Can electric energy be directly measured?

[NotionCommotion]

I expect that power monitors directly measure the current, voltage change, and maybe phase to directly calculate current power use.

Can electric energy directly be measured, or do meters just calculate power, and integrate it to obtain energy?

 

[russ_watters]

I expect that power monitors directly measure the current, voltage change, and maybe phase to directly calculate current power use.

Can electric energy directly be measured, or do meters just calculate power, and integrate it to obtain energy?

I guess it may depend on what you mean by "directly", but old style meters spun like clocks.

 

[NotionCommotion]

Thanks Russ, Sounds like the old meters just measured power, and used this spinner as an integrator. I suppose someone could make some direct monitoring energy meter by comparing the energy to a known energy value (which doesn't sound like a good idea), but I am really interested in "typical" meters. I will conclude that power is measured and energy (change) is just the integral of power.

 

[sophiecentaur]

Thanks Russ, Sounds like the old meters just measured power, and used this spinner as an integrator.

Energy is not transferred instantly so a conventional spinning disc will give the Energy supplied in a given time. (Integrated, as you say) Isn't that what you need?
Electricity companies swear by them. They have a very good range of measured power, from much less than 1W to tens of kW.
Edit: These days you can get AC power meters that measure V and I and do the calculation V.I, which is power, of course.

 

[Merlin3189]

I guess it may depend on what you mean by "directly", but old style meters spun like clocks.

Counting the rotations integrates the speed of rotation of the disc, which is proportional to power derived from current and voltage coils..

But the original question seems odd to me. What does he want the meter to show? And what is it he's objecting to in the measurement - is it the integral or the fact that power is found by measuring current and voltage?

If he wants it to show energy transferred along the wires, then that has to be over a period of time (maybe, though not necessarily, from switch on up to the current time.) But that is implicitly an integral, no way round it. (If power is constant or varying linearly with time, you can do the maths by multiplication, but that's just because it's a trivial integral.)

If he wants to measure energy directly*, not via voltage and current vectors, then can he show me some energy and tell me about its measurable properties? What would it mean to measure energy directly? Or momentum or velocity or resistance ...? Even a fundamental quantity like current is measured in terms of a force. Very few things we can actually measure directly - only mass and length spring to mind - and that's because there's an actual lump of mass we can pick up and compare: even length is dubious, because I can't imagine how we can compare lengths with the modern standard metre.

* I see OP now suggests "comparing the energy to a known energy value" The question is not hether it is a good idea, but what is energy? How can you have a standard amount? How can you compare energy. And remember, in your context, the energy that went down the wires may have been rotational KE when it started and may now be something else, heat, chemical, KE, gravitational PE, light, probably some mixture of them. So what are you comparing to what?

I can't think of any situation where you can measure energy or even power directly. You measure other quantities and infer energy or power.

 

[russ_watters]

Thanks Russ, Sounds like the old meters just measured power, and used this spinner as an integrator... I will conclude that power is measured and energy (change) is just the integral of power.

That seems arbitrary and argumentative at best. Since power and energy are so related, you could take any meter that measures one and argue it is "really" measuring the other, using the same logic. But really, I think it is wrong:

Counting the rotations integrates the speed of rotation of the disc, which is proportional to power derived from current and voltage coils.

By what means is it measuring time?

 

[russ_watters]

Here's a similar device:

Can someone explain to me how this device, with no electronics, measures speed and time and then performs multiplication or integration?

 

[Merlin3189]

By what means is it measuring time?

Sorry, don't follow that one?
I just thought that number of rotations = ∫ ω dt so the spinning disc rotates the reducing gears to count turns, 10x turns, 100xturns, 1000x turns, etc for the dials
Since ω ∝ power, energy used = ∫ ω dt
We get the definite integral by subtracting the previous reading from the present reading.

Why do I need to measure time? The energy meter is just recording the amount of energy that has passed through. It starts at an arbitrary value, which is the amount of energy passing through during adjustment and acceptance testing before installation. (or much higher if it is a reconditioned unit.)
I guess that modern digital meters actually can be reset to zero at the start of a measurement period.

 

[Merlin3189]

Can someone explain to me how this device, with no electronics, measures speed and time and then performs multiplication or integration?

I've no idea how it measures time.

