Emerson Rosemount 1066 Instruction Manual page 91

Instruction Manual
LIQ-MAN-1066
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 transmitter monitors the sensor
current. When the current is stable, the transmitter 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 transmitter 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
transmitter calculates the partial pressure of oxygen. Once the transmitter 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 mm Hg, 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 from the main
screen, select and press ENTER.
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 nA/ppm Entering a known slope value for sensor response
To calibrate oxygen:
1. Press the MENU button
2. Select Calibrate. Press ENTER.
3. Select Oxygen. Press ENTER.
The adjacent screen will appear. To calibrate Oxygen or Temperature,
scroll to the desired item and press ENTER
Calibration
Section 9: Calibration
April 2017
1.234 µS/cm 25.0 ºC
SN Calibrate?
Oxygen
Temperature
81