21
MODEL 1181pH/ORP
SECTION 3.0
START UP, AND CALIBRATION
3.2.3 Electronic Calibration using a DC Voltage
Source.
A ±2.500 volt DC input is required for calibrat-
ing a 0 to 14 range of the Model 1181 pH. Refer to
Table 3-3 for the appropriate voltage required to simu-
late other pH values .
1.
For pH range of 0 to 14 pH, install the connections
from the DC voltage source to TB2-1 and TB2-2 of
the Model 1181 pH. Be sure to remove sensor
wires from TB2-1 and TB2-2 while applying this
voltage source.
2.
Connect an ammeter between TB1-2 and TB1-3
to indicate the 4 to 20 mADC output of the trans-
mitter. Be sure to disconnect the 1181 indicating
meter first.
3.
Generate a +2.500 VDC signal from the voltage
generator. Adjust ZERO control (under serial
label) for a 4 mADC indication.
4.
Generate a –2.500 VDC signal from the voltage
generator. Adjust SPAN control (under the serial
label) for a 20 mADC indication.
5.
Repeat Steps 3 and 4 until no further calibration is
required.
6.
Reinstall sensor wires and indicating meter (if nec-
essary) .
TABLE 3-3.
pH vs Voltage Input
3.3 SYSTEM CALIBRATION.
The pH sensor and Model
1181 pH Two-Wire Transmitter must be standardized as
a system. Either the standard buffer solution method or
the grab sample method may be used.
3.3.1 Buffer Solution Method.
The following is the
buffer solution method for standardizing the pH sensor
and Model 1181 pH:
1.
Obtain two buffer solutions. One should represent
a low range, and the other a high range value.
Typical buffer solution values are 4 pH, 7 pH, and
10 pH.
2.
Remove the sensor from the sample and rinse it
with clean water.
3.
Immerse the sensor in a low range buffer solution
and allow the reading to stabilize.
NOTE
The sensor must be at the same temperature
as the buffer solution to obtain maximum
accuracy. Allow several minutes for the sen-
sor to reach ambient temperature.
4.
Adjust the ZERO control (under the serial label)
until the reading agrees with the buffer solution
value.
5.
Remove the sensor from the low range buffer
solution and rinse it in clean water.
6.
Immerse the sensor in the high range buffer solu-
tion .
7.
Adjust the SPAN control (under serial label) until
the reading agrees with the buffer solution value.
8.
Repeat Step 3 through 7 until no further adjust-
ments are required. Rinse the sensor in clean
water prior to immersing it in a different buffer
solution.
3.3.2 Grab Sample Method.
The following procedure
should be used to standardize the Model 1181 pH with
grab sample.
NOTE
Make sure the temperature of the grab sample
and the compensation of the analyzer used for
calibration are the same. Since the indications
are temperature sensitive, the reading will
vary if the temperature is different.
1.
With the sensor on-stream, wait at least two to
three minutes for the meter reading to stabilize,
particularly in very hot or cold streams, and the
automatic temperature compensator to reach
equilibrium.
2.
When the meter reading is stable, make a note of
its value and immediately obtain a sample of the
solution from the stream as close to the sensor as
possible.
3.
Check the noted reading against the reading
obtained from the other analysis method and stan-
dardize the transmitter by adjusting the ZERO
control (under serial label).
4.
To ensure greater accuracy, take at least three
grab samples and repeat Steps 2 and 3 for each
sample.
pH
Voltage Input
pH
Voltage Input
0
+2.500VDC
8
–0.357VDC
1
+2.143VDC
9
–0.714VDC
2
+1.786VDC
10
–1.072VDC
3
+1.429VDC
11
–1.429VDC
4
+1.072VDC
12
–1.786VDC
5
+0.714VDC
13
–2.143VDC
6
+0.357VDC
14
–2.500VDC
7
0.000VDC