Why don't multimeters measure watts? - Page 1
Forum section: electronics / beginners

author: N0_W4RN1NG
author_rank: Newbie
timestamp: February 11, 2023, 09:38:04 pm
content: It seems super basic but I've trawled all over google and can't find a straightforward answerThere are many power meters that plug into a wall socket or in your breaker box which can measure watts, volts, amps, frequency and power factor simultaneously. They are also very affordable ($10 for a small DC one that's essentially a screen and two leads, $20 for an AC wall outlet one and $80 for a HOPI like big Clive uses, all AUD.)This seems to be a very handy feature that is missing from multimeters. You can't even measure volts and amps simultaneously on clamp meters, which have two methods of measurement, so in theory (in my mind at least) they should be able to do it.So why don't any DMMs do this? Even the highest end ones with big screens don't have these features. Is it perhaps due to historical reasons? Technical challenges? Is it impractical for some reason?I'd love to know.

author: themadhippy
author_rank: Super Contributor
timestamp: February 11, 2023, 09:44:38 pm
content: youd need to double up a lot of the circuitry,one half to measure the current and the other half the voltage,and thats before you get to the risk of getting the probes mixed up

author: artag
author_rank: Super Contributor
timestamp: February 11, 2023, 09:51:04 pm
content: I've got a Moshimeter. It's a neat little bluetooth meter that can measure two parameters at the same time. Never tried to use it for watts but it should be able to do it, I guess.

author: Monkeh
author_rank: Super Contributor
timestamp: February 11, 2023, 09:52:53 pm
content: Quote from: N0_W4RN1NG on February 11, 2023, 09:38:04 pmThis one does:https://www.gmc-instruments.de/en/products/measuring-and-test-technology/multimeters-and-handheld-calibrators/special-applications/multimeters-for-power-and-energy-measurement/metrahit-energy/Oh, and this one:https://www.fieldpiece.com/product/sc480-wireless-power-clamp-meter/Then there's this:https://www.hioki.com/euro-en/products/pqa/portable/id_6649Oh, right, Metrel:https://www.metrel.si/en/shop/DMM/clamp-meters/md-9272.htmlhttps://www.metrel.si/en/shop/DMM/clamp-meters/md-9240.htmlThe list goes on. There's a number of options on the market.Now, as to the why you don't see it in every meter: Because it isn't really needed. There are much more convenient tools for measuring the power consumption of appliances, and there are specific instruments for proper measurements of AC power on the bench. There's a small need in industrial applications.. which is met.

author: joeqsmith
author_rank: Super Contributor
timestamp: February 11, 2023, 09:54:56 pm
content: An don't forget Dave's 121GW but its pretty limited:

author: mwb1100
author_rank: Frequent Contributor
timestamp: February 11, 2023, 10:08:12 pm
content: Quote from: themadhippy on February 11, 2023, 09:44:38 pmWouldn't it be done with the meter probes in the current positions? If so there's no more risk than already exists.

author: Monkeh
author_rank: Super Contributor
timestamp: February 11, 2023, 10:10:50 pm
content: Quote from: mwb1100 on February 11, 2023, 10:08:12 pmYou can't really measure power accurately down several metres of relatively thin cable. Plus you invite error - 'oh it reads voltage in power mode' *bang*.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 11, 2023, 11:15:38 pm
content: The reasonable quality AC power measurement works quite a bit different from most DMMs:The DMMs usually convert the things to measure to DC voltage and than have a relatively slow, high resolution ADC.The power meters usually have a specialized chip made for energy metering with 2 relatively fast ADCs and than digital math to calculate the average power over some time.So there is quite some extra effort for measuring AC power. This usually takes extra chips and these take extra power, which can be a problem with battery supply.There is some extra risc involved in using the meter with 3 cables on mains - little more tricky than measuring mains current. The ready made power meters are quite a bit safer, though the cheap ones are missing a lower current range. The more realistic point would be building such a cheap meter with also a low current range.

author: golden_labels
author_rank: Super Contributor
timestamp: February 12, 2023, 12:03:42 am
content: DMMs would need to have additional circuitry to measure phase difference between the inputs. And that for pure sine waves only. For True RMS meters this would become much worse. So while certainly possible, I am not surprised manufacturers aren’t flocking to implement the idea.Quote from: mwb1100 on February 11, 2023, 10:08:12 pmIf you connect current probe to where voltage probe goes, your power measurement will be somewhere between a loud soud and a bright flash, depending on the quality of the meter.

author: bdunham7
author_rank: Super Contributor
timestamp: February 12, 2023, 12:49:49 am
content: Quote from: N0_W4RN1NG on February 11, 2023, 09:38:04 pmTo reiterate and rephrase what Kleinstein said, most DMMs actually don't have anything in them that is useful for power-related measurements. In order to get power, you have to measure current and voltage simultaneously, and to get AC power right, you have to make these simultaneous measurements very rapidly--up to thousands of times per second. Typical handheld DMMs make just a few readings per second on one channel. Some DMMs do have power capability, as listed above, but that is really a separate feature using additional circuitry, not just leveraging features of an ordinary DMM.

author: mikerj
author_rank: Super Contributor
timestamp: February 12, 2023, 10:20:20 am
content: Quote from: mwb1100 on February 11, 2023, 10:08:12 pmYou need to measure voltage and current simultaneously to calculate power, so you would need probes in the current, voltage and common connectors. Getting the voltage and current probes swapped on a high energy supply would be a noisy and exciting event. The majority of common DMM chipsets can't measure two parameters at once, and their sampling rate is pretty low.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 12, 2023, 10:40:15 am
content: There are a few of the moden DMM chip sets that do digital RMS with a 2nd relatively fast (e.g. 10 kHz range ?) ADC. The step from digital RMS to doing power measurements is not that large.It is even possibly to do a reasonable, though not good power measurement with just a 10/12 bits ADC of a µC.The DMM part at least already contains the current shunts and votlage divider. So some parts can be reused, but it is still quite some extra effort.With mains voltage the noise from the extra connection to the voltage and using the COM terminal for both current and voltage is not that bad, as the error is relative to a large voltage. This can however be an issue for lower voltage uses (e.g. 3.6 or 5 V input to an SMPS). In the SMPS example there is an additional difficulty with the higher frequency part and thus the need to measure voltage and current quite fast. In addition the inductance of the cables and burden voltage can become an issue.For measuring mains power consumption the purpose made energy meters are just much more convenient from the connectors. It is just a pitty the cheap one are limited to a single current range and just not good enough to check low power, like standby power or small LED lamps.

author: bdunham7
author_rank: Super Contributor
timestamp: February 12, 2023, 03:15:46 pm
content: Quote from: Kleinstein on February 12, 2023, 10:40:15 amYou would need a second digital RMS ADC or else be able to switch the inputs very rapidly. I don't know how the DMMs that do have it accomplish this. The 121GW has published schematics and such, so perhaps we could look. In any case, as long as you can properly scale the inputs I think a good 10-bit ADC is probably adequate since the LSB would represent ~0.1% error, as good or better than many current shunts and mid-level DMMs. I don't feel there's much market for this capability other than the low-end and hobby business or some specialized field work because other instruments are available. Any DSO that can do basic math can do power. A more advanced one that has a deskew function can do power really well, even at SMPS frequencies.QuoteThe P3 KillAWatt is single range but has resolution of 10mA, 100mV and 100mW/VA and works pretty well for all but the tiniest of loads. Some others, like those modules you get on eBay, are horribly terrible in comparison.