Emerson Rosemount 400 Instruction Manual page 17

Instruction Manual
LIQ-MAN-400
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3.1.3 Calibrating using a grab sample
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Calibration and Maintenance
Orient the sensors so that air bubbles always have an easy escape path and cannot get
trapped between the electrodes.
Turn off automatic temperature compensation in the transmitter.
Almost all process conductivity transmitter feature automatic temperature compensation
in which the transmitter applies one of several temperature correction algorithms to
convert the measured conductivity to the value at a reference temperature, typically 25 °C.
Although temperature correction algorithms are useful for routine measurements, they
should not be used during calibration.
There are two following reasons:
a. No temperature correction is perfect. If the assumptions behind the algorithm do not
perfectly fit the solution being measured, the temperature-corrected conductivity will
be in error.
b. If the temperature measurement itself is in error, the corrected conductivity will be in
error.
The purpose of calibrating the sensor is to determine the cell constant. To minimize the
error in the cell constant, all sources of avoidable error, e.g., temperature compensation
should be eliminated.
Keep tubing runs between the sensors short and adjust the sample flow as high as possible.
Short tubing runs and high flow ensure the temperature of the liquid does not change as
it flows from one sensor to another.
If the process temperature is appreciably different from ambient, high flow may not be
enough to keep the temperature constant. In this case, pumping sample at room
temperature from a reservoir through the sensors might be necessary. Because such a
system is likely to be open to atmosphere, saturate the liquid with air to prevent drift
caused by absorption of atmospheric carbon dioxide.
To prevent contamination of low conductivity (<1 µS/cm) process liquids, use clean tubing
to connect the sensors. To prevent drift caused desorption of ionic contaminants from
tube walls, keep the sample flow greater than 6 ft/sec (1.8 m/sec).
Use the grab sample method when it is impractical to remove the sensor for calibration or
to connect a reference sensor to the process line.
Take a sample of the process liquid, measuring its conductivity using a reference
instrument, and adjusting the reading from the process transmitter to match the measured
conductivity.
Take the sample from a point as close to the process sensor as possible.
Keep temperature compensation turned on. There is likely to be a lag time between
sampling and analysis, so temperature is likely to change.
Be sure the reference and process instruments are using the same temperature correction
algorithm.
Calibration and Maintenance
May 2017
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