Why can't a multimeter measure current and voltage simultaniously?

[staze]

Subject says it all. Why can't a dual display multimeter, with 3 probes (shared common), display both voltage, and current. Am I missing something really obvious here?

Edit: obviously a clamp multimeter will, but I'm talking more theoretically about standard bench/handheld multimeters.

[gxti]

Some do: http://www.gossenmetrawatt.com/english/produkte/metrahitenergy.htm

A simple reason is cost (and those meters aren't cheap). You need two analog pathways, both capable of meeting the specified accuracy. And if you want to measure true power, it's even more complicated.

[Lightages]

Well there is one relatively cheap one, the Uni-T UT71E. Not the best quality or safest in the world but it does do this.

[staze]

Oh, okay. *phew* I thought it was something really bloody obvious that I was missing... here, it's just cost. =P

Yeah, I've kept looking at my Fluke going "huh, it's got mA and A separate, why not just allow another probe, and display V while you're displaying current"... though, thinking about it, I could see where this might end up being a 4 wire operation. But no reason a bench meter couldn't do this. =/

[c4757p]

But no reason a bench meter couldn't do this. =/

Fluke 45 can.

[staze]

Does it?

[staze]

wow, it sure can. Page 4-2 of the manual... damn.

We should start a thread arguing with Dave's hatred of bench meters. =P

[c4757p]

We should start a thread arguing with Dave's hatred of bench meters. =P

As usual, Dave will jump in saying he doesn't hate them in 3... 2...


(I agree with him, I don't think he hates them, just finds them inconvenient. It depends on your working style. I'm actually the exact opposite - handheld meters make an ungodly clutter of my bench and I hate them with a burning passion. I have shelves for a reason. Also, batteries suck.)

[gxti]

I'm normally the type to complain about batteries, but I've had my 87-V for nearly 3 years now and haven't had to change it yet. I even bought new ones in anticipation of wearing it out quickly, maybe it's staying alive to spite me. Granted, I don't do this full time. I'm sure it's more of an issue for someone who has it powered up for hours a day.

[staze]

*nods*

If I had a huge workspace, handheld meters are far superior. You can take them with you, put them where you need them, etc. That said, I have a tiny workspace (it's my home office desk currently), which I share with other uses. So having something live on a shelf is much easier... but, it also means it's not at eye level.... but then my scope won't be either.

Damned if you do, damned if you don't.

All that said, you CAN power the Fluke 45 from a battery. =P

[jarvis]

You could refer to
http://fieldpiece.com/clamp-meters/sc53

[staze]

I should have indicated that I didn't mean clamp meters, obviously they'll do it just fine. =P

[NiHaoMike]

The input amplifiers for voltage and current are very different (the current channel needs a lower noise floor and offset voltage, while the voltage channel needs a high impedance), so I wouldn't expect too much extra cost there for a quality meter. A second ADC will add a lot of cost, or they can multiplex the ADC the way many DSOs do. It would also require a DSP that is twice as fast, but processing power is really cheap nowadays.

[Wytnucls]

Well there is one relatively cheap one, the Uni-T UT71E. Not the best quality or safest in the world but it does do this.

To be clear, AFAIK, the UT71E doesn't display the current on the screen. It only shows the measured real power W, apparent power WA (RMS) VA and power factor cos Phi (phase angle) on the triple display.
It also only works with AC power, with a voltage between 50V and 250V.
The current shunt is also limited to 10A for 15 sec (5A continuous), otherwise the shunt will overheat and the meter could be damaged and at best, be out of specs.

To obtain the true RMS current (A), one would have to divide the displayed apparent power (VA) by the known true RMS voltage.

[Electro Fan]

Here are the Amprobe ACD-14 Plus and ACD-14 TRMS-Plus:
http://www.amprobe.com/Amprobe/usen/Clamp-Meters/Multimeter-Clamps/ACD-14-PLUS.htm?PID=73068

The have the ability to display current and voltage simultaneously.  One thing that could be better though would be to make the displays larger and more easily readable.

