Teledyne MD300 - Oxygen analyzer and monitor (Diving) - Manual (Page 25)

Portable Oxygen Monitor Operation
Teledyne Analytical Instruments 25
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 correct itself in about 30
minutes.
2.3.2 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 AD/MD300 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 sample gas that is flowing under
pressure, the sensor may read high. 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 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. An un-
pressurized reading of 50% oxygen will increase to 55.3% if the sensor is
subjected to a pressure of 100cm H
2
O. The reading will rise proportionally
less for smaller pressures.
2.3.3 Humidity
Humidity does not directly affect the accuracy of the sensor's
measurement. However, when a nebulizer or other device is used to
increase moisture levels in gas mixtures, the moisture actually dilutes
the mixture. This dilution effect decreases the oxygen concentration.
For example, if an 80% oxygen gas mixture is humidified to
saturation at room temperature, the resulting gas mixture will contain
only 77.5% oxygen. Your portable oxygen monitor accurately measures
decreases in the oxygen concentration due to the dilution effects of
moisture added to gas mixtures.
As with all oxygen sensors, excessive condensation on the sensing
surface of the R17S will block the diffusion of oxygen to the sensor,
rendering it inoperative. We recommend installing the sensor on the dry
side of the breathing circuit at all times.