Thermo Scientific AquaSensors
™
AnalogPlus
™
Differential pH Sensor User Guide
27
pH Offset Test
1.
Disconnect the RED and GREEN sensor wires from the analyzer terminal block while leaving the other
wires connected. Connect the (+) lead of a millivolt meter to the RED wire and the (-) lead of the millivolt
meter to the GREEN wire.
2.
Place the sensor in 7 pH buffer. Stir the sensor for 10 to 15 seconds and then allow the sensor to stabilize
in solution for about 2 minutes. The meter should read 0.0mV +/- 50 mV. If the reading is higher or lower
than specified, replace the salt bridge and filling solution and try again. If on the second attempt the offset
does not meet specification, replace the sensor.
pH Span Test
If the RED and GREEN wires have already been disconnected to run the offset test, skip step 1.
1.
Disconnect the RED and GREEN sensor wires from the analyzer terminal block while leaving the other
wires connected. Connect the (+) lead of a millivolt meter to the RED wire and the (-) lead of the millivolt
meter to the GREEN wire.
2.
Rinse the sensor in clean water. Put the sensor in 4 pH buffer. Stir the sensor for 10 to 15 seconds and
then allow the sensor to stabilize in solution for about 2 minutes. The meter should read a change of at
least –150mV from the offset reading. For example, if the offset is +20mV the reading in 4 pH buffer should
be at least –130mV.
After testing offset and span, the RED and GREEN wires should be reconnected to the analyzer terminal
block.
Sensor Power Test (For Sensors that Connect to the P99 or GLI/Hach Analyzers)
If the sensor does not respond with any reasonable millivolt signal in the offset and span tests, it may be
worthwhile to verify that power is being supplied to the sensor from the analyzer. To do this, connect a
voltmeter from the BLACK lead to the WHITE lead (It is not necessary to disconnect the wires from the
terminal block). The meter should read –5 VDC +/- 0.3 VDC. If the voltage is low, disconnect the white wire
from the terminal block and measure the voltage on the terminal block. If the voltage is within specification
there is a defect in the sensor that is loading down the analyzer. If the voltage remains low then the analyzer
may be defective.
If the sensor passes the temperature, offset, span and power tests but the analyzer is not displaying the
correct values, the analyzer may be defective.
