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The fluke 87-V is advertised as a "four and a half digit" meter. What does that mean, how does half a digit fit into the equation?

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  • \$\begingroup\$ I would suggest to edit the title "4.5 digit" to "four and a half digit" as well. I came across situations where 4.5 digit can be a notation that there are 4 possible digits on the left and 5 on the right side (0000.00000). \$\endgroup\$ – Rev1.0 Oct 9 '13 at 6:45
  • \$\begingroup\$ The spec describes it as "6000 counts, 3-3/4 digits. 4-1/2 digit mode for precise measurements (20,000 counts)" - but remember precision isn't accuracy, it's only for monitoring slow changes. I find it more useful to look at the counts but consider the basic DC accuracy to be more important to me (i.e. to prefer a 6000 count 0.1%+1 meter over a 200000 count 1%+3 meter) \$\endgroup\$ – RedGrittyBrick Oct 9 '13 at 8:22
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At most ranges, the smallest reading is 1/19,999 of the maximum (e.g. on the 20 volt range, values range from 2.000 to 19.999 in steps of 0.001). Although it may seem odd to regard a meter that can read up to 19,999 as being a full "half digit" better than one which can read up to 9,999, common terminology for many decades has been to use the term "1/2 digit" to mean a leading digit that's zero [blank] or one, "2/3 digit" to mean 0, 1, or 2, and "3/4 digit" to mean 0, 1, 2, or 3. The fraction is explained thus: the numerator is the maximum display value for the MSB - '1' or '1/2', '3' for '3/4'; the denominator is the total number of possible display values '0, 1' (hence '2' for '1/2'), '0, 1, 2, 3' (hence '4' for '3/4'). Thus, "3 3/4 digit" doesn't mean that the numerical significance is 3.75 times as much as much as for a 1-digit meter, but instead that there are three full digits, plus a digit that shows a value 0-3.

Source: What's a half digit anyway?

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  • \$\begingroup\$ Why do you say 1/3, 1/2 and 2/3 would be more accurate? What is it based on? I'd suppose it should instead be like 1/9, 2/9 and 1/3, since the digit is able to describe these parts of nonzero range. \$\endgroup\$ – Ruslan Dec 20 '15 at 7:18
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    \$\begingroup\$ The number of digits in fractional terms would be the base-10 logarithm of maximum reading, in lowest-digit units. Each full digit increases the reading by a factor of 10, thus increasing the logarithm by 1. Adding a leading 0 or 1 doubles the number of readings, and the base-10 logarithm of 2 is about 0.301 [about 1/3]. Adding a 0-2 triples the number of readings; the base-10 logarithm of 3 is 0.477 [about 1/2]. Adding a 0-3 quadruples the number of readings, and the base-10 logarithm of 4 is 0.602 [about 2/3]. \$\endgroup\$ – supercat Dec 21 '15 at 16:57
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The final answer given in not completely representative of what will be found on a meter. A small correction to the description is needed.

The '19,999' really means 20,000: 4 digits for the 0,000 and an implied 1. Because it is implied it is give 9,999 but really means 10,000:1 of the full scale value. That is the missing bit.

If the scale is 0-5 volts, then the reporting display show 0-5 volts divided by 10,000 for 4 digits, and 20,000 for 4.5 digits. In other words the extra 'half digit' means it can display a change of 1/2 the amount indicated by a 4 digit machine.

If it was a 0-10,000 volt meter, it can show 1 volt steps with 4 digits (really only going up to 9,999 volts).

An extra half digit means it can show 0.5 to 9,999.5 volts, but not 9,999.2 or 9,999.8. The last digit is either 0 or 5. That is the '1/2'.

A 5-digit machine could display 9,999.1 and 9,999.7 and 9,999.5. That is a 'full 5 digits'.

Some scales show 0.2 g resolution, not 0.1 so they can show 999.2, 999.4, 999.6, 999.8 and 999.0.

How many digits is that? :)

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That means that the maximum reading on the display is 19999 - the first digit can only be 0 or 1.

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  • \$\begingroup\$ Or -1, because of the sign segment. \$\endgroup\$ – Ignacio Vazquez-Abrams Oct 8 '13 at 23:01
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I posted this answer on the 4.75 digit topic, but I thought it would be useful here too.

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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