Emerson Rosemount 1056, Инструкция по эксплуатации

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7.7 Calibration — Oxygen
7.7.1 Description
Oxygen sensors generate a current directly proportional
to the concentration of dissolved oxygen in the sample.
Calibrating the sensor requires exposing it to a solution
containing no oxygen (zero standard) and to a solution
containing a known amount of oxygen (full-scale
standard). The zero Calibration is necessary because
oxygen sensors, even when no oxygen is present in the
sample, generate a small current called the residual
current. The analyzer compensates for the residual
current by subtracting it from the measured current
before converting the result to a dissolved oxygen
value. New sensors require zeroing before being placed
in service, and sensors should be zeroed whenever the
electrolyte solution is replaced. The recommended zero
standard is 5% sodium sulfite in water, although
oxygen-free nitrogen can also be used. The Model
499A TrDO sensor, used for the determination of
trace (ppb) oxygen levels, has very low residual
current and does not normally require zeroing. The
residual current in the 499A TrDO sensor is equivalent
to less than 0.5 ppb oxygen.
The purpose of the In Process Calibration is to
establish the slope of the calibration curve. Because the
solubility of 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 deter-
mines 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 calcu-
lates 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 tem-
perature. At 25°C and 760 mm Hg, the equilibrium sol-
ubility is 8.24 ppm. Often it is too difficult or messy to
remove the sensor from the process liquid for calibra-
tion. In this case, the sensor can be calibrated against
a measurement made with a portable laboratory instru-
ment. The laboratory instrument typically uses a mem-
brane-covered amperometric sensor that has been cali-
brated against water-saturated air.
THIS SECTION DESCRIBES HOW TO CALIBRATE
THE MODEL 1056 WITH AN OXyGEN SENSOR.
THE FOLLOWING CALIBRATION ROUTINES ARE
COVERED.
Instruction Manual
LIQ-MAN-1056
Section 7.0: Calibration
April 2017