Hach-Lange ORBISPHERE A1100 Family - Manual (Page 18)

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Introduction
3.3 Basic principle of operation
In its simplest form, the electrochemical sensor consists of one center electrode (cathode) and
one counter electrode (anode) immersed in an electrolyte solution which is separated from the
gaseous or liquid sample by a gas permeable membrane. An electronic circuit is linked to the
anode and cathode. Through an applied voltage, current will flow between the anode and the
cathode.
A guard ring electrode surrounds the center electrode in order to reduce the influence of other
gases on the center electrode, and therefore improving analysis stability.
The sensor head is covered with a protection cap and, in some applications, a grille to protect
the membrane. Materials used for the components of the sensors differ with the application.
Gas penetrating through the membrane into the cell dissolves in the electrolyte. It undergoes a
reaction at the cathode, causing a measurable electric current to flow. This current is
proportional to the amount of gas entering the cell, which in turn is proportional to the partial
pressure of gas in the sample outside the cell.
The result is shown as gas concentration, which can then be displayed with a choice of several
measuring units, according to instrument setup.
The sensor also includes “smart sensor technology”, implemented using an RS485 interface.
The sensor electronics perform four functions:
• Apply constant voltage to the anode
• Measure the current flowing through the sensor
• Compensate for temperature variation in the gaseous or liquid sample
• Convert the cell’s electric current into an analog signal for sensor output