Which Meter. Extech EX330 or Amprobe AM240 ?

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march.brown
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Which Meter. Extech EX330 or Amprobe AM240 ?

I have decided to go for a better voltmeter than my cheapies (3 digit) and would like one that gives an extra digit.

I want to measure my 18650’s (hopefully) more accurately.

I have been considering the Extech EX330 and the Amprobe AM240 … Both are 4000 count … I have a 5.0000V reference and at present my meters read 5.00V … I would like the new meter to read 5.000V … Most meter ranges seem to be 2.000V , 20.00V , 200.0V or 4.000V , 40.00V , 400.0V … If I measure my reference , it would be displayed on the 20.00V or 40.00V ranges as it is higher than the lower ranges … I would like to display on a 5.000V or 6.000V range to get that extra digit.

If I measure my 5.0000V reference on these two meters , will there only be two decimal places displayed because they would be measuring on a 20.00V or 40.00V range ? … If so they would not give the extra digit that I want.

Unfortunately there is nowhere in the area that I can try out these models against my reference.

I can’t really justify spending over £50 on a new meter , so that excludes the 6000 counts and above.

I probably have got it all wrong in my understanding of these meters … Will these two show 5.000V when measuring my reference (or not) ?

Any help will be appreciated.
.

march.brown

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march.brown wrote:
I have decided to go for a better voltmeter than my cheapies (3 digit) and would like one that gives an extra digit.

I want to measure my 18650’s (hopefully) more accurately.

I have been considering the Extech EX330 and the Amprobe AM240 … Both are 4000 count … I have a 5.0000V reference and at present my meters read 5.00V … I would like the new meter to read 5.000V … Most meter ranges seem to be 2.000V , 20.00V , 200.0V or 4.000V , 40.00V , 400.0V … If I measure my reference , it would be displayed on the 20.00V or 40.00V ranges as it is higher than the lower ranges … I would like to display on a 5.000V or 6.000V range to get that extra digit.

If I measure my 5.0000V reference on these two meters , will there only be two decimal places displayed because they would be measuring on a 20.00V or 40.00V range ? … If so they would not give the extra digit that I want.

Unfortunately there is nowhere in the area that I can try out these models against my reference.

I can’t really justify spending over £50 on a new meter , so that excludes the 6000 counts and above.

I probably have got it all wrong in my understanding of these meters … Will these two show 5.000V when measuring my reference (or not) ?

Any help will be appreciated.
.

That last digit isn’t really useful for our purposes anyway. There’s far more variation (>0.1V) from the various other error factors in the measurement. A meter which happens to have the right range might offer a tiny bit more precision, but who knows if it’s accurate to what you’re trying to achieve.

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march.brown
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agenthex wrote:

That last digit isn’t really useful for our purposes anyway. There’s far more variation (>0.1V) from the various other error factors in the measurement. A meter which happens to have the right range might offer a tiny bit more precision, but who knows if it’s accurate to what you’re trying to achieve.

I have a very accurate five volt reference to check the meter against.

It is 5.0002V at 72F … My other meters read 5.00V , but I would like a meter that would read 5.000V so would be within 0.5mV of the reference.

If it is going to be too expensive to buy a higher resolution meter then I will stick with the old ones which are pretty accurate anyway … When I read 4.20V at the moment , it could be anywhere between 4.195V and 4.205V … Perhaps being within five millivolts is OK with my 18650’s.

I might be just wasting my money on a more expensive meter when it isn’t really needed.
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march.brown

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If you have an accurate reference to check against, you can just calibrate to two reference points and do linear interpolation in between them. Meters tend to be much more precise than accurate.

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march.brown
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agenthex wrote:
If you have an accurate reference to check against, you can just calibrate to two reference points and do linear interpolation in between them. Meters tend to be much more precise than accurate.

