Emerson Rosemount5081 - Sensor Not Responding to Ph Changes; Process Ph Slightly Different; Process Ph Reading Changes with Flow; Process Ph Grossly Wrong or Noisy

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