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TELEDYNE ELECTRONIC TECHNOLOGIES
Analytical Instruments
SECTION EIGHT EFFECT OF PRESSURE
Virtually all gas sensors and analysers measure the partial pressure, not th e percentage, of the
gas that they sense. The only time that these instruments can accurately read perc entages is
when the total pressure does not vary over time between calibration and use. This is why it is
important to calibrate the TED 191 Portable Oxygen Monitor at regular in tervals.
It is recommended that the instrument 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 ox ygen percentage (especially if
the sensor is fast enough to sense the changes). In reality, the percenta ge of oxygen is not
changing; it is the total pressure that is increasing, producing a c orresponding increase in the
partial pressure of oxygen. A hundred-centimeter water pressure pulse will produce a .11
atmosphere, or an 11% increase in the total and therefore partial pressure of ox ygen. Assuming
that the sensor is fast enough to track this pressure pulse, an unpressuris ed reading of 50%
oxygen will increase to 55.3% if the sensor is subjected to a pressure c ycle of 100cm H2O. The
reading will rise proportionally less for smaller pressures.