Hach-Lange ORBISPHERE 31xxx Installation and Maintenance - Manual (Page 18)

16
Introduction
The sensor is designed with one center electrode (cathode) and one counter electrode (anode)
immersed in an electrolytic solution. The electrodes and the electrolyte are separated from the
gaseous or liquid sample by a membrane permeable to gas.
Note: Anode and cathode positions are inverted on the H
2
sensor
Additionally, the sensor's design includes a guard ring electrode which surrounds the sensor's
center electrode. Its role is to reduce the influence of other gases on the center electrode,
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. The amount of gas entering the cell is
proportional to the partial pressure of this 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 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
Electrochemical sensor assembly Protection cap and cable removed
Sensor cell, with membrane and electrolyte Membrane and its support removed
Figure 4 Electrochemical sensor overview