Thermo Scientific AquaSensors™ DataStick™ Measurement System User Guide
108
For example, if the free chlorine concentration is 0.88 ppm as measured by the colorimeter, the temperature is 25 °C as measured by
the Chlorine DataStick, and the pH is 6.73 as measured by the pH DataStick, then the hypochlorous acid concentration value in ppm
that must be written to the Chlorine DataStick will be
[
]
76
.
0
10
10
476
.
1
25
10
056714
.
0
1
88
.
0
73
.
6
8
8
=
×
+
×
×
+
=
−
−
−
HOCl
7.3 PLC Programming Guidelines
This section provides some guidelines for the PLC programmer.
Assume that the following floating-point registers are allocated within the PLC:
Register
Variable
F
8:1
[
]
HOCl
as read from the Chlorine DataStick
F
8:2
T
as read from the Chlorine DataStick
F
8:3
pH
as read from the pH DataStick
F
8:4
[
]
[
]
−
+
OCl
HOCl
for displaying to the user
F
8:5
[
]
[
]
−
+
OCl
HOCl
entered by user at calibration time
F
8:6
[
]
HOCl
to be written to the Chlorine DataStick at calibration time
The PLC reads
[
]
and
HOCl
T
from the Chlorine DataStick and stores the values in
F
8:1 and
F
8:2, respectively, and it reads
from the pH DataStick and stores the value in
F
8:3.
pH
7.3.1 Measurement
The PLC calculates [
HOCl
] + [
OCl
−
] and stores the value in
F
8:4 using equation (4):
(
) (
)
(
)
(
)
⎭
⎬
⎫
⎩
⎨
⎧
×
+
×
×
+
×
=
−
−
−
3
:
8
8
8
10
10
476
.
1
2
:
8
10
056714
.
0
1
1
:
8
4
:
8
F
F
F
F
(6)
The free chlorine concentration value in
F
8:4 can then be displayed to the user.
7.3.2 Calibration
At calibration time, the user enters a free chlorine value that the PLC stores in
F
8:5. The PLC then calculates
using
equation (5) and stores the value in
F
8:6:
[
]
HOCl
(
)
(
)
(
)
(
)
3
:
8
8
8
10
10
476
.
1
2
:
8
10
056714
.
0
1
5
:
8
6
:
8
F
F
F
F
−
−
−
×
+
×
×
+
=
(7)
The hypochlorous acid concentration value in
F
8:6 can then be written to the Chlorine DataStick to calibrate it.