Like the disc in the meter, some external agency makes it rotate. In the watt meter it is electricity. Here it is someone pushing it along the road.
The little counter attached is made of reducing gears which the wheel makes to rotate.
The speed of the rotating wheel ω(t) is integrated by counting the rotations. So the counter tells you $\int_{t=0}^{now} ω(t) \, dt$
You do need to read the counter at the start of your trip, to find $\int_{t=0}^{start} ω(t) \, dt$
Then $\int_{t=start}^{now} ω(t) \, dt = \int_{t=0}^{now} ω(t) \, dt - \int_{t=0}^{start} ω(t) \, dt$ = distance travelled, which was what was wanted.

It doesn't tell you the speed (well, any speed) but if you had a watch and recorded the time at the start and end, you could divide the distance by the difference in time to find your average (mean) speed.

If you wanted it to multiply, the only way I can think of off hand, would be to mark out a length representing one number, then walk the device along it the other number of times. Of course you want the latter number to be an integer, so you have to scale it as we had to on computers when we had only integer arithmetic available.

Edit 1: corrected error in w⁵
Edit 2: I forgot to put constants in my integrals to allow for conversion from radians to turns (2π) and from turns to metres (circumference of wheel.)

 

[NotionCommotion]

Hi All!

It has been a while since I studied this this subject. A real long time! My purpose of this question was just to reafirm my understanding.

@sophiecentaur, Yes, integrated is what I need. Actually, just confirmation that such is what I need.

@Merlin3189. Even mass and length are just viewed under our perceptive viewpoints, so they too are relative. But if we take typical normal assumptions of our view point, I agree energy can't be directly measured, but why not power?

@russ_watters. Do you know of a practical direct energy meter? It is measuring time applied by given energy. And given average power and agreed upon units, we can determine time.

@russ_watters Because of the diameter of the circle and assigned units.

@Merlin3189. No comment.

Thanks!

 

[Windadct]

I am wondering in what case can any form of energy be measured in a single parameter, i.e. directly.

Kwh meters have a voltage and current coil, and use a magnetic circuit to multiply them.

 

[NotionCommotion]

Thanks Windadct, i misspoke about my perceived definition of "directly". I acknowledge that I am measuring current and voltage but see that as directly. I feel when I ride my bike, I can measure distance and measure time and measure speed directly. But not the same with energy, and can only monitor current and voltage (or heat created) but need to apply time to get energy (unlike my bike distance analogy).

 

[russ_watters]

I've no idea how it measures time.

The answer is that it doesn't measure time.

Like the disc in the meter, some external agency makes it rotate. In the watt meter it is electricity. Here it is someone pushing it along the road.
The little counter attached is made of reducing gears which the wheel makes to rotate.
The speed of the rotating wheel ω(t) is integrated by counting the rotations. So the counter tells you $\int_{t=0}^{now} ω(t) \, dt$

Not by that device it isn't. The fact that the mathematical relation between speed and distance exists does not imply that any particular device is using it.

It doesn't tell you the speed (well, any speed) but if you had a watch and recorded the time at the start and end, you could divide the distance by the difference in time to find your average (mean) speed.

Right. It measures distance and in order to find speed, you need to separately measure time and divide.

Why do I need to measure time? The energy meter is just recording the amount of energy that has passed through.

The OP claimed that an energy meter measures power and integrates over time (which it also must measure in order to do the integration) to get energy. I disagree: I think it measures energy directly. It appears you agree with me...

 

[russ_watters]

@russ_watters. Do you know of a practical direct energy meter?

Certainly:
http://www.gordonelectricsupply.com/index~text~5777163~path~product~part~5777163~ds~dept~process~search?gclid=Cj0KCQjwh_bLBRDeARIsAH4ZYEPt_K8PYUzzBwHgZcBXZldh0EmsOjF-DzgfM7GexAKDJ-CerZQ5K6AaAmVDEALw_wcB

It is measuring time applied by given energy.

I'm afraid that makes no sense.

And given average power and agreed upon units, we can determine time.

Since it only measures energy, not power, you can't use it to measure time in that way. If you connect a load with a known constant power, then you could.

@russ_watters Because of the diameter of the circle and assigned units.

