Voltage Differences ?

[DCarlton]

My EDM 730 and AV 30 both display voltage but the AV 30 reads .5 volts lower.  Is that common?  How can the difference be explained if they’re wired to the same buss?  I’m haven’t checked with a volt meter to see who’s providing ground truth.  

[Ragsf15e]

51 minutes ago, DCarlton said:

My EDM 730 and AV 30 both display voltage but the AV 30 reads .5 volts lower.  Is that common?  How can the difference be explained if they’re wired to the same buss?  I’m haven’t checked with a volt meter to see who’s providing ground truth.  

You can definitely have voltage loss where there’s resistance through a bad/corroded connection or circuit breaker.  The higher one is likely the voltage on your bus.  Trace the lines and check each connection with a dvm to find where the drop is for the lower one.  I had a similar problem and found a looses connection to a circuit breaker.  It doesn’t take much.

[DCarlton]

48 minutes ago, Ragsf15e said:

You can definitely have voltage loss where there’s resistance through a bad/corroded connection or circuit breaker.  The higher one is likely the voltage on your bus.  Trace the lines and check each connection with a dvm to find where the drop is for the lower one.  I had a similar problem and found a looses connection to a circuit breaker.  It doesn’t take much.

The AV30 is on an older 2 amp breaker that hasn’t been used in years.  I wondered if the breaker could be the cause.  Also wondered if a poor crimp could cause it or a poor ground?  Yes I do believe the EDM is accurate.  Will break out the DVM.  Thanks.  

[N201MKTurbo]

There is an old saying that if you have a clock, you always know what time it is. If you have two clocks, you have no idea what time it is.

The only way the real voltage can be different between the two voltmeters is if there is a significant current flow between the two meters. The two volt meters draw very little current themselves, so it is most likely a calibration error on one or both of them. The only way to know is to measure the voltage at each of them with an independent volt meter such as a handheld meter and see how they compare. 

[ArtVandelay]

My AV20S reads lower, but not that much.

[DCarlton]

8 hours ago, N201MKTurbo said:

There is an old saying that if you have a clock, you always know what time it is. If you have two clocks, you have no idea what time it is.

The only way the real voltage can be different between the two voltmeters is if there is a significant current flow between the two meters. The two volt meters draw very little current themselves, so it is most likely a calibration error on one or both of them. The only way to know is to measure the voltage at each of them with an independent volt meter such as a handheld meter and see how they compare. 

Gotta figure out where the EDM taps power to measure; AV30 is obvious.  One thing I've noticed after crawling around under the panel, is that grounds have been added over the years using many different methods.  If someone wanted to add a best case ground or multi-pin ground block somewhere forward on a Mooney, what would be the best attachment point?  The skin or the steel frame?  And if the frame, what would be the best attachment method?   Or does it not matter that much?...  

[N201MKTurbo]

My 77 J has a ground lug welded to the steel frame at the top of the avionics area. When I did my avionics, I just used it as a star ground for all the boxes up front. I just screwed all the ground wires to that one lug with a long screw. 
 

I think the steel cage is a better choice for a ground than the aluminum, but I can’t justify that statement. The engine ground strap goes to the firewall, which is connected to the aluminum at its edges and to the steel cage at the motor mount bolts. The battery is grounded to the aluminum in the tail. The aluminum and steel cage are connected at many places. So the aluminum may actually be a better choice. I don’t think it really matters. 
 

It would be best if all the grounds behind the panel went to the same place though.

[MikeOH]

@DCarlton

TL;DR

But, it sounds like the EDM 730 and AV-30 while wired to the same positive bus location have different ground points.  I suspect that is responsible for the 1/2 volt difference.  It is possible that one of the meters is not properly calibrated but that is not as likely, IMHO.

The idea of a single point ground, whether the steel frame or aluminum skin, is to provide a stable ground reference for all loads.  The 'best case' is only ONE common ground point for all loads rather than exactly where it is located. As you have noted, practically speaking, that is far from the truth when one looks under their panel!  Grounds added wherever is most convenient!

The difficulty with multiple ground points is that current flows between them, and since all these connections (be they wire, aluminum, or steel) have some amount resistance, those different ground points are NOT at the same voltage, and even worse that voltage changes as load currents vary with time.  Thus other loads see changing voltages which is not ideal and can affect performance.  

[ArtVandelay]

Modern avionics all have voltage regulators that’s why they can handle 12 or 24 volt systems. As long as the current is steady it shouldn’t be an issue.

