Hach SIGMA 900, Руководство пользователя

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Page 52
Flow Proportional Operation
8837prg.fm
Section 5
Flow Proportional Sampling, External—Using 4–20 mA Signal Converted to Pulses, Flow Signal Interface
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 ten 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.
Use the following examples to help you determine the value that you need to
enter when programming for sampling based on counts.
Example 1
You want to collect 24 samples over a 24-hour period on a 4–20 mA flow
proportional basis.
Average Flow Rate = 3.5 mgd (average flow rate over the 24-hour period)
Maximum Flow Rate = 10 mgd
a = 1440 minutes
1. Calculate Q .
2. Calculate t .
3. Multiply Q x t x 10 .
Therefore, the value entered for
INTV = _ _ _ _ CNTS
is 210.
Q
Average Flow Rate
Maximum Flow Rate
------------------------------------------------------
3.5 mgd
10 mgd
--------------------- 0.35 mgd = = =
t
a
n
---
1440 min
24 samples
------------------------------ 60 min/sample = = =
0.35 mgd 60 min/sample
×
10
×
210 =