Frequency  counters:

The measurement of energy used in our home is an application to which digital metering is suited. It is easier to read the drum type, digital kilowatt-hour meter than to read pointer type meter. While measuring frequencies of signals, digital metering is not more convenient only, but more accurate also.

The frequency counter measures by counting pulses actually, in a manner similar to the method the utility meter counts the number of turns of a motor. But frequency counter works electronically, in absence of any moving parts. It measures frequency by tallying up the number of pulses in the oscillating wave, even when the number of pulses per second is large.

The accuracy of the frequency counter is the function of lock-in time. Lock-in is done in 0.1 second, 1 second or 10 seconds.  Increasing lock-in time by a factor of 10 will cause accuracy to be better by one additional digit. Modern frequency counters are good to 6, 7 or 8 digits; sophisticated lab devices will show frequency to 9 or 10 digits.