Bought a clamp meter, question.

[agent00f]

Depends what you're trying to do. If you're just measuring current then a multimeter is probably more accurate, but if you want to measure power then a multimeter is inappropriate because it can't measure power factor.

From what I read the Killawatt is not very accurate. I've never tested one myself though. For power factor you just want to make sure you have a true RMS meter.

The killawatt I recollect was just a simple shunt design, it's generally speaking no more or less accurate than in-lining through a typical dmm given that does the same. I don't know how good these cheap clamps are, hard to imagine it's any better.

Isin't that what a true RMS meter does though? I thought that was the whole idea of getting one, so that the amp reading you get is corrected for the power factor.

True RMS is for non-sinusoidal waves, not the same thing as power factor for nonaligned V/I waves.

 

[John Connor]

This is my meter. Although not a Fluke, it works well.

http://www.eevblog.com/forum/testgear/uni-t-ut204-clamp-meter-review-and-tear-down/

 

[Mr Evil]

Isin't that what a true RMS meter does though? I thought that was the whole idea of getting one, so that the amp reading you get is corrected for the power factor.

I probably can't explain it any better in a single forum post than the Wikipedia article I linked to above, but imagine connecting a capacitor to an AC voltage source. As the voltage rises, current flows into the capacitor, storing energy. When the voltage falls, current flows out again and the energy is returned to the source. Thus no power is used, but you can measure a current. If you multiply that by the voltage you get a number that looks like power, but isn't. It's called "reactive, or "imaginary" power, and has units of VA (volt-amps). This is true even if the voltage and current are both sine waves, so a true RMS meter doesn't help you there.

Elecrical appliances will use some imaginary power, and some real power. The ratio between them is the power factor. Good quality computer power supplies under normal loads have a power factor close to 1, so simply multiplying current by voltage will be close to the real power. Under other conditions the power factor can be a lot less than one, which will make it appear as if its using more power than it really is (for instance you may be able to measure a significant current when the power supply is off, due to the filter components).

The killawatt I recollect was just a simple shunt design, it's generally speaking no more or less accurate than in-lining through a typical dmm given that does the same...

I have a power meter very similar to a kill-a-watt, and it measures watts, VA, and power factor. I can't vouch for its accuracy, but it gives believable readings.

 

[agent00f]

I probably can't explain it any better in a single forum post than the Wikipedia article I linked to above, but imagine connecting a capacitor to an AC voltage source. As the voltage rises, current flows into the capacitor, storing energy. When the voltage falls, current flows out again and the energy is returned to the source. Thus no power is used, but you can measure a current. If you multiply that by the voltage you get a number that looks like power, but isn't. It's called "reactive, or "imaginary" power, and has units of VA (volt-amps). This is true even if the voltage and current are both sine waves, so a true RMS meter doesn't help you there.

Elecrical appliances will use some imaginary power, and some real power. The ratio between them is the power factor. Good quality computer power supplies under normal loads have a power factor close to 1, so simply multiplying current by voltage will be close to the real power. Under other conditions the power factor can be a lot less than one, which will make it appear as if its using more power than it really is (for instance you may be able to measure a significant current when the power supply is off, due to the filter components).

Personally I don't like the "imaginary" nomenclature because it's more the result of insufficient measurement or mismeasurement than anything to do with abstract imagination; kind of gives the wrong idea.

I have a power meter very similar to a kill-a-watt, and it measures watts, VA, and power factor. I can't vouch for its accuracy, but it gives believable readings.

I meant the current/voltage measurement method is the same, in contrast to a clamp. These kill-a-watts probably have some simple sampling or one of those multi-dmm tricks to measure pf.

 

[John Connor]

So I measured the current from a Dell netbook. The amps were about .130 milliamps. This is about 15 watts. Is this wrong?

 

[agent00f]

Seems feasible. Most of the power in modern portable computer goes to the backlight.

 

[John Connor]

Except I'm using an external monitor and keyboard.

 

[agent00f]

Modern portables don't use that much power on idle. Switching power supplies can have strange waveforms, but shouldn't be off by too much even if it's off.

 

[NeoPTLD]

Go buy a Kill-A-Watt which has a DSP to compute true wattage if you want to measure computers. You need the voltage, RMA amps and PF and Kill-A-Watt is one of the few tools available that can do this.

 

[CountZero]

Personally I don't like the "imaginary" nomenclature because it's more the result of insufficient measurement or mismeasurement than anything to do with abstract imagination; kind of gives the wrong idea.



I meant the current/voltage measurement method is the same, in contrast to a clamp. These kill-a-watts probably have some simple sampling or one of those multi-dmm tricks to measure pf.

Its been almost 20 years since I took the class but I thought it was called imaginary due to the vector math used to calculate this stuff where some portion is on the real axis and some on the imaginary axis.

 

[agent00f]

Its been almost 20 years since I took the class but I thought it was called imaginary due to the vector math used to calculate this stuff where some portion is on the real axis and some on the imaginary axis.

Yeah, it's used a lot as a tool in in ee but the nomenclature tends to give students the wrong idea.

 

[Bryf50]

So I measured the current from a Dell netbook. The amps were about .130 milliamps. This is about 15 watts. Is this wrong?

Except I'm using an external monitor and keyboard.

Seems about right. What are you expecting, more or less?

 

[John Connor]

Well, given that all the "experts" were saying that I wouldn't get a true watt reading, or amp reading in this case, I was thinking perhaps my netbook wasn't consuming 15 watts?

 

[agent00f]

In most cases it should be reasonably accurate.