My meters are reading correct at zero (leads shorted) and at 5.00 volts (as checked against my 5.0002V reference) … As such , I have assumed (rightly or wrongly) that they read correctly at 4.20V … One of my previous meters was slightly out (+10mV) against the reference , so I corrected the Li-Ion reading by a factor of 4.2 divided by 5.0 × 10mv … I subtracted 8.4mV from the reading … Since the meter only read to two places of decimals , I subtracted 0.01V … It was easy to do , but I gave the meter away as the others were reading exactly 5.00V on my reference.

I was hoping to get a cheap meter (under £50) that would read 5.000V rather than the 5.00V , but I might be spending money unnecessarily for use on Li-Ions.

I think that to get what I want at 4.200V , I would need a 5000 or 6000 count minimum … They are not available in my price bracket.

Looks as though I will be sticking with my 2000 count meters.
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march.brown

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If you are simply taking the DC voltage of 18650s I’d suggest you don’t bother to upgrade, I can’t really see a need to be that accurate.

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Try finding a VICHY VC99 DMM, it has a 6000 display and is fairly cheap.

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Check out the stuff at http://www.voltagestandard.com/ He sells some very nice voltage references and meter standards for cheap. If you have a 4000 count meter, he has a 3.0000V reference for 25 bucks.

I have the equivalent of a national standards quality calibration lab (8.5 digit meters, sub-microvolt accurate references and dividers, 1E-13 accurate frequency, etc). For 25 bucks, it’s hard to beat those little references.

march.brown
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HKJ wrote:

Try finding a VICHY VC99 DMM, it has a 6000 display and is fairly cheap.


This was originally on my short list till I looked at reviews on meters under $50 (on the internet) and it came in the bottom two … I would like to find somewhere in South Wales that sells them so I can try one against my 5.0002V reference … If this meter gave a reading of 5.000V from my standard , I would buy one … I’m not worried about the physical size as it wouldn’t be carried about … It would be in my desk drawer.

I did also consider the Amprobe PM55A as it is a 5000 count meter … It is very small but only uses a CR2032 battery and has a drain of 6mA on auto range … So I didn’t know how long the battery would last.

Decisions , decisions.
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texaspyro wrote:
Check out the stuff at http://www.voltagestandard.com/ He sells some very nice voltage references and meter standards for cheap. If you have a 4000 count meter, he has a 3.0000V reference for 25 bucks.

I have the equivalent of a national standards quality calibration lab (8.5 digit meters, sub-microvolt accurate references and dividers, 1E-13 accurate frequency, etc). For 25 bucks, it’s hard to beat those little references.


My five volt reference is 5.0002V , so I don’t really need another reference … $25 is a good price for what you got though.
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Jeansy wrote:
If you are simply taking the DC voltage of 18650s I’d suggest you don’t bother to upgrade, I can’t really see a need to be that accurate.

I’m beginning to think that way … For mains use , I use my Avo 8 … I’ve had it since 1975 and it is still working OK … Good quality leads on it for any mains voltage work too.

I’m wondering if I really do need to go to the third decimal place when reading voltages on my Li-Ions … I’ve managed for a few years now with my 2000 count meters (on the 20V range) … There would be no advantage in going for a 4000 count meter (once the voltage goes over 4.00V).

Decisions , decisions.
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march.brown

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march.brown wrote:
This was originally on my short list till I looked at reviews on meters under $50 (on the internet) and it came in the bottom two ... I would like to find somewhere in South Wales that sells them so I can try one against my 5.0002V reference ... If this meter gave a reading of 5.000V from my standard , I would buy one ... I'm not worried about the physical size as it wouldn't be carried about ... It would be in my desk drawer.

 

I would not call it a bad DMM, compared to my 6½ digit Fluke it is only 3 mV wrong at 5 volt, i.e. it show 5.003 when the other meter shows 5.00010. The 5 volt is from a DMMCheck.

 

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march.brown wrote:
Jeansy wrote:
If you are simply taking the DC voltage of 18650s I’d suggest you don’t bother to upgrade, I can’t really see a need to be that accurate.