[MikeOH]

11 minutes ago, ArtVandelay said:

Modern avionics all have voltage regulators that’s why they can handle 12 or 24 volt systems. As long as the current is steady it shouldn’t be an issue.

While true that modern avionics are less susceptible to power supply variations, not all of us have the luxury of entirely modern avionics

[DCarlton]

3 hours ago, MikeOH said:

@DCarlton

TL;DR

But, it sounds like the EDM 730 and AV-30 while wired to the same positive bus location have different ground points.  I suspect that is responsible for the 1/2 volt difference.  It is possible that one of the meters is not properly calibrated but that is not as likely, IMHO.

The idea of a single point ground, whether the steel frame or aluminum skin, is to provide a stable ground reference for all loads.  The 'best case' is only ONE common ground point for all loads rather than exactly where it is located. As you have noted, practically speaking, that is far from the truth when one looks under their panel!  Grounds added wherever is most convenient!

The difficulty with multiple ground points is that current flows between them, and since all these connections (be they wire, aluminum, or steel) have some amount resistance, those different ground points are NOT at the same voltage, and even worse that voltage changes as load currents vary with time.  Thus other loads see changing voltages which is not ideal and can affect performance.  

I have a starboard side AMP ground block in a very difficult to access location for most of radio stack.  It's very difficult to see or chase the wires.  That would be ideal if there were unused ground wires.  I'll do a little work with the volt meter and may have an opportunity to improve some of the grounds.  But yes, currently I doubt the EDM and AV are wired to nearby or the same ground points.  Perhaps that can be improved.  This voltage difference may just be a nit annoyance but I'm trying to improve the plane wherever possible when making any changes.  

[MikeOH]

I can appreciate wanting both “watches” to read the same, but unless they are wired directly to the battery posts their absolute accuracy isn’t critical. You are looking for deviations from what is normal; downward trend indicating a failing charging/regulating system or sudden excessive load, or an upward reading that might indicate a problem with the regulator.

The time for accuracy is when setting the voltage regulator output. Then, based on temperature, the voltage at the battery terminals is what matters.

[bcg]

It's not uncommon to see a different reading from different parts of the wiring. Voltage drop across the wire, the EDM should be grounded directly to the engine vs the AV20 likely being grounded to a convenient spot on the airframe, etc. I wouldn't worry about it, the EDM is the one you should trust.

Sent from my Pixel 6a using Tapatalk

[dkkim73]

If one path is grounded through an aluminum alloy and then to steel, and one directly to steel, might there be a galvanic gap? 

Agree old long wiring and connections are more common and universal explanations. 

D

 

[N201MKTurbo]

All this supposition is fun, but it will never solve the problem. You need to get in there with a voltmeter and see what is happening.

[DCarlton]

2 hours ago, N201MKTurbo said:

All this supposition is fun, but it will never solve the problem. You need to get in there with a voltmeter and see what is happening.

On the to do list.  Thanks.  

[Yetti]

Rounding error.  Go fly the plane.

[MikeOH]

4 hours ago, N201MKTurbo said:

All this supposition is fun, but it will never solve the problem. You need to get in there with a voltmeter and see what is happening.

Yeah, I guess; but, I would like to point out he is already 'in there' with TWO voltmeters

[N201MKTurbo]

1 hour ago, MikeOH said:

Yeah, I guess; but, I would like to point out he is already 'in there' with TWO voltmeters

I mean an independant voltmeter so he can check the voltage at the two power wires and see if there is really a difference or if one (or both) of his meters are lying.

[MikeOH]

1 minute ago, N201MKTurbo said:

I mean an independant voltmeter so he can check the voltage at the two power wires and see if there is really a difference or if one (or both) of his meters are lying.

@N201MKTurbo

The  was my indication of an attempt at humor; I see that I have failed

[DCarlton]

There was a lot of abandoned wiring removed during this project leaving a couple of ground wires unused.  One went to improved panel lighting and one went to the AV30.  Based on this discussion I suspect the ground wire used for the AV30 was not the best choice; it was hard to physically trace to ground.   I think the one that went to the panel lighting went to a newer AMP ground block.  That might be a better choice.  Or perhaps installing a new dedicated ground with solid pedigree.  This may be a nit but I like to take a look after my knees recover.  It’s the only minor “discrepancy” after several weeks of work.  Sill need an inspection, sign off, and pitot static cert.  This discussion has helped.  Thanks.