8 A-96.250.531 / 300617
AMI Oxytrace QED
Product Description
2. Product Description
This manual describes the functions of the following monitors
AMI Oxytrace
AMI Oxytrace QED
Both monitors are mainly identical except that AMI Oxytrace QED
includes a faraday verification.
2.1. Description of the System
Application AMI Oxytrace is used to measure low levels of oxygen in high-puri-
ty water. Especially in power plant water cycles (e.g. feedwater), a
very low level of oxygen is needed to prevent corrosion.
Measuring
principle
Clark principle:
The sensor consists of one noble metal electrode (e.g. platinum or
gold), a reference electrode (mostly Ag/AgCl) and optionally a met-
al guard electrode.
The Clark-type electrode is the most widely used oxygen sensor for
measuring oxygen dissolved in a liquid. The basic principle is that
there is a cathode and an anode submersed in an electrolyte and a
voltage is applied between the two parts. Oxygen enters the sensor
through a permeable membrane by diffusion, and is reduced at the
cathode according to
O
2
+ 4e
-
+ 2 H
2
O --> 4 OH
-
This reaction creates a measurable current. There is a linear cor-
relation between the oxygen concentration and the electrical cur-
rent.
The guard electrode is on the same voltage level as the cathode
but there is no current measurement. Oxygen which diffuses from
the electrolyte to the cathode is consumed by the guard electrode.
As a consequence, residual oxygen in the electrolyte will no more
disturb the measurement signal and the response time to low oxy-
gen levels will be shorter.
Temperature
compensa-
tion
The measuring signal depends on temperature, but is automatically
compensated to 25 °C. The sample temperature is determined con-
tinuously by a temperature sensor inside the oxygen electrode.
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