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SIGMA Manual Number 41019
Revision AA
(CW) signal centered at a nominal 293 KHz. This signal is generated by exciting a piezoelectric crys-
tal near its natural resonant frequency. The ultrasonic signal propagates from the sensor housing lat-
erally through the administration set tubing. The ultrasonic receiver is a piezoelectric crystal
matched in frequency to the transmitter. The differential output of the receiver is amplified, envelope
detected, and filtered before being presented to a 12-bit A/D converter where it is converted to a 12-
bit digital word. Three samples are taken for each pump revolution. The digital words are processed
by the SIGMA Spectrum processor and fed into a detection algorithm. The detection algorithm com-
pares successive samples and looks for a trend of decreasing signal amplitude caused by a reduction
in pressure against the sensor housing. If the trend continues through a predetermined threshold over
a predetermined unit of time, a proximal occlusion is declared and a visual and audible alarm is pre-
sented to the user.
Air Sensor
The air detector uses the same sensor hardware as described in the Proximal Occlusion Detection
section. The air detection algorithm uses an “air” threshold that is set by a calibration routine at the
factory. If the signal amplitudes received by both sets of ultrasonic receivers are below the air thresh-
old, an air alarm is declared and a visual and audible alarm is presented to the user. The SIGMA
Spectrum air sensor is designed to detect air bubbles greater than 1 inch of tubing (125–150 µl
depending on tubing calibration).
Downstream Sensors
Set Loading
The downstream sensor consists of a channel piezoelectric bridge force transducer and is located
between loading points 3 and 4 inside the pump door. This sensor has two basic functions. The first
is to acknowledge an IV administration set as being installed at load points 3 and 4. Acknowledge-
ment is identified on the display with a third green bar and check mark. The second function is to
detect distal occlusions.
Distal Occlusion Detection
The distal occlusion sensor uses a piezoelectric bridge force transducer. A pusher connected to the
door assembly assures that a constant force is exerted to push the administration set tubing against
the sensor housing. The detection algorithm compares successive samples and looks for a trend of
increasing/decreasing signal amplitude caused by a increase/reduction in pressure against the sensor
housing. Four samples are taken for each pump revolution. The differential output of the force trans-
ducer is amplified, envelope detected, and filtered before being presented to a 12-bit A/D converter
where it is converted to a 12-bit digital word. The digital words are processed by the SIGMA Spec-
trum processor and fed into a detection algorithm. The detection algorithm compares the received
signal levels to thresholds set by the user selected occlusion alarm trip points. If the received signal
exceeds the trip points, a distal occlusion alarm is declared and a visual and audible alarm is pre-
sented to the user.