Sparks Electrical News

When it comes to multimeters and component 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 accuracy, 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 purchase,” says Ryan Burger, divisional product manager: 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.

Know Your ToolsThe 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 measured readings. Counts tell you what the 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 ½ 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' proceeding 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 ¾ 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. Putting 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 manufacturers also use 3/4 to represent digits that can display 0 to 5. In this case, 3.75 digits can also represent 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, especially the range of the instrument. Some manufacturers 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 meter 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.

Resolution

DMM resolution depends on the maximum number of analogue-to-digital converter (ADC) counts during a full conversion. For example, the theoretical resolution of a 2 000-count meter with a 3½-digit display is (1/2 000)(100%) = 0.05%. The larger number of counts should translate into higher resolution, and usually DMMs with a higher resolution have higher accuracy. However, DMM accuracy also depends on other design factors, such as Analogue to Digital Converter (ADC) accuracy, component tolerances, noise level, and the stability of internal references. So, do not automatically assume that a 4½-digit meter is 10 times more accurate than a 3½-digit meter. In addition, since DMMs have automatic polarity detection, they display negative values equal in range to the positive values. That is, the display of a 3½-digit DMM can show any number from -1 999 to 0 and from 0 to 1999.

DEFINITIONS

Accuracy

An instrument’s degree of reliability and degree of acceptable error can occur under specific operating conditions. Alternatively, how close the measurement is to the actual or reference value of the signal being measured. Accuracy may be represented as a percentage as well as digits. Example: an accuracy of ±2%, +2 digits means 100.0 V reading on a multimeter can be from 97.8 V to 102.2 V. Accuracy is generally compared to an accepted industry standard.

Precision

An instrument’s degree of repeatability i.e. how reliably can the instrument repeat the same measurement and read the exact same value when making a series of measurements.

Component aging, battery condition, temperature and warm-up time may all affect repeatability. “Obviously, if you can have accurate and precise readings, you will have repeatability in your readings and assurance in terms of the measurements,” says Burger.

Resolution

The smallest increment an instrument can detect and display such as hundredths, thousandths, millionths.

Range

The upper and lower limits an instrument can measure a value or signal such as amps, volts and ohms. The larger number of counts should translate into higher resolution and generally a higher accuracy.

Products

“It comes down to the type of work you will be doing,” says Marius Kempenaars, Technical Specialist: Tools and Instruments at HellermannTyton. “For someone doing standard electrical work, the TBM869 Digital Multimeter would be overkill. But if it’s a consulting engineer or a design engineer, that’s the guy who would use it and he would understand the reasons why the product has 5 -4/5 digits and 500 000 counts as opposed to the TBM811, which is perfect for electricians, and which has 3 -9/10 digits and 10 000 counts.” As an example, it is recommended that multimeters for entry level/training and DIY have 2000-4000 counts and qualified electricians use meters with 6 000 counts and upwards. “The sales people therefore need to ask the right questions when approached by customers,” notes Burger, “Where are you using it, how are you using it, what’s your budget?”

Enquiries: www.hellermanntyton.co.za

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