Another product that might also be of interest to you in helping with current reading (since almost every DMM reads voltage) is the ELS2A:
http://www.amprobe.com/amprobe/usen/accessories/other/els2a.htm?PID=73229
http://www.amazon.com/Amprobe-ELS2A-Current-Clamp-Accessory/dp/B001DPR0FE
- not very expensive and pretty handy if you have a meter with a current clamp.  Some manufacturers sell their version for even less but the Amprobe seems pretty good.

[skipjackrc4]

Does it? [referring to Fluke 45]

Yes, but it uses a relay to constantly switch measurements because it has only one ADC.  I don't use it because I don't want to wear out the relay.  That's why I have multiple meters.  And yes, I prefer bench meters, though at least one good handheld is absolutely necessary.

[c4757p]

Yes, but it uses a relay to constantly switch measurements because it has only one ADC.  I don't use it because I don't want to wear out the relay.

Ah, yes, thank you - this didn't really occur to me. I've performed such extensive repairs on my 45 that I don't really care if it wears out a relay, I'll just toss in another. That thing is more like my 12-year-old car than like my "nice" meters...

Hell, not even my car has an EEPROM bodge-wired in because I didn't have one with the right pinout... 

[iloveelectronics]

Some DMM's certainly can measure V and A at the same time, such as this Prova 803.

[staze]

Another product that might also be of interest to you in helping with current reading (since almost every DMM reads voltage) is the ELS2A:
http://www.amprobe.com/amprobe/usen/accessories/other/els2a.htm?PID=73229
http://www.amazon.com/Amprobe-ELS2A-Current-Clamp-Accessory/dp/B001DPR0FE
- not very expensive and pretty handy if you have a meter with a current clamp.  Some manufacturers sell their version for even less but the Amprobe seems pretty good.

I have a craftsman AC current clamp "probe". Obviously won't do DC, but yeah. I guess this was all more of a theoretical question than a prove me wrong. =D

[staze]

Some DMM's certainly can measure V and A at the same time, such as this Prova 803.

Cool!

[Lightages]

To be clear, AFAIK, the UT71E doesn't display the current on the screen. It only shows the measured real power W, apparent power WA (RMS) VA and power factor cos Phi (phase angle) on the triple display.
It also only works with AC power, with a voltage between 50V and 250V.
The current shunt is also limited to 10A for 15 sec (5A continuous), otherwise the shunt will overheat and the meter could be damaged and at best, be out of specs.

To obtain the true RMS current (A), one would have to divide the displayed apparent power (VA) by the known true RMS voltage.

You are partially correct. I should have been clearer. It measures voltage and current at the same time but only in AC and not very low in either with any accuracy. It defaults to showing power, a VA and cos Phi, but you can switch it to display Hz on the main display and current and volts on the secondary displays. This really is not of much use but it does do it.

[BravoV]

Some DMM's certainly can measure V and A at the same time, such as this Prova 803.

Hey Franky, is that yours ? Do a teardown please !

[iloveelectronics]

Yes, it's mine, but someone else already did a teardown: https://www.eevblog.com/forum/testgear/prova-903-discussion/msg182116/#msg182116

Some DMM's certainly can measure V and A at the same time, such as this Prova 803.

Hey Franky, is that yours ? Do a teardown please !

[BravoV]

Thanks, after seeing the board & reading the impressions made there, not very convincing. 

[Wytnucls]

To be clear, AFAIK, the UT71E doesn't display the current on the screen. It only shows the measured real power W, apparent power WA (RMS) VA and power factor cos Phi (phase angle) on the triple display.
It also only works with AC power, with a voltage between 50V and 250V.
The current shunt is also limited to 10A for 15 sec (5A continuous), otherwise the shunt will overheat and the meter could be damaged and at best, be out of specs.

To obtain the true RMS current (A), one would have to divide the displayed apparent power (VA) by the known true RMS voltage.

You are partially correct. I should have been clearer. It measures voltage and current at the same time but only in AC and not very low in either with any accuracy. It defaults to showing power, a VA and cos Phi, but you can switch it to display Hz on the main display and current and volts on the secondary displays. This really is not of much use but it does do it.

OK, I wasn't aware of that feature. I think the count is 4,000 on the secondary displays. Still useful I guess, if true RMS figures.