Emerson Rosemount 56, Instruction Manual

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Instruction Manual Section 10: Calibration
LIQ-MAN-56 April 2017
atmospheric oxygen in water as a function of temperature and barometric pressure is well
known, the natural choice for a full-scale standard is air-saturated water. However, air-saturated
water is difficult to prepare and use, so the universal practice is to use air for calibration. From the
point of view of the oxygen sensor, air and air-saturated water are identical. The equivalence
comes about because the sensor really measures the chemical potential of oxygen. Chemical
potential is the force that causes oxygen molecules to diffuse from the sample into the sensor
where they can be measured. It is also the force that causes oxygen molecules in air to dissolve
in water and to continue to dissolve until the water is saturated with oxygen. Once the water is
saturated, the chemical potential of oxygen in the two phases (air and water) is the same.
Oxygen sensors generate a current directly proportional to the rate at which oxygen molecules
diffuse through a membrane stretched over the end of the sensor. The diffusion rate depends
on the difference in chemical potential between oxygen in the sensor and oxygen in the sample.
An electrochemical reaction, which destroys any oxygen molecules entering the sensor, keeps the
concentration (and the chemical potential) of oxygen inside the sensor equal to zero. Therefore,
the chemical potential of oxygen in the sample alone determines the diffusion rate and the
sensor current. When the sensor is calibrated, the chemical potential of oxygen in the standard
determines the sensor current. Whether the sensor is calibrated in air or air-saturated water is
immaterial. The chemical potential of oxygen is the same in either phase. Normally, to make the
calculation of solubility in common units (like ppm DO) simpler, it is convenient to use water-
saturated air for calibration. Automatic air calibration is standard. The user simply exposes the
sensor to water-saturated air. The analyzer monitors the sensor current. When the current is
stable, the analyzer stores the current and measures the temperature using a temperature
element inside the oxygen sensor. The user must enter the barometric pressure. From the
temperature the analyzer calculates the saturation vapor pressure of water. Next, it calculates
the pressure of dry air by subtracting the vapor pressure from the barometric pressure. Using
the fact that dry air always contains 20.95% oxygen, the analyzer calculates the partial pressure
of oxygen. Once the analyzer knows the partial pressure of oxygen, it uses the Bunsen
coefficient to calculate the equilibrium solubility of atmospheric oxygen in water at the
prevailing temperature. At 25 °C and 760 mmHg, the equilibrium solubility is 8.24 ppm. Often
it is too difficult or messy to remove the sensor from the process liquid for calibration. In this
case, the sensor can be calibrated against a measurement made with a portable laboratory
instrument. The laboratory instrument typically uses a membrane-covered amperometric sensor
that has been calibrated against water-saturated air.
To calibrate the oxygen sensor, access the Calibration screen by pressing ENTER/MENU from
the main screen, select S1 or S2 Measurement and press ENTER/MENU. Press INFO at any
time to learn more about this procedure. A yellow screen will appear with detailed
instructions and information.
The following calibration routines are covered:
1. Zero Cal Zeroing the sensor in a
medium with zero oxygen
2. Air Cal Calibrating the sensor
in a water-saturated air sample
3. In Process Cal Standardizing to
a sample of known oxygen
concentration
4. Sen@ 25 °C:2500 µA/ppm Entering a
known slope value for sensor re-
sponse.
Calibration 83