44
MODEL SOLU COMP II
SECTION 8.0
TROUBLESHOOTING
D. Is the sensor fouled? The sensor measures the pH of the liquid adjacent to the glass bulb. If the sensor is heav-
ily fouled, the pH of liquid trapped against the bulb may be different from the bulk liquid.
E. Has the sensor been exposed to poisoning agents (sulfides or cyanides) or has it been exposed to extreme
temperature? Poisoning agents and high temperature can shift the reference voltage many hundred millivolts.
To check the reference voltage, see Section 8.6.
8.3.4 Invalid Input While Manually Entering Slope.
If the sensor slope is known from other sources, it can be entered directly into the analyzer. The Solu Comp II will
not accept a slope (at 25°C) outside the range 45 to 60 mV/pH. See section 8.3.2 for troubleshooting sensor slope
problems.
8.3.5 Sensor Does Not Respond to Known pH Changes.
A. Did the expected pH change really occur? If the process pH reading was not what was expected, check the
performance of the sensor in buffers. Also, use a second pH meter to verify the change.
B. Is the sensor properly wired to the analyzer?
C. Is the glass bulb cracked or broken? Check the glass electrode impedance. See Section 8.3.2.
D. Is the analyzer working properly. Check the analyzer by simulating the pH input.
8.3.6 Buffer Calibration Is Acceptable, Process pH Is Slightly Different from Expected Value.
Differences between pH readings made with an on-line instrument and a laboratory or portable instrument are nor-
mal. The on-line instrument is subject to process variables, for example ground potentials, stray voltages, and ori-
entation effects that may not affect the laboratory or portable instrument. To make the process reading agree with
a referee instrument, see Section 6.5.
8.3.7 Calibration Was Successful, but Process pH Is Grossly Wrong and/or Noisy.
Grossly wrong or noisy readings suggest a ground loop (measurement system connected to earth ground at more
than one point), a floating system (no earth ground), or noise being brought into the analyzer by the sensor cable.
The problem arises from the process or installation. It is not a fault of the analyzer. The problem should disappear
once the sensor is taken out of the system. Check the following:
A. Is a ground loop present?
1.
Verify that the system works properly in buffers. Be sure there is no direct electrical connection between
the buffer containers and the process liquid or piping.
2.
Strip back the ends of a heavy gauge wire. Connect one end of the wire to the process piping or place it
in the process liquid. Place the other end of the wire in the container of buffer with the sensor. The wire
makes an electrical connection between the process and sensor.
3.
If offsets and noise appear after making the connection, a ground loop exists.
B. Is the process grounded?
1.
The measurement system needs one path to ground: through the process liquid and piping. Plastic piping,
fiberglass tanks, and ungrounded or poorly grounded vessels do not provide a path. A floating system can
pick up stray voltages from other electrical equipment.
2.
Ground the piping or tank to a local earth ground.
3.
If noise still persists, simple grounding is not the problem. Noise is probably being carried into the instru-
ment through the sensor wiring.
C. Simplify the sensor wiring.
1.
Disconnect all sensor wires at the analyzer except pH/mV IN, REFERENCE IN, RTD IN and RTD RE-
TURN. See the wiring diagrams in Section xx.xx. If the sensor is wired to the analyzer through a remote
junction box containing a preamplifier, disconnect the wires at the sensor side of the junction box.
2.
Tape back the ends of the disconnected wires to keep them from making accidental connections with other
wires or terminals.
