Suppose I have a 20,000 counts multimeter, and I am measuring a 220k resistance with it.

The multimeter can only display a resistance below 200k as 200.00k - supposedly because of the counts. For higher resistances, it has to switch the range and starts displaying the result as 0.2200M. But this makes no sense to me: switching range doesn't require the display to change in a major way like that.

Granted, by switching range the meter loses one digit of precision, and conjuring it up would be wrong. But why not simply hide that non-existent digit, and display 220.0 k (see how the last digit is missing) instead of the less readable 0.2200M - they both have the exact same precision!

Why don't multimeters do this seemingly very sane thing, not even the very expensive ones? Am I missing something important?

  • \$\begingroup\$ Someone I asked in person suggested that since 220.0 k and 0.2200M are exactly the same thing, this might just be because nobody cares. If you don't care, that would be a helpful answer too :) \$\endgroup\$ Apr 30 '11 at 14:33
  • \$\begingroup\$ related thread: electronics.stackexchange.com/q/94503/7036 \$\endgroup\$ Apr 29 '14 at 21:02

The display is not the limiting factor (as you point out). Fundamentally, there is a cut-off. If the display was the most expensive part in the meter, then most meters would probably read up to 9999 on all digits. However, there's a couple factors that make the meter ranges what they are.

There is a limit to how high a range goes - the count is a design decision for the meter maker. A 20... meter isn't the only type available either, I personally have a 6000 count meter.

Logarithmically, you get the most bang for your buck by having 2000 counts. See the image of the number line below. By doubling the counts from 1000 to 2000, you get a bigger bang for your buck, and you also get to use one more digit (better for marketing).

enter image description here

There's another reason for not dropping the least significant digit. Part of the gotcha with auto-ranging meters is that you can easily forget that the meter has different measurement ranges, and that the measurement accuracy changes with each range. For example, compare the following displays:

193.00 k
 .1930 M
193.0  k <----- Would you notice the missing digit as easily?

Personally, I see it as a more obvious way of conveying information about the measurement. It doesn't detract much from the readability in my opinion.

  • \$\begingroup\$ Even though I disagree on what's more readable and in which reading the precision is more obvious, your answer points out that it is indeed more subjective than I thought. Thanks! \$\endgroup\$ May 2 '11 at 8:28
  • \$\begingroup\$ What about showing digits in the same place as in the megs scale, but moving the decimal three places to the right and showing a "k" icon? \$\endgroup\$
    – supercat
    Oct 31 '13 at 19:10
  • \$\begingroup\$ @supercat I don't think that would work as well because when the count rolls over to 1.0000 M, it will look like it is changing ranges again. \$\endgroup\$
    – W5VO
    Oct 31 '13 at 20:04
  • \$\begingroup\$ @W5VO: Fair point; I guess if one doesn't want to look like one is changing ranges the question becomes one of whether it's more natural to regard values of 470,000 and 1,500,000 ohms as 470k and 1500k, or as 0.470M and 1.500M. \$\endgroup\$
    – supercat
    Oct 31 '13 at 20:14

Well, three years late, but I just got here. :-)

The reason, which no one has touched on, is that since X-1/2 digit displays can only display a "1" or a "-1" as the MSD, when the display rolls over from, say, 1.999 to 2.000 there's no place for the "2" to go except to the right.

Reconsidering, that's not what happens. Sorry about that...

When, say, a 20000 count 4-1/2 digit multimeter tries to roll over from 19999 to 20000, it can't, so the MSD stays on "1" and the remaining 4 digits blank in order to indicate OVERRANGE mode.

For example, on the 20000 ohm range, the meter will read XX.XXX k until the resistance measured is greater than 19999 ohms, when - since there is no "2" MSD - the meter will go into OVERRANGE mode and read 1BBBB, where "B"s are blanked digits.

If the meter overranged at 20000 ohms, and the resistance stayed there, changing the range switch to the 200 k range will result in the meter reading B20.00 k, a loss of one digit of resolution until the resistance increased to 100 k, when the meter would read 100.00k, and thereafter until the meter read greater than 199.99 k, when it would read 1BBBB again,

The brunt of it is that, for a 20000 count meter, on each range the resolution will only be five digits wide from 10000 to 19999, 20000 and higher being impossible to display.

  • \$\begingroup\$ Except the asker brings up a 4 1/2-digit display, with the leftmost digit blanked in their example. \$\endgroup\$ Jun 28 '14 at 12:45
  • \$\begingroup\$ That seems like a crippling limitation. I don't see a reason for that at all. \$\endgroup\$
    – sherrellbc
    Jun 28 '14 at 12:47
  • \$\begingroup\$ @sherrellbc: Not really. Unless you get a more expensive DMM with higher precision any further digits are a lie regardless. \$\endgroup\$ Jun 28 '14 at 12:54
  • \$\begingroup\$ @IgnacioVazquez-Abrams, That surprises me a bit. So anything that requires higher than a 1 in the MSD loses a digit of precision to the right. \$\endgroup\$
    – sherrellbc
    Jun 28 '14 at 13:02
  • \$\begingroup\$ @sherrellbc: Only if the DMM isn't built for it. It is perfectly possible to build a 4000- or 6000-count DMM with decent precision, but it requires better engineering than a 2000-count DMM (not to mention X0000-count DMMs or higher!). \$\endgroup\$ Jun 28 '14 at 13:10

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