Emerson Rosemount TCL, Manual

Page: 17 / 108

2 Principles of operation
Total chlorine by definition is the iodine produced in a sample when it is treated with
potassium iodide at a pH between 3.5 and 4.5. Typically, acetic acid (or vinegar) is used to
adjust the pH.
The total chlorine system consists of a sample conditioning system, which injects the
reagent into the sample, and a sensor and transmitter, which measure the amount of
iodine produced.
Figure 2-1 shows the sample conditioning system. The sample enters the
sample conditioning enclosure and flows to an overflow sampler from which the sample
pump takes suction. Excess sample drains to waste. At the same time, the reagent pump
draws reagent, a solution of potassium iodide in vinegar, from the reagent carboy and
injects it into the suction side of the sample pump. The sample and reagent mix as they
pass through the pump, and total chlorine in the sample is converted to the chemically
equivalent amount of iodine. The flow rates are 11 mL/min for the sample and 0.2 mL/min
for the reagent.
Schematic of Sample Conditioning System and Transmitter Figure 2-1:
The treated sample next enters the flow cell. Bubbles injected into the flow cell produce
turbulence, which improves the stability of the reading. A membrane-covered
amperometric sensor in the flow cell measures the concentration of iodine. The
transmitter receives the raw signal from the sensor and displays the concentration of total
chlorine. Display units are ppm (mg/L) chlorine as Cl
2
. The treated sample leaves the flow
cell and drains to waste along with the excess sample.
Principles of operation
Instruction Manual 9