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Author Topic: Power Measurement Basics  (Read 22140 times)

Offline poynt99

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Power Measurement Basics
« on: January 20, 2014, 02:27:36 PM »
Inspired by many folks here that wish to better understand some of the lesser-known points about power measurement in any circuit.

Part 1 deals with a simple DC circuit but the concepts apply to all power measurements.

This goes out to all those opposing the fact that to retain proper phase information when making power measurements with an oscilloscope, one channel of the oscilloscope must be electrically inverted (because we physically invert one channel). This video also illustrates the little known fact that power sources compute to a negative power, while devices that dissipate power compute to a positive power. These issues are of particular importance when overunity claims are being made that involve the measured polarity of the power source.

Part 1a: http://www.youtube.com/watch?v=wIbQUUp9S9o
Part 1b:

EDIT: Re-assembling the videos, as I messed up the order of several parts. Sorry. OK, half done.
« Last Edit: January 21, 2014, 02:18:17 AM by poynt99 »

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Power Measurement Basics
« on: January 20, 2014, 02:27:36 PM »

Offline TinselKoala

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Re: Power Measurement Basics
« Reply #1 on: January 20, 2014, 02:52:36 PM »
Thumbs up! These are definitely "must watch" items.

Thank you for the effort and time you put into making these clear presentations.

I'm looking forward to the next chapters!

 8)

Offline poynt99

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Re: Power Measurement Basics
« Reply #2 on: January 20, 2014, 02:57:51 PM »
Thanks TK.

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Re: Power Measurement Basics
« Reply #2 on: January 20, 2014, 02:57:51 PM »
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Offline MarkE

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Re: Power Measurement Basics
« Reply #3 on: January 20, 2014, 04:53:55 PM »
My comments:

I think that the videos do a good job of handling Kirchhoff's voltage law.  I also think that they do a good job of showing the essential importance of following consistent conventions through a circuit, for both the direction of current and the direction of EMFs.

My major dissents:

1) Power measurement convention is dissipated power.  The videos follow a convention of measuring voltage drops across power dissipating devices as negative values and the voltage drop across the battery power source as a positive voltage.  I think this potentially adds to the confusion that the audience may have that you are trying to address.

2) Voltage measurement convention across loads is from the circuit common to the powered load terminal.  This is the opposite of what was shown in the video.

3) There are two options for measuring voltage and current with non-isolated, single-ended oscilloscope channels and a current sense resistor:
a) Using the circuit common as the instrument common.  In this case the voltage drop across the CSR is included in the DUT voltage measurement introducing a small error.  This is by far the more common measurement practice used in industry as it places the instrument commons at the circuit common.  The general practice is to reduce the error term to the point that it is insignificant, and/or to compensate for it.

b) Using the junction of the DUT and the CSR as the instrument common.  This eliminates the voltage drop error of a) but inverts the sense of the measured CSR voltage with respect to positive convention current that flows through the DUT.  That inversion as noted in the video can be corrected by setting the oscilloscope to invert.  This is a:  clever, and accurate way to measure, but moves the instrument commons from the circuit common, making it an unusual, albeit useful practice.

I keep threatening to write a power measurements primer for Revolution-Green.

Offline poynt99

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Re: Power Measurement Basics
« Reply #4 on: January 20, 2014, 05:31:45 PM »
My major dissents:

1) Power measurement convention is dissipated power.  The videos follow a convention of measuring voltage drops across power dissipating devices as negative values and the voltage drop across the battery power source as a positive voltage.  I think this potentially adds to the confusion that the audience may have that you are trying to address.
If "dissipated" power is the only terminology we are permitted to use, then power sources have a "negative" dissipation. This in no way contradicts what I have illustrated in the videos.

I disagree that it is confusing to illustrate "voltage drops" and "voltage gains" in a circuit. This is basic electrical theory that everyone should be familiar with, even OU enthusiasts.

The convention that I chose makes sense from the perspective that we "align" ourselves with the supply voltage, because most people are accustomed to placing the positive DMM lead on the positive terminal of the battery. It also makes sense because it is convention to talk of "voltage drops" across resistors and diodes etc., and a negative voltage measurement across them imo coincides perfectly with this phraseology.

