LED bulb question

[Ether]

I just tested a 750/1500 watt dual heat resistive space heater with the Kill-A-Watt. The readings made sense. Power Factor 1.00 (as expected), and watts approximately 765 (on low) and 1443 (on high).

When I get a chance, I'm going to test a larger sample of incandescent light bulbs of various brands and power ratings.

[Richard Wallace]

I just tested three GE Reveal incandescent light bulbs with my Kil-A-Watt.

The 29W bulb measured 28 Watts, 0.23 Ampere, 1.0 power factor.

The 60W bulb measured 60 Watts, 0.5 Ampere, 1.0 power factor.

The 75W bulb measured 76 Watts, 0.63 Ampere, 1.0 power factor.

As expected.

[Ether]

Originally Posted by Richard Wallace

I just tested three GE Reveal incandescent light bulbs

I don't have any GE Reveal here, but I tested a bunch of GE Soft White incandescents and they all measured roughly half their wattage rating (with PF=1.00).

Do you have any GE Soft White bulbs? If so, could I ask you to test them?

[Mr V]

What about the LED bulbs that started this discussion, have you tested them? Inquiring minds want to know.

I do have to wonder if there is a problem with Ether's Kill-a-watt in accurately measuring at the lower end of the range. I was under the impression that the difference between the standard, soft white and reveal bulbs was purely the coating on the inside of the glass and not due to a different filament. So in theory they should all match their wattage ratings within the expected tolerance of a high volume mass produced item at low price point.

How about plugging a power strip in with that heater on low and a lamp with the bulb and comparing the results with and without the bulb on?

[Ether]

Originally Posted by Mr V

How about plugging a power strip in with that heater on low and a lamp with the bulb and comparing the results with and without the bulb on?

Thanks for the excellent suggestion.

34.2W bulb only (60 watt soft white incandescent)

755W heater only

811W heater plus bulb

811-755 = 56W

Power Factor was 1.00 for all readings above.

So in theory they should all match their wattage ratings within the expected tolerance of a high volume mass produced item at low price point.

Spec accuracy is listed as 0.2%

I think I may have a defective unit.

[Mr V]

I was thinking about the bulb's tolerances, but the Kill-a-watt's tolerance is an important part of the equation as well.

Is there anything in the documentation that would indicate the tolerance is different at different parts of the range? If it is just a stated .2% then I'd have to agree that you seem to have a defective unit based on Richard's results with his. +/- 1 Watt seems to be as good as would be expected on a product like an incandescent bulb.

[FrankJ]

Have you tried measuring the bulb resistance with a DVM?

[Ether]

Originally Posted by FrankJ

Have you tried measuring the bulb resistance with a DVM?

The resistance of the bulb changes substantially as the filament gets white hot.

I don't have a DVM that uses 120 volts to measure resistance.

.

[Ether]

Originally Posted by Ether

I don't have a DVM that uses 120 volts to measure resistance.

... and the inexpensive DVMs I have access to don't support measuring AC current with 120V applied.

[FrankJ]

Originally Posted by Ether

The resistance of the bulb changes substantially as the filament gets white hot.

I don't have a DVM that uses 120 volts to measure resistance.

.

There is always the amp function of your DVM.

The cold resistance gives you a reference to tell if things are really strange. You can also make some assumptions and do a little math and see if a simple physics model matches reality. Tungsten (assuming your filament is really tungsten) has a resistance temperature Cef of 4.5E-3 at 20C. (Assuming it is linear all the way to white hot is a big assumption at least for me.) Using 120 VAC for the rated voltage 2500K as the bulb temp and a little excel math I get about 22 ohm cold. Am I close? Of course with a 240ish ohm hot resistance the cold resistance doesn't tell you that much without a tested model.

[Ether]

Originally Posted by FrankJ

There is always the amp function of your DVM.

None of my 8 (inexpensive) various models of digital or analog VMs measure AC amps in the desired range.

The cold resistance of 60W soft white incandescent bulb measures approx 18 ohms.

Don, if you're still reading this thread: does the label on the back of your Kill-A-Watt say model P4400 (like Richard's) or P4400.01 (like mine) ?

[Ether]

Originally Posted by Mr V

Is there anything in the documentation that would indicate the tolerance is different at different parts of the range?

The max watts spec for the unit is 1800W.

So even if 0.2% means 0.2% of 1800, the unit is still way out of spec.

[Ether]

I just called the tech support number and the friendly fellow who answered wasn't able to shed any light on the situation.

.

[Ether]

Richard, If you are still following this thread, could you please take a peek at the label on the back of your Kill-A-Watt and compare it to this and flag any differences for me? Note especially the red arrows.

I'm wondering if your unit is an improved version.

I bought mine at Menard's, and they appeared to have been hanging on the rack for quite some time (packaging covered in fine dust). Also, the printing on the cardboard label inside the thermal-welded plastic packaging is copyright 2011.

[DonRotolo]

Data point: My Kill-a-watt is wildly inaccurate at very low (< 10W) power levels: Some of my light bulbs don't even register any reading. (This is based on measurements more than a year ago. I should try to repeat them now).