crap multimeter

[House91320]

So a hardware store sells a $5 multimeter that has a clamed acuracy of 0. 5 On dc 0. 5 On ac 1 on ohm and 1 on curent. can some one explina how this is posible

[Kiriakos-GR]

Well my friend, by reading your text I stayed frozen for five minutes before I press the reply button.

There is thousands products out there, and new products are made every day.
Why should any one care to study an 5$ multimeter ?

[House91320]

Scuse my typing i was doing it on a phone anywaye this is the meter http://www.harborfreight.com/7-function-digital-multimeter-90899.html and the nubers are the acuracys

[sacherjj]

I have two of them.  They are fairly accurate.  What you pay for in a $50 multimeter are a couple things. 

1 - They is a much less likely chance of dying when you use on mains voltage.
2 - When you drop them, they still work. 
3 - The make measurements that are accurate through ranges.  Comparing these to "real" meters, usually prove their accuracy numbers to be lies.
4 - The probes are actually insulated to a level that makes them safe to use in higher voltages.  I've had one lead on a cheap meter crack.  Had I not noticed it, that crack could jump directly to me.

If all you are doing is 12V or less hobby projects.  You can get by with a $5 meter.  Never trust it for anything past that.

[Neilm]

I have two of them.  They are fairly accurate.  What you pay for in a $50 multimeter are a couple things. 

1 - They is a much less likely chance of dying when you use on mains voltage.
2 - When you drop them, they still work. 
3 - The make measurements that are accurate through ranges.  Comparing these to "real" meters, usually prove their accuracy numbers to be lies.
4 - The probes are actually insulated to a level that makes them safe to use in higher voltages.  I've had one lead on a cheap meter crack.  Had I not noticed it, that crack could jump directly to me.

If all you are doing is 12V or less hobby projects.  You can get by with a $5 meter.  Never trust it for anything past that.

Add to that list
5 - Will be mush less likely to explode in fault conditions. See this video

Neil

[Kiriakos-GR]

According the manual of it that is available for download ,
the accuracy on DC are 1.0%   and at AC 1.2% -/+ 10 digits.

This is just terrible.    

Get the manual here.
http://manuals.harborfreight.com/manuals/90000-90999/90899.pdf

[sacherjj]

Add to that list
5 - Will be mush less likely to explode in fault conditions. See this video

Good way to turn hardware into vaporware. 

[FreeThinker]

Add to that list
5 - Will be mush less likely to explode in fault conditions. See this video

Good way to turn hardware into vaporware. 

Just how exactly would you come across 25kv under normal conditions (apart from a crt etc) ?
and how would a fluke 87 fair in the same conditions?
Main trouble with a $5 meter is it will fall to bits at the first opportunity if not before.

[Kiriakos-GR]

Main trouble with a $5 meter is it will fall to bits at the first opportunity if not before.

If you are lucky and fails in 90 days, you do get an replacement.  

[FreeThinker]

By a strange coincidence I was digging around in my junk box today, and came across an old micronta 22-188 multimeter from Tandy (Radio Shack) Put in 2x AA batteries and good as new. Must be getting on for 30yrs old if not more. Seem to remember it was not that expensive when I got it. . Had Flukes shuffle off in way less time... go figure!

[Kiriakos-GR]

Well there is an small secret about this cheap turtles,
the old ones like 15 years back, they are better made than the latest ones.

And the description "better made" , goes mostly to the inner parts, like real IC and real size resistors.
The new ones are totally trash.

[sacherjj]

I found that same Tandy meter in dad's stuff that I have been going through.  I remember working with him with it when I was really young.  Until I built my Heathkit digital VOM.

[BrickBoiler]

Its amazing to me that a company can design, market, source parts, assemble it, design packaging, buy packaging, ship across an ocean, truck across a continent, pay a distributor, advertise, and whatever other crap it takes to get that multimeter from wherever it comes from to a harbor freight store in my town, all for 4.99. It's incredible. I don't even think I can drive my little car to harbor freight and back for 4.99 in gasoline, and it is literally just on the other side of town.

When I think about it like this it seems obvious that nowhere along the way could anyone have been willing to spend even an extra nickel in the name of safety.

[House91320]

Little off topic but what doses the +-#d after the accuracy number meand and what number is beter?

[PetrosA]

Its amazing to me that a company can design, market, source parts, assemble it, design packaging, buy packaging, ship across an ocean, truck across a continent, pay a distributor, advertise, and whatever other crap it takes to get that multimeter from wherever it comes from to a harbor freight store in my town, all for 4.99. It's incredible. I don't even think I can drive my little car to harbor freight and back for 4.99 in gasoline, and it is literally just on the other side of town.

When I think about it like this it seems obvious that nowhere along the way could anyone have been willing to spend even an extra nickel in the name of safety.

I think the technical name for it is "dumping" but we owe the Chinese so much money that they kinda have us by the balls.