Quote
2) Voltage measurement convention across loads is from the circuit common to the powered load terminal.  This is the opposite of what was shown in the video.
One can establish either one of two conventions, as long as they stick with that convention throughout the measurement process (i.e. no flipping of the measurement leads is permitted, unless it is re-inverted inside the scope). I chose the convention which is established by how we would normally measure the power source; red lead on positive, black lead on negative. If one does not care about polarity (or require it) in their measurements, then of course this is all moot.

Quote
3) There are two options for measuring voltage and current with non-isolated, single-ended oscilloscope channels and a current sense resistor:
a) Using the circuit common as the instrument common.  In this case the voltage drop across the CSR is included in the DUT voltage measurement introducing a small error.  This is by far the more common measurement practice used in industry as it places the instrument commons at the circuit common.  The general practice is to reduce the error term to the point that it is insignificant, and/or to compensate for it.
Agreed, and is my preferred method for measuring LOAD power, but not for measuring power supply power. For best accuracy measuring LOAD power, one can easily compute/subtract the power in the CSR. For power supply power, your method b) is my choice. The video only addressed measuring the power supply (battery) power with the scope, not the LOAD power.

Quote
b) Using the junction of the DUT and the CSR as the instrument common.  This eliminates the voltage drop error of a) but inverts the sense of the measured CSR voltage with respect to positive convention current that flows through the DUT.  That inversion as noted in the video can be corrected by setting the oscilloscope to invert.  This is a:  clever, and accurate way to measure, but moves the instrument commons from the circuit common, making it an unusual, albeit useful practice.
If one terminal of the power source IS the circuit common (and it often is), then we are not moving the instrument common from the circuit common. As illustrated in the diagram, the probes are commoned at the negative terminal of the battery. Again, the videos have not addressed oscilloscope power measurement of the LOAD yet, only the power source (battery in this case).

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Re: Power Measurement Basics
« Reply #4 on: January 20, 2014, 05:31:45 PM »
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Offline poynt99

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Re: Power Measurement Basics
« Reply #5 on: January 20, 2014, 05:52:05 PM »
In your first diagram, how did you come to the conclusion that the battery power is -10W and the resistor is +10W?

Offline MarkE

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Re: Power Measurement Basics
« Reply #6 on: January 20, 2014, 05:57:43 PM »
If "dissipated" power is the only terminology we are permitted to use, then power sources have a "negative" dissipation. This in no way contradicts what I have illustrated in the videos.
Yes, you did a good job of showing that the sign of power from a source must be the opposite sign of power dissipated by a load. 
Quote

I disagree that it is confusing to illustrate "voltage drops" and "voltage gains" in a circuit. This is basic electrical theory that everyone should be familiar with, even OU enthusiasts.
I agree that it is an essential concept.  I think the video would have been more consistent had you chosen to follow a CCW convention rather than CW.  This would have shown positive voltage drop values across loads corresponding to positive power dissipation measurements.
Quote

