Teledyne MX300 manual Temperature, Pressure

Portable Oxygen Monitor Operation

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 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 H2O. The reading will rise proportionally less for smaller pressures.

Teledyne Analytical Instruments 27