[Mechatrommer]

According the manual of it that is available for download ,
the accuracy on DC are 1.0%   and at AC 1.2% -/+ 10 digits.
This is just terrible.    

not terrible at all! for a $5

[ilikepez]

[/quote]
Just how exactly would you come across 25kv under normal conditions (apart from a crt etc) ?
and how would a fluke 87 fair in the same conditions?
[/quote]

well its a 25 kA fault. and really you will run into the possibility of that anytime you plug it into mains voltage. It all depends on what is upstream and how good the overcurrent protection is. The Fluke 87v has 20 kA interrupt rating fuses on its current inputs. While its not rated for that, I would have much more confidence testing with the Fluke. But for limited energy stuff they aren't horrible. Not for five bucks. But what it comes down to is the understanding that when you go test something you are bringing that entire circuit out into your hands. All that is protecting you is that meter and your gloves, and this thing supposedly is good for measuring up to 750 volts. In the long run a good multimeter is cheap, and treatment for burns is expensive.

[Kiriakos-GR]

well its a 25 kA fault. and really you will run into the possibility of that anytime you plug it into mains voltage. It all depends on what is upstream and how good the overcurrent protection is.

Hi , your math about the 25.000A and the mains plug, are wrong.
I think that I an helping you by saying that you need to refresh everything you know about CAT,
by just reading them for one more time.

[ilikepez]

True. I should have said any time you go poking in a main electric panel, or in industrial situations you can run into that kind of possibility. Brain fart on my part.

[Zero999]

It's not a problem in the UK domestic setting as all mains appliances will be fitted with a 3A to 13A plug top fuse

[ejeffrey]

Little off topic but what doses the +-#d after the accuracy number meand and what number is beter?

+/- #d means an additional error in units of the last digit of precision, rather than proportional to the measured value.  For instance, consider a 3.5 digit meter with +/- 1% + 5 counts.  The 2 volt range will be used from 200 millivolts to 2 volts, and the resolution will be 1 millivolt (2 volt / 2000 counts).  If you measure 1 VDC, the maximum error is 1 volt * .01 + 5 millivolts == 15 millivolts or 1.5%.  However, if you measure 250 millivolts, the maximum error will be 0.25 volt * .01 + 5 millivolt == 7.5 millivolt or 3%.  If you turn off autoranging and measure 50 mV, the error will be 10%.

An ideal meter is 0% + 0.5 counts, which represents perfect rounding to the nearest value, but that is not actually achievable.  Good meters are usually +/- 1 count, at least on the DC volts scale.

[Neilm]

It's not a problem in the UK domestic setting as all mains appliances will be fitted with a 3A to 13A plug top fuse

Only if you are probing on something with a plug. If you are probing into the socket then there is a lot more current available.

Neil

[alm]

Even a fused plug only protects after the fuse blows, which takes time (maybe enough time for a bang). Fuses are there to protect wires from melting and causing fires, not to protect people or equipment. A fused connection to a high-energy circuit does not make a low energy circuit, only increasing the output impedance does. This is why an otherwise CAT II circuit with less than 10m (?) of wiring from a CAT III circuit is still considered CAT III, because the extra wiring will add enough resistance and inductance to limit the maximum energy to CAT II levels. Fuses don't enter the equation when determining the over voltage category.

[Zero999]

Now you're talking about overvoltage category: here in the UK, domestic is typically CAT II.

The fuse does matter as far as safety is concerned, even if it doesn't limit the fault current because it restricts the amount of time a large current surge can exist for. For example the BS1362 fuse used in a UK plug top has a breaking capacity of 6kA but even if the main circuit can provide more than that, it shouldn't matter as long as the circuit breaker can interrupt the current before any damage is done.

[alm]

Now you're talking about overvoltage category: here in the UK, domestic is typically CAT II.

IEC 61010 is also concerned with the potential energy of the fault, since this is the energy the DUT has to contain in case of a fault. This is why they specify output impedance for the tests.

The fuse does matter as far as safety is concerned, even if it doesn't limit the fault current because it restricts the amount of time a large current surge can exist for.

That helps, but may not be enough. I don't know the pulse duration used for IEC61010 tests of the top of my head, but it may be less than the time it takes to trip the fuse. A 1ms transient of 6kA into 1ohm (made up numbers, feel free to substitute any real world data you may have) is still 36kJ, a fairly significant chunk of energy.

For example the BS1362 fuse used in a UK plug top has a breaking capacity of 6kA but even if the main circuit can provide more than that, it shouldn't matter as long as the circuit breaker can interrupt the current before any damage is done.

If the damage is to a piece of 2.5mm copper wire, probably, it should be designed to take the fault current for a short amount of time (otherwise the wiring is too thin for the fuse). Not so sure if it's a flimsy crappy multimeter which might go bang at the slightest provocation.