Emerson Process Management 225 ユーザーズマニュアル
6
MODEL 225
CALIBRATION
CALIBRATION
The nominal cell constant of the Model 225 sensor is
2.7/cm. The error in the cell constant is about
2.7/cm. The error in the cell constant is about
±10%, so
conductivity readings made using the nominal cell con-
stant will have an error of at least
stant will have an error of at least
±10%. Wall effects,
discussed below, will likely make the error greater. For
higher accuracy, the sensor must be calibrated.
higher accuracy, the sensor must be calibrated.
Wall effects arise from the interaction between the cur-
rent induced in the sample by the sensor and nearby
pipe or vessel walls. As Figure 11 shows, the measured
conductivity can either increase or decrease depend-
ing on the wall material. Because wall effects do not
disappear until the Model 225 sensor is at least 1 inch
(25 mm) away from the nearest wall, wall effects are
present in most installations. Correcting for them is an
important part of calibration.
rent induced in the sample by the sensor and nearby
pipe or vessel walls. As Figure 11 shows, the measured
conductivity can either increase or decrease depend-
ing on the wall material. Because wall effects do not
disappear until the Model 225 sensor is at least 1 inch
(25 mm) away from the nearest wall, wall effects are
present in most installations. Correcting for them is an
important part of calibration.
Conductivity sensors are calibrated against a solution
of known conductivity (a standard) or against a previ-
ously calibrated referee sensor and analyzer. If wall
effects are present, calibrating a toroidal sensor with
a standard solution is possible only if the vessel hold-
ing the standard has exactly the same dimensions as
of known conductivity (a standard) or against a previ-
ously calibrated referee sensor and analyzer. If wall
effects are present, calibrating a toroidal sensor with
a standard solution is possible only if the vessel hold-
ing the standard has exactly the same dimensions as
the process piping. Model 225 sensors are often
installed in sanitary flange tees, so an appropriate cal-
ibration vessel is another tee with blank flanges on the
ends. If removing the sensor from the process piping for
calibration is impractical, calibrate the sensor in place
against a referee sensor and analyzer while both sen-
sors are simultaneously measuring the process liquid.
If this is not practical the sensor can also be calibrated
against the results of a measurement made on a grab
sample.
installed in sanitary flange tees, so an appropriate cal-
ibration vessel is another tee with blank flanges on the
ends. If removing the sensor from the process piping for
calibration is impractical, calibrate the sensor in place
against a referee sensor and analyzer while both sen-
sors are simultaneously measuring the process liquid.
If this is not practical the sensor can also be calibrated
against the results of a measurement made on a grab
sample.
For more information about calibrating toroidal conduc-
tivity sensors, refer to application sheet ADS 43-025
available on the Rosemount Analytical website.
tivity sensors, refer to application sheet ADS 43-025
available on the Rosemount Analytical website.
MAINTENANCE
Generally, the only maintenance required is to keep the
opening of the sensor clear of deposits. Cleaning fre-
quency is best determined by experience.
opening of the sensor clear of deposits. Cleaning fre-
quency is best determined by experience.
FIGURE 11. Measured conductivity as a function
of clearance between sensor and walls
of clearance between sensor and walls
FIGURE 10. Remote Junction Box (PN 23550-00)
CAUTION
Be sure sensor has been cleaned of process liquid
before handling.
before handling.