Right: so it doesn't measure speed and time to find distance, it measures distance directly. Just like an old-style electric meter.

 

[russ_watters]

I feel when I ride my bike, I can measure distance and measure time and measure speed directly.

If you are measuring distance and time and then calculating speed, doesn't that violate your criteria for "directly" measuring speed? It's time and distance that you are measuring "directly" and speed "indirectly" (by calculation), isn't it?

I acknowledge that I am measuring current and voltage but see that as directly.

But not the same with energy, and can only monitor current and voltage (or heat created) but need to apply time to get energy (unlike my bike distance analogy).

Old-style rotating meters don't measure current, voltage or time.

 

[sophiecentaur]

I think the OP is on a hopeless mission here. He hasn't really defined what he wants and is trying to scratch an intuitive itch. We have all been there at one time or another, when we think we have hit on something fundamental. That itch will go away if he reads all the above posts carefully and with an open mind.

 

[Merlin3189]

I apologise to Russ for not taking him seriously. I thought #7 was intended to be facetious and replied in similar vein.

Two things made me just take for granted the role of a such a device as an integrator. One was using a planimeter back in the 60's to measure the area under a graph. It resembled Russ' device - at least it had a wheel you slid across the paper, which made a dial go round - and the result was what we would have obtained had there been a known function for the graph and we'd integrated it.
The other thing was the descriptions of energy meters, which all seem to work by adding up the sequence of power values.

To me, both devices seem to be doing integration, or at least summation of the products of rates and a very small interval. But I can see that I need to give it a bit more thought to either come up with some more substantial argument, or realize my mistake.

 

[russ_watters]

I apologise to Russ for not taking him seriously. I thought #7 was intended to be facetious and replied in similar vein.

Fair enough...since I am often sarcastic, it sometimes gets missed when I'm not.

Two things made me just take for granted the role of a such a device as an integrator. One was using a planimeter back in the 60's to measure the area under a graph. It resembled Russ' device - at least it had a wheel you slid across the paper, which made a dial go round - and the result was what we would have obtained had there been a known function for the graph and we'd integrated it.

Without looking too much into how they work, it does suggest to me something like a slide rule, which does indeed perform calculations. The distance wheel, though, doesn't; it just counts.

 

[NotionCommotion]

@russ_watters. Do you know of a practical direct energy meter?

Certainly:
http://www.gordonelectricsupply.com/index~text~5777163~path~product~part~5777163~ds~dept~process~search?gclid=Cj0KCQjwh_bLBRDeARIsAH4ZYEPt_K8PYUzzBwHgZcBXZldh0EmsOjF-DzgfM7GexAKDJ-CerZQ5K6AaAmVDEALw_wcB

While I don't agree this device measures the absolute energy value, I agree it measures change in energy. I believe my source of confusion related to thinking change of time was the x-axis which isn't the case thus it isn't displaying power. But it still seems to me that it must be monitoring power internally, and not energy.

I've never built an energy (oops, I almost said power) meter, but if I did, I suppose I would measure the current by inducing some secondary loop, measure the voltage drop, use a chip or circuit to multiple the two values to determine power, and then integrate power over time using a chip, spinning dial, etc.

Where am I going astray?

 

[russ_watters]

While I don't agree this device measures the absolute energy value...

What is an "absolute energy value" and what does it have to do with your question.

Where am I going astray?

I think you keep moving the goalposts and are using a meaningless/reversible definition in order to keep getting the answer you want?

 

[Merlin3189]

Putting aside my theme for now,

@russ_watters. Do you know of a practical direct energy meter?

Certainly:
http://www.gordonelectricsupply.com/index~text~5777163~path~product~part~5777163~ds~dept~process~search?gclid=Cj0KCQjwh_bLBRDeARIsAH4ZYEPt_K8PYUzzBwHgZcBXZldh0EmsOjF-DzgfM7GexAKDJ-CerZQ5K6AaAmVDEALw_wcB.

Isn't that the meter that OP originally described? Or was he talking about a digital Wattmeter?

So are you saying that "old style meters <that> spun like clocks" (as in your link) are what you regard as directly measuring energy?

 

[russ_watters]

Isn't that the meter that OP originally described?

Possibly, yes - or a digital version.

Or was he talking about a digital Wattmeter?