I’m beginning to think that way … For mains use , I use my Avo 8 … I’ve had it since 1975 and it is still working OK … Good quality leads on it for any mains voltage work too.

I’m wondering if I really do need to go to the third decimal place when reading voltages on my Li-Ions … I’ve managed for a few years now with my 2000 count meters (on the 20V range) … There would be no advantage in going for a 4000 count meter (once the voltage goes over 4.00V).

Decisions , decisions.
.

Today i used my colleges 287 rather than my 187 for some very basic mains checks, it damn well annoyed me having to wait for the silly little start up, heck id of been happy with my almost retired 70 series.

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Jeansy wrote:
Today i used my colleges 287 rather than my 187 for some very basic mains checks, it damn well annoyed me having to wait for the silly little start up, heck id of been happy with my _almost_ retired 70 series.

The 287 is a great DMM for some stuff, but the slow start up, the low contrast on the display and the user interface with soft buttons is annoying.

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HKJ wrote:

march.brown wrote:
This was originally on my short list till I looked at reviews on meters under $50 (on the internet) and it came in the bottom two … I would like to find somewhere in South Wales that sells them so I can try one against my 5.0002V reference … If this meter gave a reading of 5.000V from my standard , I would buy one … I’m not worried about the physical size as it wouldn’t be carried about … It would be in my desk drawer.

 

I would not call it a bad DMM, compared to my 6½ digit Fluke it is only 3 mV wrong at 5 volt, i.e. it show 5.003 when the other meter shows 5.00010. The 5 volt is from a DMMCheck.

 


Is it user-adjustable ? … Can I use my 5.0002V as a reference and adjust the VC99 to read exactly 5.000V ?

This would be the ideal situation if there was just one “pot” to adjust to give the perfect reading … It would be a matter of which pot should I tweak ? … Unfortunately it would invalidate the warranty.
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march.brown wrote:
Is it user-adjustable ? ... Can I use my 5.0002V as a reference and adjust the VC99 to read exactly 5.000V ? This would be the ideal situation if there was just one "pot" to adjust to give the perfect reading ... It would be a matter of which pot should I tweak ? ... Unfortunately it would invalidate the warranty. .

 

It looks like it is adjustable, but I have not checked what ranges the pots affects:

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march.brown wrote:
My five volt reference is 5.0002V , so I don’t really need another reference …

If you have a 4000 count meter and want to check the accuracy on the lower voltage scales you DO need another reference. 5V will cause the meter to switch to the next higher range.

march.brown
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texaspyro wrote:
march.brown wrote:
My five volt reference is 5.0002V , so I don’t really need another reference …

If you have a 4000 count meter and want to check the accuracy on the lower voltage scales you DO need another reference. 5V will cause the meter to switch to the next higher range.


My meters are only 2000 counts.

I have been thinking of another way to get more accurate readings … My reference is known to be 5.0002V … If I connect my reference positive to my battery positive and measure between the reference negative and the battery negative with a high impedance meter , the reading with a fully charged 18650 would be about 0.800V … If I subtracted the 0.800V (or whatever) from the 5.000V then that should give me a more accurate reading than I get at the moment … I realise that this is a bit long-winded , but it would cost nothing extra to do.

I might not bother … I think that two places of decimals will be OK for checking my Li-Ion voltages.

Maybe one day I will justify a better meter.
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march.brown

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march.brown wrote:
Maybe one day I will justify a better meter.

I recommend the HP3458A. And for a better reference a Fluke 732 or 734. Add a Fluke 720A Kelvin-Varley divider to it to get any voltage you want (< 10V).

But then, that brings up the question of how do you know it is all working properly? A man with one meter never knows what volt it is. A man with two meters can know something is wrong. Better get three of each… I did.