The convention that I chose makes sense from the perspective that we "align" ourselves with the supply voltage, because most people are accustomed to placing the positive DMM lead on the positive terminal of the battery. It also makes sense because it is convention to talk of "voltage drops" across resistors and diodes etc., and a negative voltage measurement across them imo coincides perfectly with this phraseology.
This is a point where we differ.  When you measure the voltage across a circuit, do you place your instrument black lead on the circuit common or on the supply rail?  I am sure that you place the reference black lead on the circuit common like we all do.  So now when thinking about Kirchhoff there is a decision to make:  Is that voltage that you read on the voltage rail the voltage that is being supplied by source, or is it the total voltage that is being dropped by the loads?  Convention says that it is the voltage dropped by the loads.  Think about what you would say to a colleague:  "I have 10V across my circuit."  IE: "I have 10V drop across my loads."  The black reference lead is the lead that stays in place.  As we go around the loads in a circuit we observe 10V, 8V, 2V etc all with reference to the circuit common.  This is what I contend your audience has been doing, conforms with convention, and is what they understand.  The mental stumbling block is that the same positive rail voltage is for purposes of KVL a negative voltage drop from the reference to the supply rail.
Quote
One can establish any one of two conventions, as long as they stick with that convention throughout the measurement process (i.e. no flipping of the measurement leads is permitted, unless it is re-inverted inside the scope). I chose the convention which is established by how we would normally measure the power source; red lead on positive, black lead on negative.
We absolutely agree on this.  Where we disagree is which of the two conventions is the more common and familiar.  I've made my argument on that point.
Quote
Agreed, and is my preferred method for measuring LOAD power, but not for measuring power supply power. For best accuracy, one can compute/subtract the power in the CSR. For power supply power, your method b) is my choice. The video only addressed measuring the power supply (battery) power with the scope, not the LOAD power.
I don't think that point of view came out in the video.  I think it is important that you make that clear.
Quote
If one terminal of the power source IS the circuit common (and it often is), then we are not moving the instrument common from the circuit common. As illustrated in the diagrams, the probes are commoned at the negative terminal of the battery.
Right, so think about this whether you are using a meter or a scope: the CW convention that you followed demonstrating voltage drops according to KVL eventually placed the instrument red lead on the circuit common while looking at the loads.  But, what you have said is that you wanted to look at this from the standpoint of the power source.  I submit that for that purpose you should have used a stack of batteries and measured those individual drops, and not gone around the loads.  There is ultimately no mathematical difference in magnitude whether you choose positive voltage drop for dissipation or source.  There is a difference in sign.  If everyone rigorously follows one convention the opportunity for confusion as to whether a particular circuit branch is dissipating or sourcing power is minimized.
Quote
Again, the videos have not addressed oscilloscope power measurement of the LOAD yet, only the power source (battery in this case).
I appreciate the time and effort that goes into making a video.  It's obvious to me that you know your engineering.  My comments are directed at helping you to convey your intent in a way that your intended audience is likely to understand.

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Re: Power Measurement Basics
« Reply #6 on: January 20, 2014, 05:57:43 PM »
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Offline poynt99

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Re: Power Measurement Basics
« Reply #7 on: January 20, 2014, 06:37:52 PM »
I think the video would have been more consistent had you chosen to follow a CCW convention rather than CW. This would have shown positive voltage drop values across loads corresponding to positive power dissipation measurements.
Yes, perhaps you and I would start with the 2k resistor to see what the voltage drop is, and knowing that it was connected to the positive terminal of the battery, we would place the red lead first, then the black lead second, which would result in a + voltage drop across it.

But we are not strictly trying to measure voltage drops here, we are trying to perform power measurements, and a convention to do so must therefore be established. For the battery power measurement, the scope probe is placed across the battery source with the probe tip on the + battery terminal, not the opposite as you seem to be suggesting. Since we have established the orientation of the probes by how the voltage probe is placed across the battery, we must follow this same convention while placing the current probe across the CSR, otherwise the phase of the power computation will be off by 180º. Since we can not actually place the CSR probe correctly due to the common gnd issue, we must physically invert the CSR probe so that the gnd leads of both scope channels are tied together. Now to correct for this physical CSR probe inversion, we simply invert the CSR channel electrically in the scope. Relative phase between the voltage and the current is now restored.

Quote
This is a point where we differ.  When you measure the voltage across a circuit, do you place your instrument black lead on the circuit common or on the supply rail?  I am sure that you place the reference black lead on the circuit common like we all do.  So now when thinking about Kirchhoff there is a decision to make:  Is that voltage that you read on the voltage rail the voltage that is being supplied by source, or is it the total voltage that is being dropped by the loads?  Convention says that it is the voltage dropped by the loads.  Think about what you would say to a colleague:  "I have 10V across my circuit."  IE: "I have 10V drop across my loads."  The black reference lead is the lead that stays in place.  As we go around the loads in a circuit we observe 10V, 8V, 2V etc all with reference to the circuit common.  This is what I contend your audience has been doing, conforms with convention, and is what they understand.  The mental stumbling block is that the same positive rail voltage is for purposes of KVL a negative voltage drop from the reference to the supply rail.We absolutely agree on this.
I think you might be missing the point and confusing the issue Mark.

We are not trying to measure nodes in the circuit relative to any other node. We are trying to measure the voltage across each individual component.

Quote
Right, so think about this whether you are using a meter or a scope: the CW convention that you followed demonstrating voltage drops according to KVL eventually placed the instrument red lead on the circuit common while looking at the loads.  But, what you have said is that you wanted to look at this from the standpoint of the power source.  I submit that for that purpose you should have used a stack of batteries and measured those individual drops, and not gone around the loads.
Not sure what point you are trying to make here.

