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What does it mean for a digital instrument when its readout is 4.75 digits "long'. And what is it like in comparison to, say a 4.5 digit meter?

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  • \$\begingroup\$ Could you post a reference to the instrument for which you saw such description? \$\endgroup\$ – Nick Alexeev Dec 19 '15 at 23:24
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    \$\begingroup\$ Possible duplicate of What does it mean that a multimeter is four and a half digit? \$\endgroup\$ – Chris Stratton Dec 20 '15 at 1:13
  • \$\begingroup\$ Yes, it looks like it has already been answered at the link posted by Chris Stratton. \$\endgroup\$ – gbulmer Dec 20 '15 at 12:32
  • \$\begingroup\$ Hi. If there's an answer that helped you solve your problem, please click Accept on that answer. Thanks. \$\endgroup\$ – Armandas Jan 1 '16 at 12:14
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Background

Digits is an old way of describing the resolution of a digital multimeter. The meaning of half-a-digit is pretty well established - it can only display the value "1". Problems started when manufacturers came out with things like 4.25 digits or 4.75 digits.

Since there is no standard as to what the 0.25 or 0.75 mean, manufacturers are free to come up with their own interpretation, which just causes confusion for everyone. For this reason, vendors started using counts.

A number of counts is the maximum value the multimeter can display, plus one. For example, a 4000 count meter can display values up to 3999.

The answer

Let's start with the most standard option of 0.5 count. A 4.5 digit meter would be capable of displaying at least 20,000 counts (max value of 19999). It means you have four full digits and one half digit, only capable of displaying 1.

The 4.75 digits is a more of a marketing term and could mean anything between 30,000 and 50,000 counts. Note that the Fluke 187, which is a 50,000 count meter is still described as 4.5 digit meter by Fluke themselves.

But wait! Then there's also 4.25 counts. Does your meter have 4.5 digits, but does not go all the way to 19999? No worries, just describe it as 4.25 digit multimeter! See this Aim-TTi 1705 spec sheet as an example. It is a 12,000 count meter, by the way.

Further reading

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Without a link to examine, I'd suggest that "4.75 digits" refers to a 5-digit meter with a maximum leading digit of 3. In other words, one with a maximum reading of 39999. This is by analogy to at 4.5 digit unit, which has a maximum reading of 19999.

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Consider that a 3 digit meter would be able to count to 1000 (or 999) -- that's 10^3. By that analogy, a 4.5 digit meter could count to 10^4.5 = 31623, or above 30,000; and a 4.75 digit meter could display up to 56234 -- perhaps 50,000 allowing for marketing exaggeration.

The B&K Precision 5491B, 4.75 Digit Digital Bench Multimeter counts to 50,000

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  • \$\begingroup\$ Yes, this would be the right mathematical interpretetation. Unfortunately, meter manufacturers don't interpret things this way, and a 4.5 digit meter can typically count up to 20000, only. \$\endgroup\$ – Massimo Ortolano Dec 20 '15 at 11:36
  • \$\begingroup\$ +1 The difference can be attributed to marketing. The same method is used to determine the number of base-2 digits (bits) - \$log_2(X)\$ where X is the 'count'. \$\endgroup\$ – Spehro Pefhany Dec 20 '15 at 12:22

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