Digital Multimeter Which One?

[crofty]

No luck find either Innova or Extech`s in UK, I guess I just need to spend a bit more than the really cheap one`s to get more reliability.

Would prefer a fluke if I can`t get the same model as you guys use (tried and tested). The 116 looks like a good alternative for the 179. Please keep your budgest suggestions coming though.


TakeTheActive, Needs and opinions change, get over it and stop over analyzing things

 

[likeguymontag]

No luck find either Innova or Extech`s in UK, I guess I just need to spend a bit more than the really cheap one`s to get more reliability.

Would prefer a fluke if I can`t get the same model as you guys use (tried and tested). The 116 looks like a good alternative for the 179. Please keep your budgest suggestions coming though.

I've gotten along just fine with my Radio Shack and Craftsman branded DMMs, each costing something like $30. You won't find either of those brands in the UK, but the point is that the choice isn't Fluke vs. crap; there's a whole range of choices.

 

[Mr Happy]

Unfortunately I can't find the Innova/Equus 3320 in the UK, though it really is a nice meter at a good price if you could find a US vendor that ships to the UK.

Here are some other places to browse for meters:

Amazon
Maplin
test4less

There's a whole range of prices and features to pick from in the sub-£100 bracket. Oddly, I find relatively few auto ranging meters on the UK market. I'm not sure why.

 

[TakeTheActive]

Which meter did you eventually buy?

If you click 'view full technical specification' you can see the accuracy is 0.5%. I remain unconvinced that you can actually believe the quoted specifications of a cheap meter but if you only want ±100mV at ~4V that is 2-3%.

I think you need to stop over analysing the issue and buy one.

...TakeTheActive, Needs and opinions change, get over it and stop over analyzing things

What are your needs and opinions now?

 

[Black Rose]

However, I have one of these more expensive Innova meters (model 3320) found in Wal-Mart, and the quality of construction is in a different league. I don't know if is available anywhere in the UK, but I would certainly recommend it.

I agree. The Innova/Equus DMMs are very good DMMs for the money.

I have a couple Innova/Equus 4320a's, which are similar to the Innova 3310.
I bought them on sale for $9.99 each and they work well.

I've already verified that the fuse does it's job well

 

[Justin Case]

This is the stuff thanks guys, I`ve learn`t a lot.


That is the question now, imagine we know a battery is 3.7v. As long as the meter reads it as 3.7v and not 3.8 or 3.6 for example that`s the accuracy I`m looking for.

I`m thinking of giving this one a go but it doesn`t list accuracy. Is there a test that can be done with the meter to check it`s reading correctly?
Somethig like what you can do with scales, weigh a ten pence coin which you know weighs 6.50 grams.

The link provided does give accuracy. It is listed as 0.5% + 2 digits (I assume that is +/-) and a 3 1/2 digit display.

This sort of accuracy probably doesn't matter unless you are measuring charge termination voltage for Li-ions. But if you are checking charge termination to avoid overcharge, I think that this level of accuracy is borderline, unless you can calibrate the meter.

Here's why. Let's assume a voltage of 4.20V (the typical recommended max charge termination voltage for Li-ions). The accuracy spec then gives

4.20*0.005 = 0.021

We round this to 0.02. Then we add the 2 digits, for a total accuracy of 0.04. For a 3 1/2 digit meter, the 2 digits really kill you.

Thus a 4.20V measurement could be 4.16V-4.24V.

This is the crux of the dilemma with these typical meters (including mine which has a very similar accuracy spec) if you are checking your Li-ion charger to make sure your cells don't get overcharged. You might think you are safe because your meter reads 4.20V or under. But the accuracy spec IMO is insufficiently narrow relative to the sensitivity of Li-ions to overcharge.

Based on observations with my meter, it appears that even inexpensive DMMs have excellent precision. It's the accuracy that is the issue. So if you can compare your meter to a very accurate meter (like one of the Flukes that specs out to something like 5 1/2 digit display and 0.05% + 1 digit) or to an accurate voltage standard, then you can have better confidence in your measurements.

 

[Justin Case]

I have an MASTECH MS8209 it does everything

Display: 3999 counts
- Auto/Manual Range
- Relative, Data Hold, Backlight
- Auto Power Off
- DC Voltage: 0.4/4/40/400/600V ± 0.7%
- AC Voltage: 4/40/400/600V ± 0.8%
- Resistance: 400/4k/40k/400k/4M Ohm ± 1.2% 40M Ohm ± 2.0%
- Capacitance: 4n/40n/400n/4micro/200microF ± 3.0%
- Frequency: 10/100/1k/10k/100kHz ± 2.0%
- Duty Cycle: 0.1% - 99.9% ± 3.0%
- Temperature: -20°C - 400°C ± 3.0% (0.1°C)
.................... -20°C - 1000°C ± 3.0% (1°C)
- Humidity: 30% - 95%RH ± 5.0%RH
- Light: 4000Lux/40000Lux ± 5.0%
- Sound Level: 35 - 100dB ± 4dB
- Continuity Test
- Diode Test
- In Accordance with IEC1010 Standard


http://www.amazon.com/dp/B0007Z0TAY/?tag=cpf0b6-20

A review is below!

http://www.devhardware.com/c/a/Mobile-Devices/Mastech-5-in-1-Multimeter-Review/

Specifically, the accuracy spec at the relevant voltage range for 4.2V Li-ions is +/-(0.7% + 2 digits).