Quote
There is ultimately no mathematical difference in magnitude whether you choose positive voltage drop for dissipation or source.  There is a difference in sign.  If everyone rigorously follows one convention the opportunity for confusion as to whether a particular circuit branch is dissipating or sourcing power is minimized.
Indeed, one convention should be followed. I presented the convention that most of us here already use, i.e. oscilloscope voltage probe tip on + battery terminal, and gnd lead at circuit common. CSR probe tip on far side of CSR, and CSR probe gnd lead at circuit common. The only caveat with this, and one that I am trying to emphasize, is that the CSR signal is now 180º out of phase and needs to be corrected by inverting the channel in the scope. Of course all this is only important IF phase (i.e. polarity) is pertinent to someone's argument or claim.

Quote
I appreciate the time and effort that goes into making a video.  It's obvious to me that you know your engineering.  My comments are directed at helping you to convey your intent in a way that your intended audience is likely to understand.
Your comments are appreciated Mark. Hopefully this discussion will help get the points across better.

Offline MarkE

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Re: Power Measurement Basics
« Reply #8 on: January 20, 2014, 07:24:39 PM »
Poynt99 we are in violent agreement that following a consistent convention is essential.  The convention that I have always seen with respect to power is with reference to the loads.  I have never seen a load referred to as a negative power source.

To establish what is needed to insure we don't mess up the phasing, take for example a sinusoidal source with an offset voltage capability.  Our load will be a simple 1 ohm resistor, and we will place in series a 1 mOhm current sense.  The current sense is low-side and we assign node 0, and the connection points for all of our instrument references to the low side of the power source.  Channel 1 measures voltage from reference to the top of the circuit where the source connects to the load.  Channel 2 measures voltage from the reference to the junction of the CSR and the load.

Set the source to 2V p-p with zero offset.
Channel 1 shows 2V p-p in phase with the power source.
Channel 2 shows ~2mVp-p in phase with the power source.
Measured power magnitude at each peak is:  2mV * 1000A/V * 2V = 4W, and -2mV * 1000A/V * -2V = 4W.

Offset the source positive by one Volt:
Channel 1 swings from -1V to +3V in phase with the power source.
Channel 2 swings from -1mV to +3mV in phase with the power source.
Measured power magnitude at each peak is:  3mV * 1000A/V * 3V = 9W, and -1mV * 1000A/V * -1V = 1W.

Offset the source negative by one Volt:
Channel 1 swings from -3V to +1V in phase with the power source.
Channel 2 swings from -3mV to +1mV in phase with the power source.
Measured power magnitude at each peak is:  1mV * 1000A/V * 1V = 1W, and -3mV * 1000A/V * -3V = 9W.

Where you have to watch yourself is when you use the trick of moving the reference node so that it is at the junction of the CSR and the load, rather than the CSR and the power source.

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Re: Power Measurement Basics
« Reply #8 on: January 20, 2014, 07:24:39 PM »
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Offline TinselKoala

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Re: Power Measurement Basics
« Reply #9 on: January 20, 2014, 07:25:44 PM »
Yes, absolutely.

I hope there will be some time for questions and answers.

For example, a proper Current Probe (Hall effect - transformer type) matched to the oscilloscope clips around a circuit wire at the measurement point and usually needs no ground reference connection to the circuit. The probe body is generally marked with the correct orientation wrt conventional current flow. How does the signal from a probe like this compare in polarity/phase with a reading of voltage drop from an inline CSR at the same location?

Another "poynt" or demonstration that might be nice would be an explanation of the use of differential voltage probes in situations like this one, and also how two passive probes can be used in place of one differential probe to measure signals between arbitrary points in a circuit.


Offline poynt99

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Re: Power Measurement Basics
« Reply #10 on: January 20, 2014, 08:38:05 PM »
Poynt99 we are in violent agreement that following a consistent convention is essential.  The convention that I have always seen with respect to power is with reference to the loads.  I have never seen a load referred to as a negative power source.
I think what you are trying to say is that in your work, you are never interested in measuring the input or source power; you are only interested in measuring the power dissipated in a circuit's loads, correct? I have never suggested that loads compute to a negative power, they don't. However, power sources DO, if measured correctly.

