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My new multimeter is here.

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Adam Munich

Tue Oct 05 2010, 02:28AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I bought this true RMS multimeter for $79 last week, and I am very impressed to say the least. Before I get started, here is the unboxing. I'm sorry the first few photos are grainy, I forgot my camera was set to ISO400.

*Front of box*

*Back of box*

I was surprised to open the box and find this.

*Inside of the box*

*Inside of the inside of the box*

*Everything laid out nicely*

Batteries included! It came with probes, RS232 cable, CD, manual and a k-type thermocouple. The probes are very well built, all rubber and no plastic. They came with dust covers for each end as well. The thermocouple is an average thermocouple.

*Probe Pic*

According to the manual this meter is optoisolated from any computer. Well seeing how the RS232 cable only has a phototransistor on the butt end, I would say it definitely is. I haven't tested the software yet though.

Now the meter itself. It is well built, rubberized and has a huge backlit screen. Although it's not as light as I thought it would be weighing in at 407g, I'd still like it to have a few more ounces. My meter had a small scratch on the rubber at the bottom to the left of the lightning bolt symbol, but I don't feel that is important seeing as it's a tool likely to get scratched up anyway.

*Front*

*Back*

The meter takes 3 samples/sec, and reaches a stable reading within 2. I am going to test the accuracy tomorrow, so "stay tuned."

Proud Mary

Tue Oct 05 2010, 03:44PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Looks fine, and just goes to show you don't have to break the bank to get a useful and versatile instrument nowadays.

radiotech

Tue Oct 05 2010, 06:34PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

That instrument appears defenseless in a hostile environment just waiting to reach out and touch something. How will you prevent it, and the computer attached to it, from following your last meter into the dustbin?

Adam Munich

Tue Oct 05 2010, 07:35PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

It is completely optoisolated from the computer, (led and phototransistor). It would take quite a lot of volts to bridge that gap. As for the meter itself, all meters are defenseless; it's up to you to defend it from the hazards of electricity.

Also I tested the frequency function with a function generator today and was shocked at the accuracy. Here is the measured % error of the meter for different frequencies. And yes I did remember to divide by 100.

5MHz ---- .04%
500kHz -- .03%
50kHz --- .03%
5kHz ---- .05%
500Hz --- .5%
50Hz ---- 1.2%
5Hz ----- 4.8%

It starts to suffer at lower freqs. There was no difference in measurement when the waveform was changed; Sawtooth Sine Square and Triangle all measure the same. It started to lose count over 5.12MHz, which is OK since it's only rated for 5.

I'll measure the accuracy of the ohmmeter, capacimeter, and voltmeter some other time.

Nicko

Thu Oct 07 2010, 06:05AM

Registered Member #1334 Joined: Tue Feb 19 2008, 04:37PM
Location: Nr. London, UK
Posts: 615

It'd be interesting to see how "true" the "trueRMS" feature is. Many (most!) such meters rely on the waveform being periodic and symmetric around 0V. Also, some assume the waveform is sinusoidal etc.

Even Flukes can have this problem - see

. I believe my trusty Tek DMM916 handles this correctly, but I haven't looked for a while...

Can you do any tests to see if it handles a DC offset in the AC waveform correctly?

Cheers

Adam Munich

Thu Oct 07 2010, 06:19AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I'll test it tomorrow, though I'm not sure if the function generator I'm borrowing has a DC offset function. I suppose I could put the generator in series with a battery to move the wave though. Since the freqmeter didn't care what kind of wave it was measuring, it might not require a sine for voltage measurement. I'll check that too. I'll also check the resistance measurements against a couple Â±.05% resistors.

Anyone know of a good way to test the capacimeter?

Edit: Just read that page, I'll use the diode method too.

radiotech

Thu Oct 07 2010, 06:50AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Check the capacimeter by seeing if it thinks diodes or resistors are capacitors.

Steve Conner

Thu Oct 07 2010, 02:23PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Nicko wrote ...

Even Flukes can have this problem - see

. I believe my trusty Tek DMM916 handles this correctly, but I haven't looked for a while...

Can you do any tests to see if it handles a DC offset in the AC waveform correctly?

Nick: As far as I understand it, that IS the correct way. It's certainly how I expect a true RMS meter to work. OK, so it's not technically true RMS, but if it weren't like that, the AC ranges would be capable of measuring DC signals too, and that would be terribly confusing: you wouldn't know if the voltage you were seeing was AC or DC. (or both...)

I guess it's no fluke (ba-boom!) that it also makes the design of the true RMS circuit easier because you don't have to worry about DC offset.

thedatastream

Thu Oct 07 2010, 08:00PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

Nice meter :)

Some true RMS meters struggle with high "crest factors" e.g. a low duty cycle high pulse amplitude

The meters that use thermal sensing to calculate true rms aren't affected by this IIRC.

klugesmith

Thu Oct 07 2010, 09:54PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Steve McConner wrote ...

Nicko wrote ...

Even Flukes can have this problem - see

. I believe my trusty Tek DMM916 handles this correctly, but I haven't looked for a while...

Can you do any tests to see if it handles a DC offset in the AC waveform correctly?

I agree that "true RMS" means just that, e.g. effective voltage or current in heating a resistive load.
But - when the range selector says DC, we properly expect (and get) just the DC component of a mixed waveform.
Nick: to use the RMS feature, you move the range selector to an AC scale. Is it then a surprise to get just the AC component of a mixed waveform? As your reference states, if the same signal gives a nonzero result on DC scale, you can externally do an RMS addition of the two readings.
[edit] And Steve: if meter indicated the true RMS of DC+AC waveform, an external RMS subtraction of the DC reading would give the correct AC value.

To find out if a meter is AC-coupled on AC ranges, you don't need a function generator. Just a transformer and battery that can be connected in series. Or, uh, just a battery.

I believe the non-thermal implementations of true-RMS generally employ a precison analog RMS converter, closely related to an analog multiplier. The input is precision rectified, then squared and averaged. Feedback is used to generate a DC signal that develops the same output from an identical squarer circuit. There is no implicit requirement for periodicity or zero-average input. I bet that's much more efficient than high-rate analog to digital conversion and digital RMS computation. Either way, it's challenging to stay accurate at high frequency or when waveforms have a high crest factor.

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