Sparks Electrical News March 2018

TOOLS OF THE TRADE

12

EXPLAINING DIGITS, COUNTS, RESOLUTION, ACCURACY AND PRECISION

W hen it comes to multimeters and com- ponent testers, what is the difference between counts and digits, and what do they represent? “For professional work, products may have all the same specifications, but when it comes down to ac- curacy, resolutions, counts and some of the other quality features which a product might have, that’s where the decisions can be made on which to pur- chase,” says Ryan Burger, divisional product man- ager: Tools & Instruments at HellermannTyton. He explains that multimeters and component testers often specify display resolution in counts or digits. It is typical to see values such as 20 000 counts or

4 1/2 digits in specifications. Manufacturers often use counts, or digits, or both to specify their display resolution. The following describes what counts and digits mean and how to convert between the two types. Counts “Counts and digits are almost synonymous with one another, they are simply referenced in a different way,” says Burger. Instruments that have display counts specified are usually multimeters and component testers. In both instrument types, there are different ranges that are used to obtain different values in measuredreadings.Countstellyouwhatthe instrument

can display before it changes to the next range. For example, suppose a multimeter has 50 000 counts. This means the range changes when it hits 50 000 on display. To illustrate, a 50 000 count meter can read 49.999 V on the display, but when it tries to display 50 V, it will read 50.00 V instead. The multimeter moves to the next range, which generally also means one digit of resolution will be lost. Digits It is often shown on DMM specifications where resolution is described in digits. It is common to see 4.5 digits, 5.5 digits, 6.5 digits, and higher. When reading this, the whole number must be split up from the fraction. For example, let’s look at 4.5 digits, or 4 and 1/2 digits. The '4' represents full digits, meaning digits that can be represented or shown from 0-9 on the display. The '1/2' or '0.5' represent the first digit of the reading display and the maximum it can read. In this case, '1/2' digit means the first digit that can represent the digit 0-1. Putting it together, 4.5 digits mean the display can show 00 000 to 19 999. This is because the first digit can show 0 and 1, and the last '4' proceed- ing digits can show 0-9. There is 4 because it is a 4 and a 1/2 digit meter. This means, if this was to be converted to count, it would be 20 000 count, because 4.5 digits mean you can display 00 000 to 19 999. The next digit up would change its range. Another example, let’s look at 3.75, or 3 and 3/4 digits. Typically, 3/4 digits can represent a digit that can read from 0 to 3, and in some cases 0 to 5. Taking the same idea as the previous example, 3 whole digits mean 3 digits can represent 0-9, and the 3/4 represents the first digit from left. Put- ting this together, 3.75 digits can represent display showing 0000 to 3999 before range is changed. Meaning, an instrument with this digit resolution would have 4000 counts. Caution should be given however as some man- ufacturers also use 3/4 to represent digits that can display 0 to 5. In this case, 3.75 digits can also rep- resent display resolution showing 0000 to 5 999. Converting this to counts, it would be 6000 counts. Because there are two different uses of '3/4', it is important to look at specifications carefully, espe- cially the range of the instrument. Some manufac- turers do not specify counts, so therefore looking at the range would be the best way to determine how much display resolution you can obtain when making measurements. For example, if the range of the multimeter for dc voltage is 2 V, 20 V, 200 V, 1000 V, it can be assumed that most likely the me- ter has a 1/2 digit. If it is 5 V, 50 V, 500 V, 1000 V, it can be assumed that most likely the meter has a 3/4 digit. Below is a table of common display resolution conversion between digits and counts:

Digits

Counts

4 1/2 (4.5)

20 000

4 3/4 (4.75)

40 000 (or 50 000)

5 1/2 (5.5)

200 000

Resolution DMM resolution depends on the maximum number of analogue-to-digital converter (ADC)

SPARKS ELECTRICAL NEWS

MARCH 2018