Unlikely since a watt meter, at face value, measures watts. Though he mixed and matched his terms, I think he was claiming an energy meter "really" measures power.

So are you saying that "old style meters <that> spun like clocks" (as in your link) are what you regard as directly measuring energy?

Yes. I would say that if a meter "actually" measures power and converts, there would be a way to intercept the power reading and read it out before the conversion. I don't think that is possible here...

...but you could add a device that measures the rotation rate and converts to watts. But it would be an additional device, either measuring energy and time and converting to power or measuring power more directly by using the rotation rate to generate magnetism to move a needle against a spring, like a speedometer.

Regardless of one's interpretation though, I can't imagine why any of this matters!

 

[NotionCommotion]

@Russ. Stated goalpost has always been "Can electric energy be directly measured?" Underlining goal was to improve my understanding. I am not necessarily claiming that an energy meter always measures power, but stating that I don't understand how any meter can directly measure energy. Maybe my use of the word "directly" is causing the confusion, and if so, I apologize, and tried to provide a better definition under my response to Merlin.

@Merlin. The link refers to the meter which Russ stated directly measures energy. No, nothing to do about spinning clocks. For an instance in time, I can measure some properties like voltage and current (and thus power), temperature, pressure, etc. Can one for a single point in time measure energy. I guess distance is kind of like energy as it must be relative to a starting point and if I could measure velocity, I could determine distance by integrating v(t)dt. However, with distance, I can also use a tape measure and get a value. Does a tape measure for energy exist which for a single moment in time spit out "the energy value is 4123 jewels"?

 

[jim hardy]

Hmmm I've been musing over this one trying to think of a mechanical analogy for Poynting vector..

 

[sophiecentaur]

Hmmm I've been musing over this one trying to think of a mechanical analogy for Poynting vector..

Force, perhaps?

 

[NotionCommotion]

Hmmm I've been musing over this one trying to think of a mechanical analogy for Poynting vector..

Never heard of a Poynting vector (or if I did, I forgot), but it sounds a lot like torque. How is it relevant?

 

[jim hardy]

Force, perhaps?

I was thinking more of a contraption to measure E and B fields and calculate the Poynting vector.
https://www.britannica.com/science/Poynting-vector

In high school(1963) we measured E and B fields along a slotted RF transmission line using a capacitive and an inductive pickup.
Seems now if we'd multiplied them we'd have been close. But analog multipliers back then were time division vacuum tube devices suited for sub-audio basically DC servo control.

Also, relation between permeability and permittivity gets down to the very fabric of space and my little brain gears start jumping teeth.

There's hope though. A year ago i didnt know the term Poynting Vector and now you PF guys have me thinking about them.

Thanks All

old jim

 

[jim hardy]

Never heard of a Poynting vector (or if I did, I forgot), but it sounds a lot like torque.

Hmm it does sort of resemble moment of inertia of a gyroscope, doesn't it ?

Thanks - more food for thought.

 

[russ_watters]

@Russ. Stated goalpost has always been "Can electric energy be directly measured?"

And yet:

Can one for a single point in time measure energy. I guess distance is kind of like energy as it must be relative to a starting point and if I could measure velocity, I could determine distance by integrating v(t)dt. However, with distance, I can also use a tape measure and get a value. Does a tape measure for energy exist which for a single moment in time spit out "the energy value is 4123 joules"?

Perhaps you intended this angle from the start, but bringing it in later makes it sound like goalpost shifting...and one I disagree with as violating the original intent: A tape measure is used for short distances, but a distance wheel is used for long distances because tape measures are impractical for long distances. That doesn't change the fact that a distance wheel "directly" measures distance.

In either case, "the energy value is 4123 joules" is not a meaningful statement unless provided with more context; energy of what? One could similarly envision asking the driver of the car you are a passenger in: "what is the distance value?" and him responding: distance of what?

Underlining goal was to improve my understanding.

I'm not sure of what, but based on the discussion I don't think it is a very enlightening discussion, so I'm not sure it is going to help much in increasing your understanding of anything except by accident. Ie, it might help you understand better how an electric meter works, but it would have been more efficient to just ask "how does an electric meter work?" Incidentally:
https://en.wikipedia.org/wiki/Electricity_meter#Electromechanical_meters