Quote
To establish what is needed to insure we don't mess up the phasing, take for example a sinusoidal source with an offset voltage capability.  Our load will be a simple 1 ohm resistor, and we will place in series a 1 mOhm current sense.  The current sense is low-side and we assign node 0, and the connection points for all of our instrument references to the low side of the power source.  Channel 1 measures voltage from reference to the top of the circuit where the source connects to the load.  Channel 2 measures voltage from the reference to the junction of the CSR and the load.

Set the source to 2V p-p with zero offset.
Channel 1 shows 2V p-p in phase with the power source.
Channel 2 shows ~2mVp-p in phase with the power source.
Measured power magnitude at each peak is:  2mV * 1000A/V * 2V = 4W, and -2mV * 1000A/V * -2V = 4W.

Offset the source positive by one Volt:
Channel 1 swings from -1V to +3V in phase with the power source.
Channel 2 swings from -1mV to +3mV in phase with the power source.
Measured power magnitude at each peak is:  3mV * 1000A/V * 3V = 9W, and -1mV * 1000A/V * -1V = 1W.

Offset the source negative by one Volt:
Channel 1 swings from -3V to +1V in phase with the power source.
Channel 2 swings from -3mV to +1mV in phase with the power source.
Measured power magnitude at each peak is:  1mV * 1000A/V * 1V = 1W, and -3mV * 1000A/V * -3V = 9W.

Where you have to watch yourself is when you use the trick of moving the reference node so that it is at the junction of the CSR and the load, rather than the CSR and the power source.
I'm not sure I understand your point. Have I shown the reference between the CSR and load?

In your scenario above, you are measuring voltage directly across the voltage source, and yes due to the probe configuration your voltage and current traces will be in-phase.  However, your probes are placed in a series-opposing configuration, which means that one channel signal is inverted wrt the other.

So since you are measuring directly across the voltage source, you are actually measuring the source power, not the load power (even though they are close to the same in magnitude). When you invert one channel in the scope to correct for the series-opposing probe configuration, your power will compute to be negative, as it should be for where you are measuring, i.e. source power.

In order to properly measure the 1 Ohm load power, you must move your voltage probes either directly across the load resistor (differential probe), or from the source to the reference, which is the connection we had previously. Now your probes are either in series-adding (differential probe scenario), or they are in a "fixed reference configuration" whereby the CSR drop must be compensated for accuracy. In either of these last two cases, there is no need to invert one channel because the probes are physically "in phase".

How would you go about measuring the power in each of several loads that were in series? What if you were only interested in the total power being used, how would you measure that?

Keep in mind that in these forums, it is important that folks understand how to measure battery/source power and load power, because often the load they are interested in may not be the only component dissipating power in their circuit. Comparison between "input" and "output" power then becomes very relevant.

And there's this:
http://www.overunity.com/14220/power-measurement-basics/msg383962/#msg383962

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Re: Power Measurement Basics
« Reply #10 on: January 20, 2014, 08:38:05 PM »
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Offline MarkE

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Re: Power Measurement Basics
« Reply #11 on: January 20, 2014, 08:48:37 PM »
Yes, absolutely.

I hope there will be some time for questions and answers.

For example, a proper Current Probe (Hall effect - transformer type) matched to the oscilloscope clips around a circuit wire at the measurement point and usually needs no ground reference connection to the circuit. The probe body is generally marked with the correct orientation wrt conventional current flow. How does the signal from a probe like this compare in polarity/phase with a reading of voltage drop from an inline CSR at the same location?

Another "poynt" or demonstration that might be nice would be an explanation of the use of differential voltage probes in situations like this one, and also how two passive probes can be used in place of one differential probe to measure signals between arbitrary points in a circuit.
I've attached an oscilloscope capture using a 100 Ohm resistor driven by a function generator and a Tektronix P6021 transformer current probe.  I have diagrammed the set-up including with the marking orientation of the P6021.  The P6021 output is in phase with the function generator voltage.  This is consistent with current flow in the diagrams that both Poynt99 and I have posted.  The issue is what convention to follow for the voltage.  My experience is that the applied voltage across a load is always used.  IE it is the voltage that with an in-phase current, results in power dissipation.

