Calibration - Temperature; Introduction - Emerson Rosemount Analytical HART 5081-A-HT Instruction Manual

MODEL 5081-A

8.1 INTRODUCTION

All four amperometric sensors (oxygen, ozone, free chlorine, and total chlorine) are membrane-covered sensors. As
the sensor operates, the analyte (the substance to be determined) diffuses through the membrane and is consumed
at an electrode immediately behind the membrane. The reaction produces a current that depends on the rate at
which the analyte diffuses through the membrane. The diffusion rate, in turn, depends on the concentration of the
analyte and how easily it passes through the membrane (the membrane permeability). Because the membrane per-
meability is a function of temperature, the sensor current will change if the temperature changes. To correct for
changes in sensor current caused by temperature, the transmitter automatically applies a membrane permeability
correction. Although the membrane permeability is different for each sensor, the change is about 3%/°C at 25°C, so
a 1°C error in temperature produces about a 3% error in the reading.
Temperature plays an additional role in oxygen measurements. Oxygen sensors are calibrated by exposing them
to water-saturated air, which, from the point of view of the sensor, is equivalent to water saturated with atmospheric
oxygen (see Section 9.1 for more information). During calibration, the transmitter calculates the solubility of atmos-
pheric oxygen in water using the following steps. First, the transmitter measures the temperature. From the tem-
perature, the transmitter calculates the vapor pressure of water and, using the barometric pressure, calculates the
partial pressure of atmospheric oxygen. Once the transmitter knows the partial pressure, it calculates the equilib-
rium solubility of oxygen in water using a temperature-dependent factor called the Bunsen coefficient. Overall, a
1°C error in the temperature measurement produces about a 2% error in the solubility calculated during calibra-
tion and about the same error in subsequent measurements.
Temperature is also important in the pH measurement required to correct free chlorine readings.
1. The transmitter uses a temperature dependent factor to convert measured cell voltage to pH. Normally, a slight
inaccuracy in the temperature reading is unimportant unless the pH reading is significantly different from 7.00.
Even then, the error is small. For example, at pH 12 and 25°C, a 1°C error produces a pH error less than ±0.02.
2. During auto calibration, the transmitter recognizes the buffer being used and calculates the actual pH of the
buffer at the measured temperature. Because the pH of most buffers changes only slightly with temperature,
reasonable errors in temperature do not produce large errors in the buffer pH. For example, a 1°C error caus-
es at most an error of ±0.03 in the calculated buffer pH.
Without calibration the accuracy of the temperature measurement is about ±0.4°C. Calibrate the transmitter if
1. ±0.4°C accuracy is not acceptable
2. the temperature measurement is suspected of being in error. Calibrate temperature by making the transmitter
reading match the temperature measured with a standard thermometer.
SECTION 8.0
CALIBRATION — TEMPERATURE
SECTION 8.0
CALIBRATION — TEMPERATURE
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