at 100 yards is about 8 feet.
 
Last edited:
You can use this cool site to find your declination. It's either a "plus" or "minus" number. For digital compasses just plug that number into the declination setting.

Then when you're compass tells you you're heading North, you'll actually be heading North, not North-by-East or North-by-West.

Magnetic Declination on Google Map
 
Last edited:
+1 on re-checking the calibration after changing the battery on a Perdix. Also +1 on recognizing most of the time I'm sighting on an object and NOT swimming to an ABSOLUTE direction relative to anything (mag north, true north, Polaris, etc.) Orienteering on land with a *real* compass is a whole 'nother kettle of fish.

BTW, batteries do not have an iron core. The central electrode is usually graphite (carbon). In fact, I can't think of any component of a dry cell battery that would affect a magnet (the outer shell is typically zinc.) I'm not sure exactly why recalibration is helpful after battery changes, but I'll pass it off as something in the electronics (I'm a chemist, not an E.E.) But last time my Perdix required an adjustment of 20 deg.

[edit: I realized after posting that directions to some dive sites from shore do involve absolute compass readings. I suppose declination would make a difference then.]
 
Last edited:
The positive pole of a DC battery is made of steel (iron). Electrons have to move for an electromagnetic field to be present. You can swipe a screwdriver tip across a magnet just twice and and it will pick up screws. A digital compass only needs a very tiny magnetic field, so literally any variance in the field will affect it's ability to determine magnetic north.

One of our "projects" in college (University of Houston physics dept.) was to observe and explain how current flowing thru a circuit affected a compass needle. Even when the circuit was open, the battery still affected the compass needle. No current was flowing, so no electrons were moving. There shouldn't have been any affect if there was no iron in the battery and the battery was not connected in a circuit. The reason was the current flowing thru the circuit when the battery was previously connected magnetized the positive anode just a tiny bit. The electrons in the anode steel of the battery all magnetically aligned when the battery was included in closed circuit. But then the question was, why does a brand-new battery just taken out of the package still affect the compass needle? According to laws of physics, if the battery was never connected in a circuit, and thus no electrons ever moved to align the electrons in the ferrocarbon material, the battery shouldn't affect the compass needle. The answer was the automated machinery the battery was manufactured on has electric motors and electric circuits, and the positive anode of the battery was magnetized during the manufacturing process.

If you put your magnetic compass beside the copper or aluminum wires in the walls in your house, and those wires are in a closed circuit with electrons flowing thru it, it will affect your compass even though copper and aluminum aren't' ferrous metals.

At the atomic level, it isn't the IRON itself that creates magnetism. It's the electrons flowing thru the iron that make the iron magnetic. The earth itself is a huge circuit, so any iron from earth is already magnetized, even in it's natural ore form. If you can get access to CHAMP images (the German space agencies satellite that constantly measures the earths magnetic field), you can actually see detailed evidence of the electron flow around the earth and how the North pole moves. I forget now, it's been 20 years, but I think it was like 1/4 inch per year or something like that.

Back in 1990 when I was in the military, we had satellites so sensitive to earths magnetic field that they could find Russian subs under the ocean by measuring the earths magnetic field in a given area and comparing it to what that magnetic field was in that same area the week (or day) before. A variance in the magnetic field meant something huge and iron was there. If it wasn't there yesterday, it shouldn't be there today. :) The Russians got smart and started building their subs with titanium in the late 70's to circumvent our magnetic satellites but couldn't get the alloy right and lost a lot of subs when cracks formed in the hulls because the titanium was too brittle. Due to the shortage of titanium in the 70's they also only used titanium for the outer pressure hull. The inner hull was still iron. So their sub was still "findable" by satellites. It got more difficult in the '90's when they perfected the alloy and began using titanium for their Alfa-class fast attack sub hulls. This time they had an abundance of titanium so they could make both the light hull and the pressure hull from titanium. The electromagnetic field variance was much less and you got a lot of false positives. Once they went to titanium hulls, the only EMF interference you detected was from the electronics (moving electrons in a closed circuit) and the small bit of collateral iron used in the sub.

EM field anomalies are one of the ways our boomers stay under water for months at a time. GPS doesn't work underwater. The radio signal only penetrates about 4 inches deep. A sub would have to surface or raise an antenna to pick up GPS radio signals. Both of those actions compromise it's location. But by mapping all the magnetic field anomalies created by shipwrecks and known iron deposits on the ocean floor, the subs computer can triangulate it's position and know pretty close where it is on the globe. Then the computer pulls up a ocean floor sonar map of that area and compares that to what it sees on it's active sonar and it knows it's exact position.
 
Last edited:
But last time my Perdix required an adjustment of 20 deg.

Wow. That sounds like a lot. Was the replacement the same brand/type? I'm interested in seeing how much MF variance I get from replacing an alkaline battery with a lithium battery.
 
I recently changed my alkaline batteries to lithium but I didn't try it before recalibrating so I can't tell you how much different it made. I do know I prefer the lithium batteries because of how much more predictable the depletion curve is.
 
https://www.shearwater.com/products/peregrine/

Back
Top Bottom