Offline MarkE

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Re: Power Measurement Basics
« Reply #12 on: January 20, 2014, 08:53:47 PM »
In your first diagram, how did you come to the conclusion that the battery power is -10W and the resistor is +10W?
The battery disssipates -10W.  That is the same as saying that it supplies +10W.

Offline poynt99

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Re: Power Measurement Basics
« Reply #13 on: January 20, 2014, 08:59:44 PM »
The battery disssipates -10W.  That is the same as saying that it supplies +10W.
You've re-stated basically what the diagram is depicting, but how did you come to the conclusion that the battery dissipates -10W, or the resistor +10W?

Why did you not conclude that the battery was dissipating +10W and the resisitor -10W?

Offline MarkE

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Re: Power Measurement Basics
« Reply #14 on: January 20, 2014, 09:42:19 PM »
I think what you are trying to say is that in your work, you are never interested in measuring the input or source power; you are only interested in measuring the power dissipated in a circuit's loads, correct? I have never suggested that loads compute to a negative power, they don't. However, power sources DO, if measured correctly.
I'm not sure I understand your point. Have I shown the reference between the CSR and load?

In your scenario above, you are measuring voltage directly across the voltage source, and yes due to the probe configuration your voltage and current traces will be in-phase.  However, your probes are placed in a series-opposing configuration, which means that one channel signal is inverted wrt the other.
They do not oppose.  Both probes are in phase.  In order to oppose one must be CCW and the other CW from the reference node.
Quote

So since you are measuring directly across the voltage source, you are actually measuring the source power, not the load power (even though they are close to the same in magnitude). When you invert one channel in the scope to correct for the series-opposing probe configuration, your power will compute to be negative, as it should be for where you are measuring, i.e. source power.
You have declared that I am measuring across the source and then introduced loss that is not shown in the diagram.  As the diagram is shown there is no distinction between the voltage across the source and the load.  If we introduce a few milliOhms of wiring resistance, then my probes would be on the load side of that wiring resistance.  The magnitudes would then change.  The signs would remain the same.

The issue entirely revolves around whether we declare positive power that which the source supplies, in which case dissipated power is negative, or do we declare positive power that which the loads dissipate, in which case sources "dissipate" negative power.  Industry convention is the latter.  Power meters indicate positive power as the power into the load.  No one refers to the power that a light bulb or any other kind of load dissipates as negative.  If we were to split the 1 Ohm load into two 0.5 Ohm resistors and probe the junction, we would note basically half the source as the positive voltage drop across the bottom 0.5 Ohm resistor.  And we would still report the same current.
Quote
 

In order to properly measure the 1 Ohm load power, you must move your voltage probes either directly across the load resistor (differential probe), or from the source to the reference, which is the connection we had previously.
You are mixing up multiple issues.  The first is what is the convention for positive versus negative power.  The second issue is one of instrumentation options of which there are many for anyone with enough budget, and fewer for those who don't.  It is critical that the convention is settled before worrying about small instrumentation errors.  The convention issue does not change whether we instrument with some piece of junk +/-20% accurate instrument or something good to six digits that is fully isolated and has an infinite CMRR.
Quote
Now your probes are either in series-adding (differential probe scenario), or they are in a "fixed reference configuration" whereby the CSR drop must be compensated for accuracy.
I described where the probes are.  They both use the same node 0 reference.  Yes, that introduces a miniscule voltage magnitude error if left uncorrected, but no it has nothing, absolutely nothing to do with the selected power convention.
Quote
In either of these last two cases, there is no need to invert one channel because the probes are physically "in phase".

How would you go about measuring the power in each of several loads that were in series? What if you were only interested in the total power being used, how would you measure that?
Dividing a branch into series pieces does not change the convention or methods.
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Keep in mind that in these forums, it is important that folks understand how to measure battery/source power and load power, because often the load they are interested in may not be the only component dissipating power in their circuit. Comparison between "input" and "output" power then becomes very relevant.
Which is the reason that I object to your choice of positive power as that supplied by a source when the common convention for positive power is the quantity dissipated by loads.  If one is intent on educating folks, which is a good thing, teaching them to go against accepted conventions is a recipe for confusion and dissent.
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And there's this:
http://www.overunity.com/14220/power-measurement-basics/msg383962/#msg383962

 

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