Model 7003M
Operator Manual
SECTION 5 |
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THEORY |
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oxygen without affecting the total | ||
The Model 7003M Percent Oxygen Analyzer | barometric pressure. | |
This relationship can be expressed as : | ||
consists of an amperometric sensor and | ||
analyzer interconnected by a |
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shielded cable. The sensor responds to the | P atm = Po2 + PH20 = P other, | |
partial pressure of oxygen and produces a |
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current signal. The analyzer amplifies and | where: | |
processes the signal, providing readout in |
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percent oxygen and current output. | P atm = total barometric pressure, | |
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a. Electrochemical Theory | Po2 = partial pressure of oxygen, | |
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With the sensor placed in the sample gas, a voltage is applied across the cathode and anode. Sample gas diffuses through the membrane and is reduced at the cathode resulting in a current proportional to the partial pressure of oxygen in the sample.
When no oxygen is present, a small residual sensor current, which can be offset during instrument setup.
a.Barometric Pressure
If ambient air is used to calibrate the sensor, a change in barometric pressure can be a significant factor. At 76 cm Hg, the partial pressure of oxygen is 15.9 cm Hg (20.9% of 76 cm Hg). As atmospheric pressure deviates from 76 cm Hg, the partial pressure of oxygen will vary proportionally. The highest partial pressure that can be read by the Model 7003M is 19 cm Hg (25% oxygen at 76 cm Hg).
b.Humidity
If calibrating with humid ambient air, the effect of the partial pressure due to water vapor content should be taken into consideration. High humidity has the effect of reducing the partial pressure of
PH20 = partial pressure of water vapor,
P other = partial pressure of all gases other than oxygen and water vapor
c.Sample Temperature
The rate of oxygen diffusion through the sensor membrane varies with temperature, with a coefficient of about +3% °C (5% °F), causing a corresponding change in sensor current. The instrument continuously monitors temperature with a thermistor in the sensor body, and compensates the output for both sample temperature and membrane permeability.
Because of a slight time lag in the response of the temperature sensor, a rapid temperature change will cause a temporary change in apparent oxygen level, such as might be encountered if the sensor is suddenly exposed to direct sunlight from a shadow.
Extended use of the sensor at low temperatures will shorten the life of the sensor. As the sensor reaches the end of its useful life, distinct step changes in the oxygen reading will be evident as the oxygen level changes.
Rosemount Analytical Inc. A Division of Emerson Process Management | Theory |