Instruction manual
94
MODEL Xmt-P SECTION 10.0
TROUBLESHOOTING
D. Is the sensor fouled? The sensor measures the pH of the liquid adjacent to the glass bulb. If the sensor is heavily
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 tempera-
ture? Poisoning agents and high temperature can shift the reference voltage many hundred millivolts.
10.5.3 Controller will not accept manual slope.
If the sensor slope is known from other sources, it can be entered directly into the controller. The controller will not accept
a slope (at 25°) outside the range 45 to 60 mV/pH. If the user attempts to enter a slope less than 45 mV/pH, the controller
will automatically change the entry to 45. If the user attempts to enter a slope greater than 60 mV/pH, the controller will
change the entry to 60 mV/pH.
10.5.4 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 perform-
ance 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.
D. Is the analyzer working properly. Check the analyzer by simulating the pH input.
10.5.5 Calibration was successful, but 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 normal. The
on-line instrument is subject to process variables, for example ground potentials, stray voltages, and orientation effects that
may not affect the laboratory or portable instrument.
10.5.6 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 elec-
trical 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, fiber-
glass 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 instrument
through the sensor wiring.
C. Simplify the sensor wiring.
1. First, verify that pH sensor wiring is correct. Note that it is not necessary to jumper the solution ground and refer-
ence terminals.
2. Disconnect all sensor wires at the analyzer except pH/mV IN, REFERENCE IN, RTD IN and RTD RETURN. See
the wiring diagrams in Section 3.0. 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.
3. Tape back the ends of the disconnected wires to keep them from making accidental connections with other wires