Hach-Lange ORBISPHERE 3650 - Manual (Page 14)

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Specifications and Certifications
2.3 Theory of operation
2.3.1 Measuring oxygen
The sensor circuitry performs four functions:
• Applying a constant voltage to the anode
• Measuring the current flowing through the sensor
• Compensating this current for sample temperature variations
• Converting these resulting signals into a scaled current or voltage
The anode is held positive with respect to the cathode. Current flowing through the sensor due
to oxygen reduction at the cathode is converted to a voltage by an amplifier, the proportionality
between voltage and current being determined by the feedback resistance of this amplifier.
The output voltage is essentially a function of oxygen activity (partial pressure), temperature and
membrane permeability. Corrections for variations in membrane permeability are made when
the sensor is calibrated. The temperature compensation circuit accounts for temperature
variations. Hence the output voltage varies only with oxygen concentration.
2.3.2 Measuring hydrogen
The sensor circuitry performs four functions:
• Maintaining a zero potential to the anode
• Measuring the current flowing through the sensor
• Compensating this current for sample temperature variations
• Converting these resulting signals into a scaled current or voltage
The anode is held neutral with respect to the cathode. Current flowing through the sensor due to
hydrogen oxidation at the anode is converted to a voltage by an amplifier, the proportionality
between voltage and current being determined by the feedback resistance of this amplifier.
The output voltage is essentially a function of hydrogen activity (partial pressure), temperature
and membrane permeability. Corrections for variations in membrane permeability are made
when the sensor is calibrated. The temperature compensation circuit accounts for temperature
variations. Hence the output voltage varies only with hydrogen concentration.