Emerson 51-DO-03, 51-DO-04 instruction manual Section Calibration Temperature, Introduction

MODEL DO-03/04	SECTION 6.0
	CALIBRATION - TEMPERATURE

SECTION 6.0

CALIBRATION - TEMPERATURE

6.1 INTRODUCTION

The Model 499ADO sensor is a membrane-covered amperometric sensor. As the sensor operates, oxygen diffus- es through the membrane and is consumed at an electrode immediately behind the membrane. The reaction pro- duces a current that depends on the rate at which oxygen diffuses through the membrane. The diffusion rate, in turn, depends on the concentration of oxygen and how easily it passes through the membrane (the membrane perme- ability). Because the membrane permeability is a function of temperature, the sensor current will change if the tem- perature changes. To correct for changes in sensor current caused by temperature, the analyzer automatically applies a membrane permeability correction that converts the sensor current to the value at 25°C. 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 atmospher- ic oxygen (see Section 7.0 for more information). During calibration, the analyzer calculates the solubility of atmos- pheric oxygen in water using the following steps. First, the analyzer measures the temperature. From the temper- ature, the analyzer calculates the vapor pressure of water and, using the barometric pressure, calculates the par- tial pressure of atmospheric oxygen. Once the analyzer knows the partial pressure, it calculates the equilibrium 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 calibration and about the same error in subsequent measurements.

Without calibration the accuracy of the temperature measurement is about ±0.4°C. Calibrate the controller if

1.±0.4°C accuracy is not acceptable

2.the temperature measurement is suspected of being in error. Calibrate temperature by making the controller reading match the temperature measured with a standard thermometer.

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