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7
Calibration
7.1
Introduction
The Calibrate menu allows you to do the following:
1.
2.
3.
7.2
Calibrating temperature
7.2.1
Purpose
The monochloramine sensor is a membrane-covered amperometric sensor. As the sensor
operates, monochloramine diffuses through the membrane and is consumed at an
electrode immediately behind the membrane. The reaction produces a current that
depends on the rate at which the monochloramine diffuses through the membrane. The
diffusion rate, in turn, depends on the concentration of monochloramine and how easily it
passes through the membrane (membrane permeability). Because membrane
permeability is a function of temperature, the sensor current changes if the temperature
changes. To account for changes in sensor current caused by temperature alone, the
transmitter automatically applies a membrane permeability correction. The membrane
permeability changes about 3%/° C at 25 °C (77 °F), so a 1 °C error in temperature produces
about a 3% error in the reading.
Without calibration, the accuracy of the temperature measurement is about ±0.4 °C.
Calibrate the sensor/transmitter unit if:
1.
2.
7.2.2
Procedure
Complete the following steps to calibrate the temperature in the transmitter.
1.
2.
Instruction Manual
Calibrate the temperature sensing element in the monochloramine sensor.
Calibrate the monochloramine sensor.
Calibrate the analog outputs.
±0.4 °C accuracy is not acceptable.
The temperature measurement is suspected of being in error. Calibrate
temperature by making the transmitter reading match the temperature measured
with a standard thermometer.
Remove the sensor from the flow cell. Place it in an insulated container of water
along with a calibrated thermometer. Submerge at least the bottom two inches of
the sensor.
Allow the sensor to reach thermal equilibrium.
The time constant for the sensor is about five minutes, so it may take as long as
thirty minutes for equilibration.
Calibration
47
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