Badger Meter B2800 - Manual (Page 18)

ADDITIONAL SCALING PARAMETERS
This information is supplied as a general introduction to the basic concepts used to scale rate displays The applicability of the
information is dependent on the type and capabilities of the specific display/monitor used
Flow meters producing an electronic signal are normally supplied in one of two output formats The pulse format generates
some form of alternating signal that can be raw, that is no amplification or wave shaping can be done prior to transmission to
the readout The output pulse rate is related proportionally to flow rate Pulses can also be modified to produce higher output
amplitudes or specific wave shapes
The other output format is an analog signal This is a continuous, variable voltage or current signal that is normally scaled to
the dynamic range of the meter Typical analog signals are 0…5V DC, 0…10V DC, and 4-20 mA The analog signals may or may
not be derived from a raw pulse signal produced by the flow meter
Scaling for any of these input signals always requires at least two scaling points for a linear process – zero or minimum flow
point and the maximum flow point Additionally, each scaling point has two components, the actual input signal value and
the desired display value at that input signal for that scaling point
For example, a pulse output flow meter has a flow of 50 gpm at a pulse rate of 100 Hz The actual input signal is the 100 Hz
figure, but allowing the display to read 100 would be meaningless to the operator The solution to this problem is to scale the
display to read 50 (gpm) when the input is 100 Hz
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
Flow Monitor, B2800 Flow Monitor – Advanced
Page 18 November 2013