Temperature Settings - Emerson Rosemount Analytical HART 5081-A-HT Instruction Manual

MODEL 5081-A

7.4 TEMPERATURE SETTINGS

7.4.1 Purpose
This section describes how to do the following:
1. Enable and disable automatic temperature compensation
2. Set a manual temperature compensation value for oxygen, chlorine, ozone, and pH measurements
3. Tell the transmitter the type of temperature element in the sensor
7.4.2 Definitions
1. AUTOMATIC TEMPERATURE COMPENSATION - OXYGEN, CHLORINE, AND OZONE. The oxygen, chlo-
rine, and ozone sensors used with the 5081-A transmitter are membrane-covered amperometric sensors. The
permeability of the membrane, or the ease with which the analyte passes through the membrane, is a func-
tion of temperature. As temperature increases, permeability increases, and the analyte diffuses more readily
through the membrane. Because sensor current depends on diffusion rate, a temperature increase will cause
the sensor current and transmitter reading to increase even though the concentration of analyte remained con-
stant. A correction equation in the software automatically corrects for changes in membrane permeability
caused by temperature. Temperature is also used in the pH correction applied to free chlorine readings and in
automatic air calibration of oxygen sensors. In automatic temperature correction, the transmitter uses the tem-
perature measured by the sensor for all calculations in which temperature is used.
2. MANUAL TEMPERATURE COMPENSATION - OXYGEN, CHLORINE, AND OZONE. In manual temperature
compensation, the transmitter uses the temperature entered by the user for membrane permeability and pH
corrections and for air calibration calculations. It does not use the actual process temperature. Do NOT use
manual temperature correction unless the measurement and calibration temperatures differ by no more than
about 2°C. Manual temperature correction is useful if the sensor temperature element has failed and a replace-
ment sensor is not available.
3. AUTOMATIC TEMPERATURE COMPENSATION - pH. The transmitter uses a temperature-dependent factor
to convert measured cell voltage to pH. In automatic temperature compensation the transmitter measures the
temperature and automatically calculates the correct conversion factor. Temperature is also used in automat-
ic buffer calibration. For maximum accuracy, use automatic temperature correction.
4. MANUAL TEMPERATURE COMPENSATION - pH. In manual temperature compensation, the transmitter 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.
SECTION 7.0
PROGRAMMING
37