Service manual

Service Manual Technical Overview 115

applications, a sample of exhaled gases is obtained through a small-diameter sampling

tube and routed to the bench located on the Stick board.

Unless otherwise noted, the following details are common for bench operation in both

sampling methods.

The bench consists of infrared (IR) light source and filter, a revolving chopper wheel, and a

photodetector.

The chopper wheel is an aluminum disk containing a sealed CO

reference cell, an open

slot, and a light-blocking area. The chopper wheel spins in the direct path of the IR light

source.

The IR source is located on one side of the bench and is projected through the gas sample

(in the airway adapter in MCO

; routed directly to the bench in SCO

) to a photodetector

at the other side of the bench. As the IR beam passes through the gas sample to the

photodetector, the components of the spinning chopper wheel are passed through the

beam with each revolution.

The amplified photodetector output signal is modulated by the revolving chopper wheel

and is referred to as VSIG. Each rotation of the chopper wheel generates two VSIG peaks

of differing size, and a low flat spot. The voltages representing these points are called

Vpeakhi, Vpeaklo, and Vdark. These correspond to the open slot, the CO

reference cell

and the light blocking areas on the chopper wheel, respectively. The VSIG output from

each complete rotation of the bench chopper wheel, occurring every thirty milliseconds,

together with a known bench temperature and calibration constants stored in an

EEPROM (located in the mainstream sensor connector or the sidestream Stick PCB),

provides all required information to determine the CO

value.