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Old 7th March 2012, 12:15 PM   #1
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Default What does it mean by saying (1%+3)

I wish to buy a low ohm measuring equipment. It says the accuracy is (1%+3). What does this mean?
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Old 7th March 2012, 12:32 PM   #2
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hello, usually means +-(percent of reading + counts)
in this case for example if your display indicates 200
the "true value" sits between
(reading-0.01*reading-3)=200-2-3=195
and
(reading +0.01*reading+3)=200+2+3=205

Fabio.
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Old 9th March 2012, 02:53 PM   #3
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Default Meter reading

Thanks.

In this case, if I need the meter to read a resistor of 0.47 ohms, it may has a reading between

0.47 * 1% = 0.4747 + 3 = 3.4747
or
0.47 * -1% = 0.4653 - 3 = value below zero!

Therefore, it can never give accurate reading. Is my understanding correct?
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Old 9th March 2012, 03:00 PM   #4
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You pays for what you gets.

A reasonable DMM will give yo +/- 1% plus +/- 1 digit.

The highest accuracy is usually on the DC voltage range. You can use this to your advantage by simply using a potential divider to measure your resistors. Even better a Wheatstone Bridge.
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Last edited by KatieandDad; 9th March 2012 at 03:08 PM.
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Old 9th March 2012, 03:09 PM   #5
DF96 is offline DF96  England
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3 counts (i.e. least significant digit), not 3 ohms!
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Old 9th March 2012, 03:22 PM   #6
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Quote:
Originally Posted by Sunsun22 View Post
Thanks.

In this case, if I need the meter to read a resistor of 0.47 ohms, it may has a reading between

0.47 * 1% = 0.4747 + 3 = 3.4747
or
0.47 * -1% = 0.4653 - 3 = value below zero!

Therefore, it can never give accurate reading. Is my understanding correct?

I'll assume that your DMM is 3 1/2 digit model, most of them are.

Therefore if you had a 100.00% accurate 0.47 Ohm resistor your meter might display:

0.47 +/- 1% = 0.478 to 0.462 Ohms

This also depends on the firmware in the DMM, does it Round-Up or Round-Down, will it remove the leading ZERO.
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Old 9th March 2012, 03:33 PM   #7
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Ah, then in my example on a meter with 2 decimal display, the 0.47 ohm will read somewhere between 0.5 to 0.44 ohm. Is this correct?
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Old 9th March 2012, 03:39 PM   #8
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Quote:
Originally Posted by Sunsun22 View Post
Ah, then in my example on a meter with 2 decimal display, the 0.47 ohm will read somewhere between 0.5 to 0.44 ohm. Is this correct?
0.47 + 1% = 0.475

Depending on whether your meters rounds this up or down could be

0.47 or 0.48 the the last digit could be up to 3 counts "OFF"

Which would give you 0.44 to 0.51

Similarly

0.47 - 1% = 0.465

Which would give you 0.43 to 0.50

So a perfectly good 1% -.47R resistor on your meter could read between

0.43 and 0.51 Ohms.

If you need better accuracy, just buy better resistors. The DMM will just serve to tell you that the resistors hasn't failed.
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Old 9th March 2012, 04:55 PM   #9
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Then, if an instrument has 3 decimal display, am I correct under the same accuracy, that the 0.470 ohm will have a reading between

0.470 + 1% = 0.4747 = 0.475 + 3 = 0.478
or
0.470 - 1% = 0.4653 = 0.465 -3 = 0.462

Therefore an instrument which has 3 decimal display will have a better accuracy than that has only 2 decimal display. Am I right?
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Old 9th March 2012, 05:02 PM   #10
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The more digits the better the accuracy.

Professional DMMs can have 6 digits or more, the most insignificant digit is then almost irrelevant.
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