Emerson Rosemount TCL - Chapter 9 Calibration; Introduction; Calibrating Temperature; Purpose

9 Calibration
9.1 Introduction
The Calibrate menu allows you to do the following:
1. Calibrate the temperature sensing element in the total chlorine sensor.
2. Calibrate the chlorine sensor.
3. Calibrate the analog outputs.
9.2 Calibrating temperature
9.2.1 Purpose
The total chlorine sensor is a membrane-covered amperometric sensor. As the sensor
operates, iodine, produced by the reaction between total chlorine and the vinegar/
potassium iodide agent, 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 iodine diffuses through the membrane. The diffusion rate, in turn, depends
on the concentration of the iodine and how easily it passes through the membrane (the
membrane permeability). Because membrane permeability is a function of temperature,
the sensor current changes if either the concentration or 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. ±0.4 °C accuracy is not acceptable.
2. The temperature measurement is suspected of being in error. Calibrate
temperature by making the transmitter reading match the temperature measured
with a standard thermometer.
9.2.2 Procedure
Complete the following steps to calibrate the temperature in the transmitter.
1. 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.
2. 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
Instruction Manual 61