Hach-Lange pHD sc - Manual (Page 32)

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Troubleshooting
Span Reading in pH 10 Buffer
With the sensor in pH 10 buffer, the sensor span reading should be at least –160 mV less
than the noted offset reading taken in step 6. Examples of typical readings:
7. If the span reading is at least +160 mV more than or – 160 mV less than the offset
reading in pH 4 or pH 10, respectively, the sensor is within factory-specified limits.
If not, contact Technical Support.
6.4 Checking ORP Sensor Operation
Simple tests using the sc controller or a multimeter and a 200 mV reference solution can
determine if the ORP sensor is operating properly. Determine if the sensor has integral
digital electronics or uses an external digital gateway. If the sensor uses a digital gateway,
it will be hard-wired to the digital gateway through terminal connections within the digital
gateway enclosure. If the sensor uses a digital gateway proceed with
section 6.4.1 . If the
sensor has integral digital electronics, move to section 6.4.2 on page 26 .
6.4.1 Troubleshooting the ORP Sensor without Integral Digital Electronics
1. Disconnect the red, green, yellow, and black sensor wires from the digital gateway.
2. Place the sensor in a 200 mV reference solution and allow the temperature of the
sensor and reference solution to equalize to approximately 25 °C (70 °F).
3. Verify that the sensor temperature element (300 ohm thermistor) is operating by
measuring the resistance between the yellow and black wires. The reading should be
between 250 and 350 ohms at approximately 25 °C (70 °F).
4. Reconnect the yellow and black wires.
5. Connect the multimeter (+) lead to the red wire and (–) lead to the green wire. With the
sensor in the 200 mV reference solution, measure the dc millivolts. The reading
should be between 160 and 240 mV. If the reading is outside these limits, contact
Technical Support.
6.4.2 Troubleshooting the ORP Sensor with Integral Digital Electronics
1. Place the sensor in 200 mV reference solution and allow the buffer and sensor to
reach temperature equilibrium. This can be verified by monitoring the sensor
temperature value for a stable temperature measurement. This value is located on the
sc100 display when it is in measurement mode.
2. From the Sensor Setup Menu on the sc controller, select “Diag/Test” and confirm.
Select “Sensor Signal” and confirm. The reading should be between 160 and 240 mV.
If the reading is outside these limits, contact Customer Service.
Table 9 Typical Span Reading Examples (pH 10 buffer)
Offset Reading (in pH 7 buffer) Span Reading (in pH 10 buffer)
–50 mV –210 mV
–25 mV –185 mV
0 mV –160 mV
+25 mV –135 mV
+50 mV –110 mV