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When the microprocessor commands the mecha-
nism to pump, the camshaft begins to rotate,
thus controlling the following pump cycle:
1. The inlet valve closes.
2. In synchrony with the expulsor moving
down to compress the tubing, the outlet
valve opens, expelling 0.050 ml of fluid (or
0.100 ml of fluid with a CADD-Prizm
®
high
volume administration set) toward the
patient.
3. The outlet valve closes.
4. The inlet valve opens as the expulsor is
retracted, causing fluid from the reservoir to
again fill the pump tubing segment.
5. The camshaft rotation stops after half a
revolution and the cycle is completed.
Pumping Characteristics
If the fluid path to the patient becomes blocked,
the pump tubing will expand as pumping
occurs. When there has been an amount of
inflation corresponding to 124 ± 62 kPa
(1.24 ± 0.62 bar, 18 ± 9 psi), the occlusion
analog sensor trips, whereupon the micropro-
cessor stops the pump mechanism and issues
visual and audible alarms. Thus the maximum
pressure which can be developed is 186 kPa
(1.86 bar, 27 psi).
To deliver the amount of drug specified by the
parameter settings, the pump’s microprocessor
causes the pump mechanism to deliver 0.05 ml
(or 0.1 ml with a CADD-Prizm
®
high volume
administration set) fluid “pulses” timed
according to the desired rate. At rates higher
than 3 ml/hr, 2 pulses in succession will be
given. Thus, to deliver 20 ml/hr, for example,
the microprocessor solves these equations:
Mechanism activations per hr
= 20 ml per hr/0.1 ml per activation
= 20/0.1
= 200
Time (seconds) between activations
= 3600 sec per hr/number of activations per hr
= 3600/200
=18
Rate Volume
(ml/hr) Resolution (ml)
Cassette or 0 - 3 0.050
Admin Set 3.1 - 125 0.100
Hi Vol 0 - 3 0.100
Admin Set 3.1 - 350 0.200
The microprocessor uses its timer circuits to
accurately time the 18 seconds (in this ex-
ample) between mechanism activations. The
timebase accuracy is ultimately determined by
the 3.6864 MHz quartz crystal oscillator.
Figure 7. A simulated pumping mechanism in a CADD-Prizm
®
pump.
Lock Button
Latch Button
Pressure Plate
Inlet Valve
Outlet Valve
Pump Tubing
Cassette Hinge
Occlusion Sensor
Expulsor
Pump Housing
Camshaft
Motor