Emerson ROSEMOUNT 5081-A Instruction Manual page 116

MODEL 5081-A
15.8 pH MEASUREMENT AND CALIBRATION PROBLEMS
Problem
SLoPE HI or SLoPE Lo message is showing
-0- oFFSEt message is showing
transmitter will not accept manual slope
Sensor does not respond to known pH changes
Process pH is slightly different from the expected value
Process pH reading changes when flow changes
Process pH is grossly wrong and/or noisy
Process readings are noisy
15.8.1 SLOPE HI or SLOPE LO message is showing.
Refer to Section 15.2.9 for assistance.
15.8.2 -0- OFFSEt message is showing.
Refer to Section 15.2.10 for assistance.
15.8.3 Transmitter will not accept manual slope.
If the sensor slope is known from other sources, it can be entered directly into the transmitter. the transmitter 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
transmitter will automatically change the entry to 45. If the user attempts to enter a slope greater than 60 mV/pH, the trans-
mitter will change the entry to 60 mV/pH. See Section 14.8.1 for troubleshooting sensor slope problems.
15.8.4 Sensor does not respond to known pH changes.
1. 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.
2. Is the sensor properly wired to the transmitter?
3. Is the glass bulb cracked or broken? Check the glass electrode impedance. See Section 14.1
4. Is the transmitter working properly. Check the transmitter by simulating the pH input.
15.8.5 Process pH is slightly different from the 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. to make the process reading agree with a reference instrument, see
Section 13.4.
15.8.6 Process pH reading changes when flow changes.
the 399 pH sensor recommended for use with the 5081A transmitter has some degree of flow sensitivity, i.e., changing
the sample flow causes the pH reading to change. Flow sensitivity varies from sensor to sensor. Flow sensitivity can be a
source of error if the pH and chlorine sensor flow cells are connected in series. the chlorine sensor requires a fairly rap-
idly flowing sample, and high flows may affect the pH reading. typically, the difference in pH reading from a 399 pH sen-
sor in a rapidly (16 gph) and slowly (<2 gph) flowing sample is less than about 0.05. If the change is greater than 0.05, the
pH and chlorine sensors should be installed in parallel streams.
110
SECTION 15.0
TROUBLESHOOTING
See Section
15.8.1
15.8.2
15.8.3
15.8.4
15.8.5
15.8.6
15.8.7
15.8.8