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Old 22-12-2017, 19:47   #1
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Unhappy How to calibrate Speed sensor with Garmin GNX 20

Hi!
I purchased a Garmin GNX20 with a Airmar DST800 paddlewheel speed sensor. I noticed that the boatspeed value is always 0.5/0.8 higher than the SOG (from my GPS).

When I tried to calibrate it, the menu says "bring the boat to cruising speed, note down, stop and press ok". What I do is:
1) set the engine to work at 1500 rpm, so the gnx20 displays boatspeed 4.5 kts (sog 4kts)
2) I stop the boat
3) I press ok --> 4,5 value appears on screen
4) press ok --> "calibration error" appears on screen.

What am I doing wrong? Thxs.
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Old 12-08-2019, 22:28   #2
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Location: Brisbane
Boat: Lightwave 45, sold nov 2020.Previous self built Roberts 36
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Re: How to calibrate Speed sensor with Garmin GNX 20

Hi did this ever get sorted. My calibration went off a while ago and I haven’t been able to get it close since
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Old 20-07-2022, 11:18   #3
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Location: Nova Scotia, Canada
Boat: Westerly Merlin 28'
Posts: 1
Re: How to calibrate Speed sensor with Garmin GNX 20

Like duckpaint, I could not calibrate my GNX 20 for speed through the water using the instructions on the device display. I called Garmin Support and was told that those instructions are incorrect.

Here is the correct Garmin GNX 20 speed calibration procedure: (Do on a calm day with no currents)

First, go to the speed calibration section of the device menu. ( Menu > Setup > Data Sources > Select Data Source > Select Depth > Select numeral representing Speed Sensor

Now ignore instructions on device and follow these:

1> Bring the boat to a steady speed between 4-6 knots.
2> Maintain that speed.
3> Note that speed. (Use a GPS, such as Navionics or other GPS app on your phone.)
4> Enter the noted speed into the GNX 20.
5> Maintain speed until calibration is complete.

That should do it; your GNX 20 should now report an accurate speed through water. That will, of course, not always be the same as the speed over ground from the GPS; there will be variances due to wind, tides and other currents, turbulence around the speed sensor and hull, etc.
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Old 20-07-2022, 15:06   #4
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Boat: Lightwave 45, sold nov 2020.Previous self built Roberts 36
Posts: 217
Re: How to calibrate Speed sensor with Garmin GNX 20

Hi , I replaced the spindle on the paddle wheel and all was perfect again. Interestingly, the wheel spindle didnt look too bad.
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