Table of Contents
4.
SLOPE HI or SLOPE LO
Once the two-point (manual or automatic) pH calibration is complete, the transmitter
automatically calculates the sensor slope at 25 °C. If the slope is greather than 62 mV/pH,
the transmitter displays the SLOPE HI error. If the slope is less than 45 mV/pH, the
transmitter displays the SLOPE LO error. The transmitter will not update the calibration.
1.
2.
3.
4.
5.
6.
7.
-0- OFFSEt
The -0- OFFSEt message appears if the standardization offset (in mV) exceeds the
programmed limit. The default limit is 60 mV, which is equivalent to about a unit change in
pH. Before increasing the limit to make the -0- OFFSEt message disappear, check the
following:
1.
2.
3.
4.
5.
Instruction Manual
Replace the sensor.
Check the buffers. Inspect the buffer solutions for obvious signs of deterioration,
such as turbidity or mold growth.
Neutral and slightly acidic buffers are highly susceptible to molds. Alkaline buffers
(pH 9 and greater), if they have been exposed to air for long periods, may also be
inaccurate. Alkaline buffers absorb carbon dioxide from the atmosphere, which
lowers the pH.
If a high pH buffer was used in the failed calibration, repeat the calibration using
fresh buffer. If fresh buffer is not available, use a lower pH buffer.
For example, use pH 4 and pH 7 buffer instead of pH 7 and pH 10 buffer.
Allow adequate time for temperature equilibration. If the sensor was in a process
liquid substantially hotter or colder than the buffer, place it in a container of water at
ambient temperature for at least 20 minutes before starting the calibration.
If manual calibration was done, verify that the correct pH values were entered.
Verify all wiring connections, including connections at a junction box.
Check the pH sensor for cleanliness. If the sensor looks fouled or dirty, clean it.
Refer to the sensor instruction manual for cleaning procedures.
Replace the sensor.
Verify that the reference pH meter is working properly and is properly calibrated.
Verify that the process pH sensor is working. Check its response in buffers.
If the transmitter is standardized against pH determined in a grab sample, be sure to
measure the pH before the temperature of the grab sample changes more than a
few degrees.
Verify that the process sensor is fully immersed in the liquid.
If the sensor is not completely submerged, it may be measuring the pH of the liquid
film covering the sensor. The pH of this film may be different from the pH of the bulk
liquid.
Check the pH sensor for cleanliness. If the sensor looks fouled or dirty, clean it.
Refer to the sensor instruction manual for cleaning procedures.
Diagnostics and troubleshooting
97
- Quick Links:
- 5081 Programming
- Manual Calibration
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Troubleshooting
