Welch Allyn LC101 CO2 Module - User Manual - Manual (Page 9)

LC101 CO
2
Module OEM Implementation Manual Section 1 - Functional Description
Welch Allyn OEM Technologies Confidential Page 9
The basic components of the CO
2
sensor (C-Cap bench) include
• IR source
• Dual element IR detector
• optical filter
• non-volatile memory
The IR source emits energy that is directed toward a dual element thermopile IR detector. The
dual element design uses two opposing thermopiles connected in series. As the ambient
temperature changes, the two elements change in similar fashion resulting in an output near
zero. Only one element is exposed to energy from the IR source, resulting in a voltage change
due to only that energy. The detector generates a voltage based on the amount of energy it
receives. In the IR path between the IR source and the detector is an optical filter which allows
only a specific IR wavelength to pass and the gas sample within the sensor chamber.
CO
2
Measurement Method
A temperature sensor is attached to the detector housing for temperature compensation beyond
the thermopile’s designed compensation, including temperature changes in the system
sensitivity. The detector generates a voltage in the mV range and an Op-amp on the circuit board
amplifies the signal.
The non-volatile memory is an EEPROM containing calibration and manufacturing data specific
to the CO
2
sensor.
Measurement Calculation
Measurements provided to the host system by the LC101 Module include ETCO
2
, Inspiratory
CO
2
(InsCO
2
) and respiratory rate (RR). These three measurements are collectively referred to
as breath data. A proprietary breath algorithm is used to calculate the breath data.
Breath Algorithm
Welch Allyn OEM Technologies’s breath algorithm incorporates an initial learning period which,
based on certain assumptions of CO
2
waveform morphology, establishes CO
2
reference points
for threshold determinations. A sliding window is used to detect a stable maximum, or ETCO
2
value, and a baseline, or InsCO
2
value. Thresholds are updated in real time with each breath. A
signal averaging technique is used to calculate the RR based on this set of measurements.
By incorporating these adaptive and signal averaging techniques, the breath algorithm effectively
reports accurate CO
2
measurements while maintaining a high level of noise immunity.
element 1
element 2
+
+
foil cover
Dual Element Thermopile
narrowband
IR pass filter
energy
from IR
source
detector
output