Dynasonics TFXP Series Transit Time Ultrasonic Flow Meter - Manual (Page 107)

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 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 x 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 meter 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
1) 700 Hz x 60 sec = 42,000 pulses per min
2)
42,000 pulses per min
K-factor = = 875 pulses per gallon
48 GPM