# Two meters show different amperage

#### steve66

##### Senior Member
They are both listed as True RMS meters, they both seem to have similar specifications, and their frequency responses should be wide enough to pick up any significant harmonics that typical line voltage appliances would create. So if they are doing a basic current measurement, they should read the same.

But you have the "MAX" function on - that is not giving a real time reading of the current.

So my question is why do you have that "MAX" function on in the first place? That's not reading the current flowing at the moment you are looking at the meter, its reading the highest current that has flowed since the meter was turned on. Again, not a real time reading.

Here is what I mean:

Assume some appliance takes a 50 amp inrush for 0.5 seconds, and then it draws a steady 20 amps as long as its on.

Assume the metes are turned on before the appliance, so they see this inrush. One meter might average readings every 0.5 seconds. So its going to see 50 amps for one sample, and its going to read a MAX of 50 amps.

Another meter might read a little slower, and it might take a sample over 1 second. Then it will see 50 amps for 0.5 seconds, and then 20 amps for 0.5 seconds. If we average those we get (50+20) / 2 = 35 amps. Hey, that's not too far from the actual readings. So that's quite possible.

And I see you have a standard 15 or 20A line cord and plug. One appliance that plugs into that cord should not be drawing more than 20 amps. That's a good clue that all three meter reading we are looking at are not the real time current readings.

So in short, you just need to turn off the MAX function.

#### gar

##### Senior Member
211028-1609 bEDT

I don't believe most responders on this thread really understand RMS, half wave averaging, full wave averaging, calibrated to read the calculated (scaling factor) RMS value from an average value assuming a sine wave, peak value, or maximum value, etc.

Beyond this is how a meter is designed to perform various measurements.

Of the responders here who can reasonably describe how to calculate an RMS value, or how such is measured by various instruments?

What is meant by maximum reading? What is meant by peak reading? What is a ballistic galvanometer?

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#### gar

##### Senior Member
211029-0607 EDT

steve 66:

What does the letter "M" in RMS stand for?
What does MAX or PEAK mean?
What does "real time current" mean?
Why is it not possible to safely have a 50 A peak load on a 20 A circuit? There are may cases where this is a common occurrence. My DeWalt radial arm saw for example.

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#### steve66

##### Senior Member
Gar; You are making this way too complicated.

Lets see some photos of the meters with the Max function turned off.

#### Sahib

##### Senior Member
gar:
Per OP, CL390 and CL900 show almost same MAX readings but not CL700 or CL800 even though all have same sampling frequency. So why don't you consider calibration problems with CL700 and CL800?

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#### gar

##### Senior Member
211029-1011 EDT

steve66:

Why turn off MAX? Joey94 seems to have little knowledge on how his meters work. He is experimenting to see how they function. His post numbered 20 indicates this.

With little other information we can reasonably assume that in normal averaging mode the different meters will be fairly close on a steady state signal.

Thus, it seem reasonable to study how the different meters work in a MAX or Peak mode. Which Joey94 is doing.

I have started some simple experiments with my Fluke 27 in MAX mode. With an unloaded single phase induction motor my starting current is quite constant for about 3 cycles. The next 3 cycles drop a little, then the centrifugal switch drops out. The Fluke 27 reading is way low compared to what it should read for MAX. There is much too long of an averaging time in that meter for MAX to be of much use.

Later I will bring home a Fluke 87 and see how it reads. I believe it is faster from prior experiments.

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#### Sahib

##### Senior Member
There is much too long of an averaging time in that meter for MAX to be of much use.
The meter designer may not be a fool. He might have specified applications for MAX function of the meter to be useful.

#### Joey94

##### Member
They all show almost the same value when measuring in regular mode. Only max mode is different. That’s what I found interesting and thought they should show the same measurements if they have the same sample frequency. If it would be only one of them showing different i would think it’s a problem of the clamp meter or just not calibrated or something. But both cl800&cl700 show the same but almost double than cl900 or cl390. It was all just curiosity to understand why this is happening. I’m trying to find answers to everything that I don’t understand and make sense of it.

