Hach 900 MAX Instrument Manual page 136

Appendix D
Flow Proportional Sampling, External—Using 4–20 mA Signal Converted to Pulses, Flow Signal Interface
Page 134
How to Calculate Pulses/Counts
1. Determine the flow increment between samples:
Q 85,000 gallons
f
= --- - = -------------------------------------- -
n 24 samples
2. Multiply the flow increment, f, by the pulse frequency output of the flow
meter.
×
3,542 gallons/sample
Therefore, the value for
Example 3
You want to collect 48 samples over a 16-hour period. The total expected flow
over this period is 1,750,000 gallons. The flow meter pulse frequency output is
one pulse for every 1,000 gallons.
1. Determine the flow increment between samples:
Q 1,750,000 gallons
f
= --- - = ---------------------------------------------- -
n 48 samples
2. Multiply the flow increment, f, by the pulse frequency output of the flow
meter.
×
36,458 gallons/sample
Therefore, the value for
The Flow Signal Interface (Cat. No. 2021) is an optional 4–20 mA interface
that converts 4–20 mA current signals (from a flow meter) to 12 V dc pulses.
At 20 mA, the interface transmits 10 12-volt pulses per minute. As the current
signal decreases, the 12 V pulses decrease proportionally. Typically, users
collect a certain number of samples over a given period of time when
sampling in proportion to the flow rate.
To calculate the number of 12 V dc output pulses between samples,
determine the total number of samples (represented by n in the following
examples) that you want to collect, and the period of time over which you want
to collect them. Use the three-step method presented below to determine the
"Total Count" value that you will enter while programming the Setup section of
the sampler.
1. Calculate Q, where Q is the average flow rate (during the sampling
program) divided by the maximum flow rate. (The maximum flow rate
corresponds to the 20 mA output of the flow meter.)
2. Calculate t, where t is defined as a/n; n is the total number of samples
collected over a given period of time; and a represents time in minutes,
over which n samples are collected.
3. Multiply Q x t x 10. Enter the result in the programming step:
.
INTV = _ _ _ _ CNTS
Note: The product of Q x t x 10 is not necessarily a whole number. You must round off
the result to the nearest whole number.
3,542 gallons/sample
=
1 pulse
-------------------------- - 70.84 pulse/sample
=
50 gallons
is 71.
INTV = _ _ _ _ CNTS
36,458 gallons/sample
=
1 pulse
---------------------------------- -
= 36.458 pulses/sample
1,000 gallons
is 36.
INTV = _ _ _ _ CNTS
8990apdx_pulsecounts.fm