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MODEL SOLU COMP II
SECTION 5.0
PROGRAMMING THE ANALYZER
5.6 CHOOSING TEMPERATURE UNITS AND MANUAL OR AUTOMATIC TEMPERATURE
COMPENSATION
5.6.1 Purpose
This section describes how to do the following:
1.
Choose temperature display units (°C or °F).
2.
Choose automatic or manual temperature compensation for membrane permeability and for air calibration.
3.
Choose automatic or manual temperature compensation for pH.
4.
Enter a temperature for manual temperature compensation.
5.6.3 Definitions — dissolved oxygen
1.
AUTOMATIC TEMPERATURE COMPENSATION - DISSOLVED OXYGEN. Temperature is important in the
amperometric determination of dissolved oxygen for two reasons.
The oxygen sensor is a membrane-covered amperometric sensor. The permeability of the membrane is a func-
tion of temperature. As temperature increases, membrane permeability increases. Thus, an increase in tem-
perature will cause the sensor current and the analyzer reading to increase even though the dissolved oxygen
level remained constant. A correction equation in the analyzer software automatically compensates for
changes in membrane permeability caused by temperature. In automatic temperature compensation, the ana-
lyzer uses the temperature measured by the sensor for the correction.
The analyzer also uses temperature during air calibration. From the temperature the analyzer calculates the
saturation pressure of water and using the barometric pressure calculates the partial pressure of atmospher-
ic oxygen. Once the analyzer knows the partial pressure of oxygen, it calculates the equilibrium solubility of
oxygen in water using a temperature-dependent factor called the Bunsen coefficient. In automatic temperature
compensation, the analyzer performs the calibration using the temperature measured by the sensor.
2.
MANUAL TEMPERATURE COMPENSATION - DISSOLVED OXYGEN. In manual temperature compensation,
the analyzer uses the temperature entered by the user for membrane permeability correction and for air cali-
bration. It does not use the actual process temperature. Do NOT use manual temperature compensation
unless the measurement and calibration temperatures differ by no more than about 2°C. Manual temperature
compensation is useful if the sensor temperature element has failed an a replacement sensor is not available.
5.6.3 Definitions — pH
1.
AUTOMATIC TEMPERATURE COMPENSATION — pH. The analyzer uses a temperature-dependent factor
to convert measured cell voltage to pH. In automatic temperature compensation, the analyzer measures the
temperature and automatically calculates the correct conversion factor. For maximum accuracy, use automat-
ic temperature compensation.
2.
MANUAL TEMPERATURE COMPENSATION — pH. In manual temperature compensation, the analyzer con-
verts measured voltage to pH using the temperature entered by the user. It does not use the actual process
temperature. Do NOT use manual temperature compensation unless the process temperature varies no more
than about ±2°C or the pH is between 6 and 8. Manual temperature compensation is useful if the sensor tem-
perature element has failed and a replacement sensor is not available.