Calibration values, who to trust?


I recently purchased the emonTx V3 and I’m trying to calibrate the readings. I’m running a space heater and have a few monitoring devices setup for reference, but I’m not sure to trust. Here’s the readings:

5.91A - Fluke 15B RMS meter in series
6.08A - Cheap clamp-on RMS meter
7.10A - “Killawatt” device
7.07A - emonTX out of the box

I wanted to trust the two meters, but out of the box, the emonTX is really close to the killawatt device. Any ideas on who to trust?


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According to page 15 of the Fluke Manual for the 15B ( the current accuracy is +/- 1.5% + 3 up to 1 year from last calibration. 5.91 +/- 1.5% + 3 = 5.82A - 6.00A 5.79A - 6.03A
If the EmonTX was correct at 7.07A, that means your Fluke is at -19.63% accuracy.
So unless the Fluke is WAAAAY out of calibration, I’d probably be trusting that over the others…


[EDIT] Corrected accuracy details and calculation.

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Welcome, John,

You’re probably finding your way around here, so you’ve not found everything. It’ll take a while - there’s a lot of information here.

Can I point you at the ‘Learn’ section - there are two articles in there, one about the emonTx accuracy and another about meter accuracy and how to interpret it.

So a possibly not minor point, @Greebo, the accuracy isn’t ± 1.5%, it’s ±(1.5 % + 3 digits) - and the 3 digits can make a difference, depending on the actual indicated value.

But as @Greebo says, I’d trust the Fluke - even if it is out of calibration (meaning more than 1 year since it was bought/calibrated).

Which is why I quoted and linked to the manual. All of the supplied values were 3 digits and within the “3 digit” range of the meter, so I didn’t think it necessary to complicate my reply with the additional detail…
I do however, stand corrected, and will try to remember to be more complete in future.

[EDIT] these are the two Learn articles @Robert.Wall mentioned:


Given the choices, I’d go with the Fluke too. A co-worker of mine used to work in our local
calibration lab. He tells me Fluke multi-meters tend to stay close to calibrated settings.

I’ve got a Kill-a-Watt, a cheap clamp-on meter and a Fluke 27.
From what I’ve seen using the KaW and the el cheapo clamp-on meter, both consistently
read different from each other as well as different an emonTx that I’ve calibratedd with the “27.”


I think it’s all subject to suspicion. Kill-a-Watt is usually pretty good. I’ve found the voltage to be very good, and the current is usually in the ballpark. Here are some numbers I just sampled with my [incandescent] lightbulb array on a 120V 60Hz circuit:

Fluke 175 = 2.86A
Kill-a-Watt - 2.84A
WhatsUpPro - 2.89A
IoTaWatt - 2.85

More like your load I connected a space heater:

Fluke 175 = 5.98A
Kill-a-Watt - 5.99A
WhatsUpPro - 6.11A
IoTaWatt - 5.97A

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Were the readings all jumping around or were they pretty steady? If your grid voltage was jumping around a lot then so would the current be, and then it depends on the averaging time of your various meters and whether they were sync’d up to look at the exact same line cycles.

My old Fluke177 hasn’t been calibrated in years, and here’s what it reports when I put exactly 7A through it:

Even the cheap-n-cheery non-true-RMS meter did a half decent job on that measurement. The short cuts it takes to measure AC are designed to work well with the pure sine wave I was feeding it.

Here’s the same Fluke measuring the output of a 20A/333mV CT wrapped around the same 7A signal:

116.6/333*20 = 7.003A.


Measurements were steady.

I’m dealing with an ESP8266 issue, so if I can regain connectivity, I’ll revisit.