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Portable Oxygen Monitor Operation
Teledyne Analytical Instruments 27
2.3.2 Temperature
The R17MED oxygen sensor adjusts for ambient temperature
changes in the range of 0–40°C (32–106°F). Since the thermistor that
compensates for these changes is located in the rear of the sensor
assembly, it is important that gas mixtures, flowing over the front of the
sensor, be at room temperature. Reading errors may occur if hot gases
from a heated humidifier are directed past a sensor teed into a breathing
circuit.
A small thermal tracking error may be encountered in application
areas where the entire sensor assembly is placed in the gas mixture to be
analyzed (e.g., incubators). Holding the sensor in your hand for more
than a few minutes can also affect the temperature tracking which
appears as a slow drift on the LCD. No adjustments should be made
during this period since this error will be eliminated when both the
thermistor and sensing electrode have had sufficient time to come to
thermal equilibrium. This can take up to 2 hours.
2.3.3 Pressure
Virtually all gas sensors and monitors measure the partial pressure, not
the percentage, of the gas that they sense. The only time that these
instruments can accurately read percentages is when the total pressure does
not vary over time between calibrations and use. This is way it is important to
calibrate the MX300-I oxygen sensor at regular intervals. It is recommended
that the unit be calibrated prior to each use or every 8 hours.
When the sensor is connected to a ventilator circuit, the alternating
“breathing” pressure cycles generated by the ventilator will be sensed as
an increase in the oxygen percentage (especially if the sensor is fast
enough to sense the changes, as is the R17MED). In reality, the
percentage of oxygen is not changing; it is the total pressure that is
increasing producing a corresponding increase in the partial pressure of
oxygen. A one hundred centimeter of water pressure pulse will produce
a 0.11 atmosphere, or an 11% increase in the total and therefore partial
pressure of oxygen. Assuming that the sensor is fast enough to track this
pressure pulse, an un-pressurized reading of 50% oxygen will increase
to 55.3% if the sensor is subjected to a pressure cycle of 100cm H
2
O.
The reading will rise proportionally less for smaller pressures.