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Unless this is necessary for some other enterprise, a super-accurate DMM to measure battery voltage is entirely pointless. The Vout is variable depending on load, so any crapo-meter with a “battery” mode (like the $3 harborfreight ones, which btw can be calibrated inside) are going to be more “accurate” than a fluke.

It’s also why getting thick leads are pointless for measuring current, too. The thin wire spring inside a tailswitch is going to offer far more resistance than any DMM probe.

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agenthex wrote:
It’s also why getting thick leads are pointless for measuring current, too. The thin wire spring inside a tailswitch is going to offer far more resistance than any DMM probe.

Nope, the thin wire spring is very short and has a low resistance compared to long cheap meter leads (which are almost always made of some thin stranded non-copper mystery-metal wire). But the real killer in measuring current is usually the current shunt resistor in the meter.

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texaspyro wrote:
agenthex wrote:
It’s also why getting thick leads are pointless for measuring current, too. The thin wire spring inside a tailswitch is going to offer far more resistance than any DMM probe.

Nope, the thin wire spring is very short and has a low resistance compared to long cheap meter leads (which are almost always made of some thin stranded non-copper mystery-metal wire). But the real killer in measuring current is usually the current shunt resistor in the meter.

The leads on the $3 meter have too low resistance to meaningfully measure with a meter. What’s the shunt resistor value on DMMs?

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agenthex wrote:
Unless this is _necessary_ for some other enterprise, a super-accurate DMM to measure battery voltage is entirely pointless. The Vout is variable depending on load, so any crapo-meter with a "battery" mode (like the $3 harborfreight ones, which btw can be calibrated inside) are going to be more "accurate" than a fluke.

On a LiIon you uses the unloaded voltage, that makes a DMM perfect for the job.

For everyday use I do agree that a 4.xx reading is enough, but the tolerance on that reading is fairly high on some DMM's.

agenthex wrote:
It's also why getting thick leads are pointless for measuring current, too. The thin wire spring inside a tailswitch is going to offer far more resistance than any DMM probe.

Not all tail springs are thing and long, on the more powerful lights they are usual with thicker wire.

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agenthex wrote:
The leads on the $3 meter have too low resistance to meaningfully measure with a meter.

Yes, it requires another method, you can see one in my DMM guide.

agenthex wrote:
What's the shunt resistor value on DMMs?

That varies with range, usual you can expect between 0.3 and 1 volt burden voltage when the current is maximum for that range. Unfused cheap DMM's can be than the more expensive DMM's.

With auto ranging DMM, you might get a lover voltage drop in the low uA and low mA range, because they uses the same resistor as the high uA and mA range.

I.e. the high uA/mA has 0.3 to 1 volt burden, the low uA/mA has 1/10 of that.

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HKJ wrote:

agenthex wrote:
Unless this is necessary for some other enterprise, a super-accurate DMM to measure battery voltage is entirely pointless. The Vout is variable depending on load, so any crapo-meter with a “battery” mode (like the $3 harborfreight ones, which btw can be calibrated inside) are going to be more “accurate” than a fluke.

On a LiIon you uses the unloaded voltage, that makes a DMM perfect for the job.

For everyday use I do agree that a 4.xx reading is enough, but the tolerance on that reading is fairly high on some DMM’s.

It’s certainly not on the cheapest possible DMM available on the mass market today, so I can’t imagine it would be on a more expensive one.

Quote:

agenthex wrote:
It’s also why getting thick leads are pointless for measuring current, too. The thin wire spring inside a tailswitch is going to offer far more resistance than any DMM probe.

Not all tail springs are thing and long, on the more powerful lights they are usual with thicker wire.

Neither is going to be that meaningful for the 1A or some measurements we do.

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[quote=agenthex]It's certainly not on the cheapest possible DMM available on the mass market today, so I can't imagine it would be on a more expensive one. [quote]

I am going on the specifications, not on a random test with a reference.

