Welch Allyn PROPAQ CS - Manual (Page 132)

128 Technical overview Welch Allyn Propaq CS Vital Signs Monitor
IPC expansion
The Inter-Processor Communications (IPC) bus, originating in the monitor, is passed to the
MSP Board from the expansion connector. The two-wire bus is expanded to the printer
and to the Mainstream Processor via opto-couplers.
Power distribution system
Power for CO
2
and SpO
2
circuit operations is derived solely from the expansion
connector. The principal supply comes from the monitor battery connections and an on-
board switching power supply.
CO
2
analysis chamber (bench)
Analysis of the expired CO
2
occurs in the bench. The bench is located in the Mainstream
sensor for MCO
2
or on the Sidestream Stick Board for SCO
2
.
In Mainstream applications, the sensor mounts over, and connects to, a disposable airway
adapter (cuvette) through which the patient’s exhaled gases flow. In Sidestream
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
2
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
2
; routed directly to the bench in SCO
2
) 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
2
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 Board),
provides all required information to determine the CO
2
value.