Welch Allyn Duet CO2 Module - User Manual User Manual
Page 9
Duet CO
2
Module OEM Implementation Manual
Welch Allyn OEM Technologies
Confidential
Page 9
The basic components of the CO
2
sensor include
§ IR source
§ chopper wheel
§ IR detector
§ thermistor
§ optical filter
§ non-volatile memory
§ heater (mainstream only)
The IR source emits energy that is directed toward an IR detector. The detector generates a voltage
based on the amount of energy it receives. In the IR path between the IR source and the detector are
three components: an optical filter, which allows only a specific IR wavelength to pass, a gas sample
in the airway adapter and a chopper wheel. The chopper wheel has three distinct areas: a reference
gas cell, an open area and a dark, or closed area.
The chopper wheel rotates at 33 revolutions per second. As the single IR beam encounters the three
areas of the rotating chopper wheel, a signal is generated to create a ratio.
REFERENCE +
SAMPLE
DARK
SAMPLE
TIME
PHOT
ODETECT
OR VOL
T
AGE
This ratio of “reference + patient sample/patient sample” is used to determine the CO
2
concentration in
the patient’s respiratory gas. This reduces error due to the variability in system components and
minimizes the effects of patient obstructions and secretions on the measurement. The dark area of the
chopper wheel provides a zero reference for re-calibrating the detector with each rotation. This results
in a self-calibrating system.
Since mainstream applications typically involve humidified circuits which can result in fog or
condensation on the airway adapter windows, it is necessary to use a heater to elevate the window
temperature above that of the exhaled respiratory gas. By doing so, the heater keeps the airway
adapter windows clear of condensation while also maintaining a stable detector temperature.
NOTE
The optimal operating temperature for the mainstream sensor is 42
o
C.
The thermistor provides temperature feedback to the heater control circuitry.
The non-volatile memory is an EEPROM containing calibration and manufacturing data specific to the
CO
2
sensor.