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#### Joey94

##### Member

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#### Sahib

##### Senior Member
They all show almost the same value when measuring in regular mode. Only max mode is different. That’s what I found interesting and thought they should show the same measurements if they have the same sample frequency. If it would be only one of them showing different i would think it’s a problem of the clamp meter or just not calibrated or something. But both cl800&cl700 show the same but almost double than cl900 or cl390. It was all just curiosity to understand why this is happening. I’m trying to find answers to everything that I don’t understand and make sense of it.

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You may also want to get a clarification from the meter dealers/suppliers and reproduce it here.

#### ptonsparky

##### Senior Member

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Entirely acceptable.

Maybe.

I ran those numbers on a spreadsheet that would calculate the current imbalance as if those were the current levels of a 3 phase motor.
Those readings would show a 2.3% imbalance. You would not be able to use those three meters to take simultaneous readings of each leg.

IMO, Even for the average joe, they don't fill the bill. Then again, if you aren't involved in looking for current or voltage imbalance, they are good enough. You need three of one model to compare.

#### LarryFine

##### Master Electrician Electric Contractor Richmond VA
"A man with one watch can tell you what time it is. A man with two watches is never sure."

#### Sahib

##### Senior Member
Those readings would show a 2.3% imbalance.
It is current unbalance and not voltage unbalance to be alarmed.

#### Dsg319

##### Senior Member
"A man with one watch can tell you what time it is. A man with two watches is never sure."
The be safest of the two, I never wear one

#### synchro

##### Senior Member
"A man with one watch can tell you what time it is. A man with two watches is never sure."
But the guy with 10 watches up his arm will tell you that they all keep good time.

#### ptonsparky

##### Senior Member
It is current unbalance and not voltage unbalance to be alarmed.
I'm not an EE by any stretch of the imagination but my little charts and consequently spreadsheet flags me if either is 2% or greater.

Sort of a chicken or egg thing for me. Does the V cause the I, or I cause the V. I suspect either.

When I walk up to an unloaded panel expecting 277 to ground, I do not want to see, 265, 277, and 268. 2.59% = Investigate.

A motor with 23.4, 22.5 and 22.7 amps respectively. 2.33% = Investigate. Voltage may or may not be all 277 to ground or 480 phase to phase.

#### Sahib

##### Senior Member
Better start investigation beyond 10% current unbalance per NEMA MG1.

#### gar

##### Senior Member
211030-1309 EDT

I have now run some limited controlled tests on two Fluke meters. These meters are somewhere in the 20 to 30 year age range. One is a 27 and the other an 87. These were 120 V AC 60 Hz nominal, and 12 V DC nominal. AC actual was close to 123 V, and DC about 12.4 V.

The results were as expected:

Closure AC ....... 87 ............. 27
Time mS

120 ms ........ 98.4 ........... 42
200 mS ........ 119.2 ....... 74
300 ms ......... 122.4 ....... 92
350 ms ......... 122.4 ....... 105
500 mS ........ 123.2 ....... 122

Closure DC

120 mS ........ 11.0 .......... 4.0
200 mS ........ 12.0 .......... 6.2
300 ms ........ 12.2 .......... 8.9
350 mS ........ 12.2 ......... 9.4
500 mS ........ 12.3 ......... 12.2

You can easily see that the 27 has a longer averaging time constant than the 87.

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#### gar

##### Senior Member
211030-2145 EDT

Since there have been no comments on my previous post I don't know whether anyone understood the results, or implications, or how or when these measurements would be useful.

If you do not understand my data, then I need to know this. Also when would any of you use these measurement modes in practice? This question is based on the long time average of the measurement. Meaning short duration signals would give grossly incorrect readings. So I would not get a good starting current reading for my test induction motor with the Fluke 27, and not real good with the Fluke 87.

Note: the 27 is a full wave rectified average reading meter in AC mode, but it is calibrated in RMS for a sine wave. If the waveform was non-sinusoidal, then an adjustment factor would be required. The 87 is a so-called true RMS meter. For both of these meters the displayed numeric value is RMS.

Only the 87 has a peak mode, and it is implied as 1 mS response time. I tested this with a 12.4 V battery. My first test was with a 1 k ohm resistor shunting the meter, and a 1 ufd capacitor charged to 12.4 V. The reading was 10 V, made sense. Changed the resistor to 10 k and the reading was higher as expected. Note: the time constant of 1 ufd and 1 k is 1 mS. Thus, the meter response time was faster than 1 mS.

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