The MS8209's 3999 display count probably means that the DMM will read out 4.2000V as 4.20V (2 decimal places). Thus, the accuracy spec gives

4.20*0.007 = 0.0294, or 0.03 rounded.

Adding in the 2 digits to the least significant place gives a final accuracy of 0.05V. Thus the reading of 4.20V may actually be 4.15V-4.25V.

IMO, 4.25V is excessively high if you are checking for Li-ion termination voltage to avoid overcharging. If your charger happens to charge your cells such that your DMM reads 4.20V, then IMO you can't be sure if you are charging safely or unsafely.

 

[HKJ]

Check a company like RS for a selection of multimeters.

 

[Justin Case]

The B&K 2880B looks like the one to get on the RS Electronics web site if you need the accuracy to measure 4.2V Li-ion terminating charge voltage. It reads out to four decimal places on the DMM's 5V scale, with accuracy of 0.03% + 5 digits. Thus, if the B&K reads exactly 4.2000V, the accuracy spec gives

4.2000*0.0003 = 0.00126 = 0.0013 (rounded)

Adding the 5 digits to the least significant place gives +/-0.0018V.

Thus, the reading could be 4.1982V-4.2018V.

The similarly-priced Fluke 87-5 that I had been considering from Amazon has a high-res mode that gives four decimal places on the DMM's 6V scale, with an accuracy spec of 0.05% + 1 digit. That translates to +/-0.0022V spread, or 4.1978V-4.2022V.

 

[TakeTheActive]

Interpreting DMM Accuracy Specifications...

The link provided does give accuracy. It is listed as 0.5% + 2 digits (I assume that is +/-) and a 3 1/2 digit display.

This sort of accuracy probably doesn't matter unless you are measuring charge termination voltage for Li-ions. But if you are checking charge termination to avoid overcharge, I think that this level of accuracy is borderline, unless you can calibrate the meter...

Justin Case,

THANK YOU for the education on interpreting DMM Accuracy Specifications (% + Digit). Previously, I didn't understand the significance of the 'Digit'.

I spent an hour GOOGLEing and learned about ppm, step and XX-bit ADCs. I found a good 'hobbyist-level' explanation at Twisted Pair Forum:

...Back to the DMM, it's that "± digit" specification that's the killer if you don't watch what you're doing. You have to add this second specification on to the %reading specification to see the full effect of the meters accuracy. For instance, that same 100.0 reading on the 200v range that gave us ±0.1v has to be modified to include ±1 digit. Since the last digit on our reading represents tenths of a volt, that means that the reading can bobble ANOTHER 0.1v up or down from that point for a total of ±0.2v. This gives an overall percentage accuracy for that reading of ±0.2%, still not bad at all for our meter.

It's when you try to read a lower voltage on that same 200v range that you can get into trouble. Yes, the overall accuracy specification was only ±0.01v in our example, but we have to add that ±1 digit onto that reading. This translates into ±0.11v total and for a 10v reading, this means that our overall accuracy specification just slipped to a lousy ±1.1%! Use the 20v range for this same 10v measurement, and you accuracy will pop back up the ±0.2%. So, the same rule applies to DMMs as it does to analog meters: use the lowest range that you can without going overrange for the best accuracy.

The ± digit portion of the spec will usually be minimal for the higher dc voltage ranges and begin to increase as you go to the lower ranges. The ac ranges, even the upper ones, will probably have a higher figure here to begin with and really get nasty as you hit the lower ranges.

A DMM is basically a dc voltmeter with a basic 200mv or 2v range with a lot of conversion circuitry to get our other ranges and functions. This is why the dcv spec is most accurate. dca is probably the second-most accurate function as better meters have laser-trimmed current shunts. The acv function has the added fudge factor of the ac-dc converter which often skews a basic ±0.1 dcv spec to ±2% or more for acv readings. A lousy meter right off Radio Shack's shelf may have a basic dcv spec of ±0.5% and an acv spec of ±4%, getting worse as the frequency of the ac voltage increases. The ohmmeter conversion circuits are often even worse than the acv converter on a cheap meter, but may be better than the acv ranges on a better meter. Watch your specs closely as you move from function to function. Just because a DMM is "deadly accurate" when measuring dc volts doesn't mean that it can accurately measure a 1% resistor for being within tolerance!...

Reference: Twist Pair Forum: Meter Specifications: VOM vs. DMM

I'm now also going to look harder at X 3/4 digit displays with ranges that end in other than (some form of) 1999.

...Based on observations with my meter, it appears that even inexpensive DMMs have excellent precision. It's the accuracy that is the issue. So if you can compare your meter to a very accurate meter (like one of the Flukes that specs out to something like 5 1/2 digit display and 0.05% + 1 digit) or to an accurate voltage standard, then you can have better confidence in your measurements.

