Sensor not responding to pH changes
1.
Did the expected pH change really occur? If the process pH reading was not what
was expected, check the performance of the sensors 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.
4.
Is the transmitter working properly? Check the transmitter by simulating the pH
input.
Process pH slightly different
Differences between pH readings made with an online instrument and a laboratory or
portable instrument are normal. The online instrument is subject to process variables, for
example, ground potentials, stray voltages, and orientation effects that may not affect the
laboratory or portable instrument.
Process pH reading changes with flow
The 399 pH sensor recommended for use with the 5081-A 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
rapidly flowing sample, and high flows may affect the pH reading. Typically, the difference
in pH reading from a 399 pH sensor 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.
Process pH grossly wrong 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 transmitter by the sensor cable. The problem arises from the
process or installation. It is not a fault of the transmitter. The problem should disappear
once the sensor is taken out of the system. Check the following:
1.
Is a ground loop present?
a. 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.
b. 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.
c. If offsets and noise appear after making the connection, a ground loop exists.
2.
Is the process grounded?
Diagnostics and troubleshooting
110
Rosemount 5081
Summary of Contents for Rosemount5081
Page 4: ......
Page 10: ...Contents vi Rosemount 5081 ...
Page 12: ...Startup procedure 2 Rosemount 5081 ...
Page 23: ...Mounting 5081 on a pipe Figure 3 2 Installation Instruction Manual 13 ...
Page 24: ...Installation 14 Rosemount 5081 ...
Page 28: ...Wiring 18 Rosemount 5081 ...
Page 50: ...Programming basics 40 Rosemount 5081 ...
Page 130: ...Diagnostics and troubleshooting 120 Rosemount 5081 ...
Page 136: ...Digital communications 126 Rosemount 5081 ...
Page 140: ...Engineering drawings 130 Rosemount 5081 ...
Page 141: ...Engineering drawings Instruction Manual 131 ...
Page 142: ...Engineering drawings 132 Rosemount 5081 ...
Page 143: ...Engineering drawings Instruction Manual 133 ...
Page 144: ...Engineering drawings 134 Rosemount 5081 ...
Page 145: ...Engineering drawings Instruction Manual 135 ...
Page 146: ...Engineering drawings 136 Rosemount 5081 ...
Page 151: ...EU Declarations of Conformity 138 Rosemount 5081 ...
