What does Count / Digits refer to on a Multimeter?
The terms Count and Digits are used interchangeably, and refer to the way a digital multimeter can display a result. They describe the maximum displayable value in each measuring range, and by default the resolution displayable for each range. When understood, it can help significantly in choosing the correct DMM for your needs.
Before we go into this, we’ll have a look at the terms ‘range’ and ‘resolution’. These two terms need to be understood, as they refer to digital multimeters, in order to get a good grasp of count and digit.
Range. Depending on how big or small a quantity that you are measuring, the multimeter needs to use the circuitry that is best suited to that particular quantity. This is known as range. Each range is described on the meter in terms of the maximum value measurable for that range.
For example, if you are measuring between 0V and 10V the multimeter should be set on a range that uses circuitry best suited for these values, eg 20V range. Here, 20V is the maximum measurable value on that range, and 0V -10V falls comfortably in that range.
However, if you are measuring 50V - 100V, then the circuitry that is suitable to measure the 0V-10V range would be unsuitable. Another range, say 200V should be selected.
With most digital multimeters these days, automatic ranging is performed by the meter.
Resolution. The term resolution refers to how many significant figures can be displayed by the DMM. That is, the smallest fraction of the measurement that can be shown. A meter displaying a value of 1.0198V has better resolution than one that can only display 1.020V on the same test. Here, the resolution would be better by a factor of 10.
Understanding the impact of the Count
Count is a figure used to describe how well a digital read out can display the measurement abilities of a digital multimeter, and gives an indication of the likely limit of each range.
The count number refers to the limit of the highest value the DMM will display for each range, and the number of digits displayed for these values. As soon as a measured value reaches this limit, the range will move up and the resolution will be downgraded by a factor of 10, that is, the decimal place will be moved to the right one spot and the last digit on the right will drop off.
Count becomes important if high resolution results are required when measuring.
Looking at an example – if we have a multimeter with a count of 40000, then the highest digit readout for each range would be 39999 (not withstanding the decimal point). Note that the display shows one less than the count specification. This is because the number 40000 is easier to work with!
If we measure 1V the screen will show 1.0000V. However, if we increase the voltage being measured until it goes above 3.9999V to 4V, then the screen will display 4.000V (which is actually 04.000V in the count). It cannot display 4.0000V because that is the cut off for that range.
Now that the range has changed you can see that the resolution displayed has been downgraded by a factor of 10. The decimal place has moved to the right one spot causing the far left digit to be dropped.
Now we can move up in this range from 4.000V until we reach 39.999V, at which time we have again reached the limit for this range. At 40V the display will move to 40.00V (Which is actually 040.00V in the count).
Again the range has changed and the resolution has been downgraded by another factor of 10. This change in range when the measurement passes each range limit, will continue to occur until the measurement extends beyond the testing abilities of the multimeter.
If we had a multimeter that was designed to measure up to 1000V with a 40000 count, then, depending on the actual circuitry, the ranges and resolution could go;
0 to 3.9999 with resolution of 0.0001
4 to 39.999 with resolution of 0.001
40 to 399.99 with resolution of 0.01
400 to 1000 with resolution of 0.1
Knowing your Count Requirements
When buying a digital multimeter, it is important to understand what you are testing, the range you will be testing in, and whether or not minor values in regards to resolution are important.
For example, if you are testing a car battery, then two decimal places at 12V will more than likely be ample resolution, so a 2000 count display will be plenty. This will give a resolution to two decimal places up to 19.99V. The same multimeter is capable of displaying up to 199.9V (1 decimal place) and 1999V (0 decimal places).
However, if you require more resolution in your results, eg. down to three decimal places for the same 12V supply, then a 2000 count will not suffice because it needs an extra digit. A 20,000 count will work here.
Do not get confused between a larger count for display resolution, and the accuracy of the multimeter. They are two different characteristics. One gives the measurement accuracy of the multimeter (see Understanding Terms - Accuracy for more info). The other (count) is the ability to display this accuracy to the user.
What about Digits?
Digits is just another way to describe the count, but in my book, a little less user friendly.
If the specification of the multimeter gives digits rather that count, it will be the number of digits that can be displayed from 0-9 plus the most significant digit, which cannot be displayed right through to 9.
The format is given as a whole number followed by a fraction, usually a 1/2 or a 3/4.
The whole number is used to represent how many digits displayed from 0-9.
The fraction is used to describe how high the most significant digit in each range can go. A 1/2 usually means the most significant digit for each range goes to 1. A 3/4 means the most significant digit for each range is greater than 1. This actual figure will change between manufacturers.
For example. A 3 1/2 digit can display up to 1999. That’s 3 digits from 0-9 AND a most significant digit from 0-1. Note that this is the same as a 2000 count.
A 4 3/4 digit may display up to 39999, 49999, 59999 or even 69999. That’s 4 digits from 0-9 and the 3/4 meaning a most significant figure is greater than one. The 3/4 fraction given leaves room for ambiguity, which is why I prefer the count.
What’s wrong with the most significant figure? Why doesn’t the multimeter display the most significant figure in digits up to 9 like the rest of the display? Common question!
It’s got nothing to do with the readout not being able to show higher digits at the most significant place. It’s actually to do with the circuitry in the multimeter and when the circuitry gets to it’s limit in each range. So, with a 2000 count the circuitry in each range gets to its limit at 1999 and has to change range.
Something to look out for with your multimeter...
Sometimes it is important to know when the multimeter actually changes range. If you are measuring a quantity that requires a certain amount of significant figures, but what you are measuring is on the border of when the multimeter changes range, then the resolution required could down grade by a factor of 10 right near the point of measurement and you lose the required lowest significant figure. Thus, this multimeter may actually not be suitable. Again, it is important to understand what you are testing, and the range you will be testing in.