A 0.5%+3 specification means that 4.20 volt can be shown as anything from 4.15 to 4.25 with a extra digit the same specification will show 4.176 to 4.224

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HKJ]<p>[quote=agenthex]It's certainly not on the cheapest possible DMM available on the mass market today, so I can't imagine it would be on a more expensive one. [quote wrote:

I am going on the specifications, not on a random test with a reference.

A 0.5%+3 specification means that 4.20 volt can be shown as anything from 4.15 to 4.25 with a extra digit the same specification will show 4.176 to 4.224

That as much as spec of accuracy as precision. I’ve never seen any meter be that imprecise.

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agenthex wrote:
HKJ wrote:

I am going on the specifications, not on a random test with a reference.

A 0.5%+3 specification means that 4.20 volt can be shown as anything from 4.15 to 4.25 with a extra digit the same specification will show 4.176 to 4.224

That as much as spec of accuracy as precision. I've never seen any meter be that _imprecise_.

That is a fairly typical specification for a low end DMM, most meters are better indoor at 25 degrees.

A fairly good DMM (Fluke 179) has 0.1%+3, for 4.2 volt that gives 4.194 to 4.206 (This specification is between 18 and 28 degrees).

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HKJ]<p>[quote=agenthex]It's certainly not on the cheapest possible DMM available on the mass market today, so I can't imagine it would be on a more expensive one. [quote wrote:

I am going on the specifications, not on a random test with a reference.

A 0.5%+3 specification means that 4.20 volt can be shown as anything from 4.15 to 4.25 with a extra digit the same specification will show 4.176 to 4.224


My meters are very cheap ones (Sinometer M-830B) and are not auto-ranging … I have to use the 20V range for my Li-Ion battery measurements … The “accuracy” on the 20V range is quoted as + /- 0.5% of the reading +/- 2 digits … My voltage reference is 5.0002V at 72F and my meters actually read 5.00V … On a 2000 count meter , it isn’t possible to get any more accurate than this … I realise that the the actual voltage could be between 4.995V and 5.005V which would show on my meter as 5.00V … Being up to 5mV out , in the measurement of an off-load Li-Ion isn’t too bad really.

I was hoping that for up to £50 I would be able to get a meter that would read exactly 5.000V , but it seems that this is not possible in my low price range without tweaking an adjustment potentiometer (on a VC99) … It seems that perhaps I should just stay with my cheap meters as I can’t justify the expenditure needed to get the 5.000V accuracy when measuring my 5.0002V reference.

Do we need this degree accuracy (4.200V) for just checking Li-Ions ? … Perhaps not.
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HKJ wrote:

agenthex wrote:
HKJ wrote:

I am going on the specifications, not on a random test with a reference.

A 0.5%+3 specification means that 4.20 volt can be shown as anything from 4.15 to 4.25 with a extra digit the same specification will show 4.176 to 4.224

That as much as spec of accuracy as precision. I’ve never seen any meter be that imprecise.

That is a fairly typical specification for a low end DMM, most meters are better indoor at 25 degrees.

A fairly good DMM (Fluke 179) has 0.1%+3, for 4.2 volt that gives 4.194 to 4.206 (This specification is between 18 and 28 degrees).

Yes, I know what the spec is, but context for why/how that’s meaningful is also relevant. 0.5% or whatever here doesn’t mean that for each arbitrary measurement of 4.20v DUT, the result will sometimes be 4.18 and then 4.22 a sec later.

In any, this isn’t particularly meaningful given that far more expensive instruments in far more professional settings are far less precise+inaccurate (often inherently, eg high freq sampling).

Similarly, I can’t imagine that the cheapo spring + various shitty contact points inside those typical 5cent switches in budget lights are as decent as the 18awg wire + stiff jacks on even the cheapest of meters.

Maybe if we’re working on the mars rover the last decimal point of accuracy would matter, but this is hardly it. To provide more context, not even the voltages inside intel chips running at >>1ghz are controlled to anywhere near that type of precision.

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