VP-X Voltage

[aguiraf]

The VP-X voltage reads 14.6 V. A Fluke voltmeter connected to the the battery input to the VP-X reads 14.4 V. Does anybody have any ideas as to why the difference?

[Raymo]

Ohm's law equation: V = I ? R

Volts = AMPS x Resistance

The VPX is likely reading from a location with less resistance than the battery.

[Brantel]

Could be tolerance/calibration differences.

If you are measuring the main power pin on the VP-X, how about the other meter lead? Are you placing it on the same ground point that the VP-X is using?

There can be slight differences in the resistance of the circuit you are measuring.

[aguiraf]

Hi Ray, thanks for the reply. I m taking the the Fluke voltmeter reading from the battery input line to the VP-X. According to the Vertical Power , VP-X takes a direct reading also from this line. They have no idea why the difference. I will try another voltmeter

[aguiraf]

I will connect ground to the same as the VP-X thanks

[pilotkms]

My voltages on my Dynon read exactly as u state. Haven?t thought much as to why. It?s been the same for 320 happy flying hours.

[aguiraf]

Thank you all for the feedback. I made sure this time the ground being used for taking the voltmeter readings was the same as the VP-X ground. The Dynon still reads 0.2 V higher than the Voltmeter reading, the engine running and the alternator on. 14.4 voltmeter, - 14.6 dynon. Not a big difference> it could be due to wiring, alligator clips used to connect the voltmeter leads to the positive terminal of the VP-X and the the common ground. Interesting

[aguiraf]

Next, plan to check Fluke 76 calibration

[MCA]

There's always some tolerance due to many of the variables discussed above.

I'd argue that 0.2v is not critical for normal aircraft operations. What you're looking for are, with engine running:

14.x volts = alternator on and working
<13.0 volts = alternator off or alternator failure

So I hope you're not too concerned about it.

Just my 2 cents.

[aguiraf]

Hi Marc,

No worries, I fully understand your comment. I have been flying with these numbers for quite sometime.The alternator is charging just fine. I have now gotten around to try to find out the reason for the small difference in the readings. Like you, and others say, it may be due to many minor issues. It has helped me question the calibration of the multi-meter. I have had this Fluke 76 for 30 years and have assumed it is always correct. I tried two other multi meters and all read anywhere within +/- 0.3 to 0.9. Thanks again for the feedback.