Dynasonics DXN Portable Ultrasonic Measurement System - Manual (Page 102)

APPENDIX E: K-FACTORS EXPLAINED
The K-factor (with regards to flow) is the number of pulses that must be accumulated to equal a particular volume of fluid. You
can think of each pulse as representing a small fraction of the totalizing unit.
An example might be a K-factor of 1000 (pulses per gallon). This means that if you were counting pulses, when the count total
reached 1000, you would have accumulated 1 Gallon of liquid. Using the same reasoning each individual pulse represents an
accumulation of 1/1000 of a gallon. This relationship is independent of the time it takes to accumulate the counts.
The frequency aspect of K-factors is a little more confusing because it also involves the flow rate. The same K-factor number,
with a time frame added, can be converted into a flow rate. If you accumulated 1000 counts (one gallon) in one minute, then
your flow rate would be 1 gpm. The output frequency, in Hz, is found simply by dividing the number of counts (1000) by the
number of seconds (60) to get the output frequency.
1000 ÷ 60 = 16.6666... Hz. If you were looking at the pulse output on a frequency counter, an output frequency of 16.666...Hz
would be equal to 1 gpm. If the frequency counter registered 33.333...Hz (2 × 16.666...Hz), then the flow rate would be 2 gpm.
Finally, if the flow rate is 2 gpm, then the accumulation of 1000 counts would take place in 30 seconds because the flow rate,
and hence the speed that the 1000 counts is achieved, is twice as great.
Calculating K-factors for Ultrasonic Meters
Many styles of ultrasonic flow meters are capable of measuring flow in a wide range of pipe sizes. Because the pipe size and
volumetric units the meter will be used on vary, it is not possible to provide a discrete K-factor. Instead the velocity range of
the meter is usually provided along with a maximum frequency output.
The most basic K-factor calculation requires that an accurate flow rate and the output frequency associated with that flow rate
be known.
Example 1:
Known values are:
Frequency = 700 Hz
Flow Rate = 48 gpm
700 Hz × 60 sec = 42,000 pulses per min
K-factor = 42,000 pulses per min = 875 pulses per gallon
48 gpm
Example 2:
Known values are:
Full Scale Flow Rate = 85 gpm
Full Scale Output Frequency = 65- Hz
650 Hz x 60 sec = 39,000 pulses per min
K-factor = 39,000 pulses per min = 458.82 pulses per gallon
85 gpm
The calculation is a little more complex if velocity is used because you first must convert the velocity into a volumetric flow
rate to be able to compute a K-factor.
To convert a velocity into a volumetric flow, the velocity measurement and an accurate measurement of the inside diameter
of the pipe must be known. Also needed is the fact that 1 US gallon of liquid is equal to 231 cubic inches.
DXN Portable Ultrasonic Measurement System
Page 102 February 2013