C

crabandy

Well Known Member
In addition to the shunt that came with the G3X sensor package I also used a 20 amp 50 mv shunt from BandC Specialty
(http://www.bandc.biz/ammeterloadmetershunts-1.aspx) to measure the output of the SD8 backup alternator.
The UMA 1C4 (50mV/100A type) shunt that came with the G3X sensor package is giving me reasonable values for my main alternator under Main Bus Amps.
When I shut my main alternator off to test the SD8 (8 amp max output), the Essential Bus Amps was showing 38 amps.
Is the shunt from BandC not compatible with the G3x? Perhaps I have something else going on?
Thanks,
Andy
 
Bill, I had briefly viewed the calibration menu but thought it was only for Hall Effect Sensors. Perhaps I bumped up the scale factor or something while I was in the calibration menu. I thought that as long as the shunt millivolts were the same the current didn't matter, but I'm new to this.
What should my calibration settings be for??
Scale Factor:
Sensor Value:
Calibrated Amps:
Thanks,
Andy
 
You should only need to set the scale factor. From what you posted....
The Garmin system is designed for a default shunt of a 50mv for 100 amps. In other words, when 100 amps is passing through the shunt, the voltage ACROSS the shunt will be 50 millivolts.
The second shunt you are using is a 50mv for 20 amps. So it will show a voltage drop across the shunt of 50mv for just 20 amps, or 5 times the voltage drop for the same amount of current passing through the Garmin provided standard shunt 100/20=5.
If you divide the 38 amps reading you are seeing displayed for the backup alternator by 5 it is 7.6 amps. That should be the same as you see, assuming all of the same equipment is connected, for your main alternator.

So your scale factor is 20/100 = .2 ........
 
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Amps Scale Factor

Hello Andy,

Bill is absolutely correct with his logic that your system is reading (100/20) or 5 times higher that it should. He is also correct that you should only need to adjust the Scale Factor on that page.

One minor correction. The G3X software multiplies the raw measured data and the Scale Factor entered on the Amps Calibration page (not divides), so you would need to enter a Scale Factor of (20/100) or 0.20 to get your system to read correctly.

38 amps x 0.20 = 7.6 amps

For those following along, the instructions for performing the Amps Calibration are located on page 18-20 of the Rev. K G3X Installation Manual. It appears we could use some additional explanation of the Scale Factor.

One additional point. The system is designed to work out of the box with the 50mV/100A shunts we sell (909-D0000-00) and no adjustment of the default Scale Factor of 1.00 is needed if these are used. There's really no advantage in using a 20A shunt since it doesn't give you any better resolution, although it can be made to work if you already have one.

Thanks,
Steve
 
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