Am I interpreting this correctly? If one were to borrow a more accurate meter and build a "Translation Table" for their less accurate meter, temporarily ignoring temperature and component aging, the results would be repeatable.

 

[TakeTheActive]

...I have one of these more expensive Innova meters (model 3320) found in Wal-Mart, and the quality of construction is in a different league...

Mr Happy,

I followed your LINK and viewed the PDF Manual. The DCV accuracy is 0.8% + 5 digits into a 3 1/2 digit display. Paraphrasing Justin Case:

...Let's assume a voltage of 4.20V (the typical recommended max charge termination voltage for Li-ions). The accuracy spec then gives

4.20*0.008 = 0.0336

We round this to 0.03. Then we add the 5 digits, for a total accuracy of 0.08. For a 3 1/2 digit meter, the 5 digits really kill you.

Thus a 4.20V measurement could be 4.12V-4.28V.

...I would certainly recommend it.

Isn't this worse for checking Li-Ions than the less expensive (~$10) '3 1/2 digit display w/0.5% + 2 digits' DMMs we've been discussing?

 

[Justin Case]

For the Mastech MS8209 that has been mentioned previously, the calibration instructions state:

"An alternative to any of these procedures is to adjust the variable resistors to match the readings of a known instrument under the same conditions."

Presumably, you'd ideally check some voltage level that falls within the lowest voltage range available to your meter. Then you would conduct your comparison to your DMM standard (e.g., a calibrated Fluke or HP) at all of the appropriate voltage ranges. For the MS8209, the voltage ranges are 400mv, 4V, 40V, 400V, and 1000V. So, you'd prefer to use a voltage source of say 100mV and conduct your comparison.

FYI, here is a 5V voltage standard. Too bad the product doesn't use the TI REF5040 or 5045. With this standard, in theory, you don't need the second, highly accurate DMM for comparison. You just use your DMM to measure the voltage output of the standard.

For this 5V standard, you'd check its voltage output on the 40V, 400V, and 1000V ranges (assuming an MS8209 DMM), and hopefully get 5V at all ranges (out to however many decimal places are available at each range).

 

[Mr Happy]

Mr Happy,

I followed your LINK and viewed the PDF Manual. The DCV accuracy is 0.8% + 5 digits into a 3 1/2 digit display. Paraphrasing Justin Case:


Isn't this worse for checking Li-Ions than the less expensive (~$10) '3 1/2 digit display w/0.5% + 2 digits' DMMs we've been discussing?

It may or may not be worse. Take note that I said the quality of construction is in a different league, not the accuracy. I stand by that opinion.

IMHO basic accuracy specs do not tell the whole story. For sure they are what they are, but you also have to consider variables like stability with temperature, calibration drift over time, variation with declining battery voltage, statistical accuracy of factory calibration, wear and tear on switches and contacts, and so on.

When you look at display counts for meters you might have "2000 count" (1999, 3 1/2 digits), "4000 count", (3999), or "6000 count" (5999). However, if you really want to move up a level you should ignore such small increments and go for more digits. E.g. "20000 count", giving you readings up to 19999, or even meters that give you 6 digits (expensive Flukes, for example). That is the way to minimize the +/- last digits effect on accuracy.

 

[was.lost.but.now.found]

 

[crofty]

So, a periodically calibrated F179, with the linked voltage reference to keep an eye on it between calibrations should do the job nicely.

Accuracy, durability, peace of mind. I`ll let you know when it`s here, won`t be for a while mind.

Thanks for all your help lads.

 

[Mr Happy]

So, a periodically calibrated F179, with the linked voltage reference to keep an eye on it between calibrations should do the job nicely.

But bear in mind that the voltage reference itself is not eternal. When you obtain one its accuracy may only be guaranteed for six months or a year, and then it will itself need re-calibrating against an external reference. (Or you could buy another pre-calibrated voltage reference each time.)

 

[Justin Case]

The fellow at voltagestandard.com offers a free re-cal service for the first year. Subsequent recals cost $5. If you go to his web site, he shows some ongoing, long term stability tests of 5 of his voltage references. He also ages his voltage references for 200 hrs before selling them. For a 3 1/2 digit meter, it's hard to imagine that the voltage reference would drift so much as to affect the second decimal place, especially since they've already been aged before trimmed. They appear to typically be trimmed to exactly 5.00000V. I got an untrimmed, unaged version that measured 5.00036V. It might drift an additional 200uV over the next 200 hrs, based on the long term stability test curves for the 5 test references. That might mean an output of 5.00056V, which is still far more accuracy than needed for a 3 1/2 digit meter.

 

[was.lost.but.now.found]

Newbie here (especially with electronics), but I got my first DMM for Christmas from my brother in law who is an electrician. He showed me how every time you take a reading you first want to touch the probes together and wait for the reading to go to zero. Call me a simpleton but is this not a free (and accurate) calibration check every time you use your instrument?

 

[Justin Case]

Touching the probes together checks for zero offset. It doesn't address gain.

 

[was.lost.but.now.found]

Touching the probes together checks for zero offset. It doesn't address gain.

I don't know what this means. Could you explain a scenario when I would check for zero offset vs. gain?