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  1. #1

    Default Scooter and Compass

    This isn't really cave diving specific, but I've not seen it discussed elsewhere.

    I have a compass for my Suex VR scooter and want to know if the motor will skew the magnetic field and throw off the compass? If so, when I am not moving will the compass return to normal behavior?

    Thanks,
    Gary


  2. #2

    Default

    I would run some experiments but keep in mind that a magnetic compass will also respond to acceleration and deceleration on certain headings as explained on the bottom of this page.

    In airplanes you have a lot of stuff that skews the earth's magnetic field and there is typically a correction card under a magnetic "wiskey" compass to account for influences that cannot be cancelled out in another way. You could make something similar for your scooter if there is any influence from the motor.

    Last edited by Head Back!; 06-13-2018 at 06:40 PM.

  3. #3

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    On scooters, there are three main causes of magnetic deviation to a compass.

    1. Hard Iron, which is primarily the steel and magnets of the motor and steel cans of the lithium battery pack. Suspend the compass and then bring the scooter close to the compass and see how much it affects the reading. This deviation is always there, even when the motor is not running.

    2. Electromagnetic field generation from the wires between the battery and motor. The more current through the wires, the stronger the magnetic field will be generated.

    3. Arcing from motor brushes when running. (Brushless motors do not have this problem, but you have a brushed motor.) New motors have some arcing, which decreases when the brushes wear in and then increases again when the brushes and contacts start to wear out.

    Run your scooter into a wall where it cannot move, then pull the trigger and adjust the power up and down to see how much deviation you get. Mounting it as close to the nose as possible is your best bet.

    _______________________________________________
    Logic Dive Gear - Genesis DPV's
    Setting the new standard for performance and technology

  4. #4

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    I use one on my Hollis with no issues. I use a kayak compass mount. I cutoff the straps it came with and attached bungee.


  5. #5

    Default

    If you ask any instrument pilot, they'll tell you how much they dislike flying a non precision approach with a failed directional gyro as the wet or "whiskey" compass is so much harder to use with any degree of precision. Vertical card compasses split the difference as they reduce some of the errors and problems with wet compasses.

    What I recommend you do with a compass on a DPV is more or less the equivalent of "swinging" the wet compass in an aircraft to quantify the errors you can expect on various headings. In an aircraft the compass must be swung around headings that are no greater than 30 degrees apart and the compass must not have more than a 10 degree error on any heading with any combination of engine and electrical systems operating. You don't need to get that deep in the weeds with a DPV, but the basic principles are still useful.

    1) Use a reel and a master compass, or the compass not attached to the DPV to establish a straight line on a 0 degree - 180 degree line (north and south).
    2) then with the compass installed on the DPV, align the DPV on the north/south line and record the compass reading with the motor stopped for both north and south.
    3) then run along the line and note the reading with the DPV under power at both high, medium and low speeds and record the readings north and south for all three power conditions
    4) rinse and repeat the process with a line laid 90 degrees - 270 degrees (east and west) and note the compass readings under the above conditions.
    5) you can repeat it again for 315 degrees - 135 degrees and 45 degrees - 225 degrees, if you like, or use 30 degree increments instead.

    In an aircraft, and in most automotive or nautical compasses, the compass can be adjusted to reduce the error in each direction by half, so that the north south and east west errors are equal before the compass is swung for the other directions. In other words, adjust the compass to read 0 degrees when facing north, and then point the aircraft, boat etc south and adjust the N-S screw so that any error is reduced by 1/2. For example, if your compass read 186 degrees when facing south, you'd adjust it to read 183, dividing the error in half. That will give you an equal 3 degree error in both north and south, and you're repeat the process facing east and west, using the e-w screw to reduce the error facing west by half.

    On a non adjustable dive compass, you'll just have to note the errors in each direction.

    One other thing to consider is that in addition to the magnetic and electrical fields of the DPV, any ferrous metals you have in your dive gear can also affect the compass - in particular your steel tanks. If you've accurately recorded your compass errors with a pair of regular back mounted double 85s, side mounting them will have an effect, as would switching to aluminum tanks, or slinging a steel 45 with a deco gas in it.

    There's also the issue of tilt angles on dive compasses. The amount of tilt they will tolerate before locking up varies but you need to be wary anytime the compass stops moving. For example, if you fail to correct for decreased buoyancy as you go deeper and your DPV ends up pointed up to produce a vertical vector to offset your negative buoyancy you might reach a point where the compass dial locks up in the case. Same with being too positive, and tilting the scooter down.

    Also, any time you tilt the DPV to turn, you may lock up the compass, and even if you don't it will swing back and forth for a bit. So you need to learn to lead any turns or pick a point on the desired heading then turn to it and then wait to verify the heading once the compass stabilizes.

    In general the utility of a compass on a DPV is primarily being able to navigate around using reciprocal courses to get you out and back in the ball park where you started, or when using a plotting board, to navigate with compass headings and time, speed and distance on the board, rather than as a precision instrument. Don't expect more from it than it can give.

    NACD Cave DPV Cert # 666: Cave DPV Anti-christ


 

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