Why accuracy matters?

The accuracy shows what is the real measured value within confidence interval.
Every DMM has accuracy specification from which can be calculated the interval in which the real value of the parameter lies within it.

The accuracy given in the DMM specification is relative, given in last digits, percents (%) or part per millions (ppm) and have to be calculated to absolute value.

The relative accuracy is given for a time period, temperature range and warm-up period. For example :

• 90 days, 1 year, 2 years from the date of the last calibration
• ±5°C difference between the real temperature and the calibrated tempearture
• 2 hours warm-up period

Here an example with 3 real DMM will be given : 

• Low end : Uni-T UT61A which cost ~ 50 USD
• Middle : Keithley 2110 which cost ~ 600 USD
• High end : Keysight 3458A which cost ~ 10000 USD

Lets assume that we measured 5V on the USB power supply with these 3 DMM. 

Tip : how to convert ppm to an absolute value -> ppm/1,000,000 * value

DMM Model	Range	Relative accuracy ±(readings + range)	Absolute accuracy
UT61A	40V	±(0.5% + 1 digit) Resolution for 40V is 0.01V	0.5% * 5V = 0.025V 1 digit * 0.01V = 0.01V Total : ± 0.035V (35mV)
Keithley 2110	10V	±(0.012% + 0.002%) 1 year, 23° ±5°C	0.012%*5V= 0.0006V 0.002%*10V= 0.0002V Total : ± 0.0008V (0.8mV)
Keysight 3458A	10V	±(8 ppm + 0.05 ppm) 1 year, 23° ±5°C	8ppm*5V = 0.00004V 0.05ppm*10V=0.0000005V Total : ± 0.0000405V (0.0405mV)

You can see the difference of absolute accuracy and the price which is paid for.

For ~50 USD, your real measurement value is between 4.965V and 5.035V.

For ~600 USD, your real measurement value is between 4.9992V and 5.0008V.

For ~10000 USD, your real measurement value is between 4.9999595V and 5.0000405V.

Because the voltmeter is used in the current and resistance measurements, the better accuracy in DC Voltage measurement, means also better accuracy of current and resistance measurements.