Pulse Output Signals for Linear Processes
Pulse output signals are related to flow rate by a constant, usually referred to as the K-factor The K-factor is reported as the
number of accumulated pulses that represents a particular volume, such as gallon or liter K-factors are indicated in pulses
per unit volume or counts per unit volume An example of a K-factor, normally supplied by the manufacturer, might be 2000
counts per gallon The K-factor is correlated to flow through a simple mathematical relationship:
Frequency = K-factor × Volume per unit of time
60
Using the previous example of 2000 counts per gallon and further assuming this meter has a maximum flow rate of 25 gpm,
the formula can be arranged to calculate the input frequency required for a scaling point as follows:
Frequency =
60
2000 × 1 (gal) = 33.333 Hz at 1 gpm
Given that the meter has a maximum flow rate of 25 gpm, the maximum frequency would then be:
Frequency =
60
2000 × 25 (gal) = 833.333 Hz at 25 gpm
A programmable display requires at least two points The first point is the zero or minimum flow and the second is normally
the maximum flow rate For the imaginary flow meter used in the example above, the scaling would be as follows:
Input Value for Scaling Point 1 = 0
Display Value for Scaling Point 1 = 0
Input Value for Scaling Point 1 = 833 33
Display Value for Scaling Point 2 = 25
Pulse Output Signals for Non-linear Processes
Few flow meters actually behave in a linear way There is always some uncertainty about the exact flow at a given reported
input value For many common flow measurement applications, the assumption of linear flow is adequate for the process
being measured When higher accuracy is required, a technique called linearization is often employed
When the flow meter is being calibrated, multiple data points are obtained for the particular meter being tested A typical five
point calibration run is displayed below
GPM (LPM)
Average Frequency
**UUT Hz Counts/GAL
UUT K (Hz × 60)/NK gpm
Error (% FS)
15 00 (59 78)
769 7
3078 59
14 90
–0 65
9 06 (34 3)
466 1
3086 75
9 03
–0 38
5 49 (20 78)
285 2
3118 64
5 52
0 65
3 32 (12 57)
171 7
3103 95
3 32
0 17
2 00 (7 57)
103 6
3101 80
2 01
0 10
**UUT = Unit Under Test
If this meter produced an actual linear output, the K-factor calculation for the Unit Under Test would be exactly the same for
each measurement point Inspection of the UUT K in the example provided shows that this is not the case and indicates that
this meter is not a perfectly linear device
Many programmable displays allow for linearization and can provide a better match of the displayed flow values with the
actual flow values by incorporating more measurement points In the example, the unit would be programmed for six points,
the five data points and a zero point, and use pairs of input values to accomplish the linearization
Additional Scaling Information
Page 15
March 2017
DSY-PM-00279-EN-03
