Cobe Centrysystem 3 Service manual
Cobe Centrysystem 3 Service Manual
Hydraulics Description of Centrysystem 3 Flow-path with BiCart Priming Enhancement:
I) “Dialysate Prep” Module
1. Water inlet supply hose with filter washers: It connects the machine to the treated
water supply, is composed of two 158-micron filter washers where one is an in
faucet connector while the other is for downstream use (protection purposes),
traps small particles that might affect the operation of the hydraulic components,
and should be inspected and cleaned after every 100 hours of service.
2. Accessory water filter (optional): Instead of using a second 158-micron filter, an
accessory water filter can be installed.
II) Dialysate Prep “Fluid” Section
1. Inlet water pressure regulator (PR-1): It is an adjustable forward-pressure
regulator which is used to reduce the pressure of incoming water (15 to 125 PSI)
to about 600 mmHg at the inlet solenoid valve V1.
2. Inlet water valve (V1) and Brown Restrictor: V1 is a normally closed 24 V
solenoid valve which opens when the main power is switched on. It allows the
supply of water to flow into the machine when opened; on the other hand, it
prevents the flow whenever the power is switched off or when the machine is in
ACDR mode (preacetate, acetyl acetate, or chloride), as in disinfection dwell
mode or chemical recirculation mode. The normal operation of the valve V1 is on
and off pulsations where a Hall-effect level sensor LS1, located in the heater and
deaeration chamber, monitors the H2O level in the machine as V1 operates. The
dialysate prep control microprocessor uses LS1’s signal to control V1. The system
generates “HEATER CONTROL DISABELED” alarm if the duty cycle of V1 is
too high or too low. Note that some system alarms may automatically override
normal operation and cause valve V1 to close. As for the brown flow restrictor, it
is located in the tubing segment between PR1 and V1. It dampens the pressure
surges when V1 opens or closes and helps in moderating the flow of water into
the heater and deaeration chamber to maintain the output signal of LS1 linear and
stable.
3. Heat Exchanger: It uses warm dialysate effluent to preheat incoming water, has a
thermally conductive stainless steel plate that separates fresh H
O from the
2
effluent side of the flow path, and uses a counter-current flow for maximum heat
transfer. It holds about 60 mL of fluid per side and operates at 70% efficiency
which allows the use of a smaller water heater and the decrease of power
consumption.
4. Pre-heater thermistor (T0): It is a control thermistor that is used to monitor the
temperature of water flowing out of the heat exchanger and into the heater and
deaeration chamber. Along with T1, it controls the heater’s duty cycle and
minimizes the system warm-up time. A “DP THERMISTOR ERROR” alarm is
generated if T0 indicates a temperature reading below 6ºC.
5. Inlet water flow switch (FS3): The flow in a Cobe Centrysystem 3 is regularly
maintained between 300 and 700 mL/min. FS3 is a normally-open flow switch
which closes when the flow rate exceeds 200 mL/min. It detects low flows (or
absence of flow) when the flow rate decreases beyond 200 mL/min. The voltage
of FS3 is sensed by the dialysate prep control and monitor microprocessor. The
voltage reads zero when the switch is closed and +4 V DC otherwise. FS3 is
controlled by V1; thus, when V1 closes, FS3 must indicate no flow for
10 seconds. If the duration for which no flow is indicated at FS3 exceeds
10 seconds, the heater is turned off, and the alarms “NO WATER” and
“HEATER CONTROL DISABELED” are indicated.
6. Heater and deaeration chamber: The chamber is consisted of a 700 mL container
with:
a. 800 W heater (H1): The heater controls the temperature of the dialysate
solution (the voltage required to operate the heater is 115 V AC or 230 V
AC since it is composed of identical partitions 400 W heating elements).
b. Small polypropylene pellets: It aids in the deaeration process by providing
nucleation sits for air to accumulate. The vacuum system maintains the
heater and deaeration chamber at a low, constant, negative pressure so that
the micro bubbles degas and form collection bead sites. When bubbles
accumulate, they break away, are pulled through a mechanical ball-valve,
and then float into the vacuum system.
c. Mechanical ball-valve and float: They release accumulated air from the
chamber.
d. Level sensor (LS1): It monitors the water level in the chamber (sensing
along analog voltage signals and sends them to a microprocessor after an
1
ADC
). The output voltage of LS1 is 4.8 V DC when the chamber is full
while it is 3.2 V DC when the chamber is empty (“TRIP POINTS” limits
exit for alarms).
7. Post-heater thermistor (T1): It monitors the temperature of the water leaving the
heater and deaeration chamber. It is the primary controller of the heater duty cycle
but coordinates its operation with T0 (for adjusting the duty cycle of the heater)
and T3 (for compensating the natural heat loss as fluid moves through the
machine).
8. Deaeration pump (GP1) with check valve: It controls the flow rate in the machine
and with GP2 it helps deaerate water coming into the machine. The flow rate is
1
ADC: Analog to Digital Converter
300 – 600 mL/min except for ACDR where the rinse flow rate is 700 mL/min.
The voltage range is 0 – 24 V DC where the gear pump head is magnetically
coupled to a 30 V DC motor.
9. Isolation check-valve (ACDR/GP1): It prevents the inlet of GP2 from recording a
negative pressure and from reaching the rinse ports when GP1 stops in specific
ACDR rinse modes (additional chemicals are drawn in “CLEAN” and
“DISINFECTION” modes).
10. Rinse ports (acetate, acid and bicarbonate) and Brown Restrictor: As illustrated by
Figure 1, the color code for the concentration lines through which the heated
water used for rinsing flows is described as follows: brown for acetate, purple for
acid, and orange for bicarbonate solution. The ACDR modes include:
a. Clean mode: Acetate solution is connected to the yellow chemical port in
Figure 1.
b. Disinfectant mode: Bicarbonate solution is connected to the green
chemical port in Figure 1.
c. Acid rinse mode: The bicarbonate solution line is connected to the
chemical container.
There are two basic procedures involved: the acetate procedure, where the flow
runs in the acid and bicarbonate concentrate lines and their corresponding ports
which are related to the acetate flow line, and the bicarbonate procedure, where
the flow runs in the concentrate lines and the ports which are related to the
bicarbonate flow line.
11. Concentrate filters (130 micron): They are used to protect the bicarbonate and
acid/acetate diaphragm pumps from being damaged by debris from concentrate
containers.
12. Bicarbonate diaphragm pump: It is otherwise recognized as DPB; it has two major
functions:
a. Bicarbonate mode: DPB delivers bicarbonate concentrate into the main
flow-path and is mixed with heated water to form the bicarbonate
dialysate.
b. ACDR mode: DPB acts as a disinfection chemical pump when the
machine is in “DISINFECT” mode.
13. Bicarbonate mixing chamber: This chamber is used to mix water with bicarbonate
concentrate.
14. Bicarbonate control conductivity cell (CC1): This sensor is used by servo-
feedback control loop to maintain proportioning of bicarbonate concentrate
(ratio 25:1). The system composing of CC1, T3, DPB, and dialysate control
microprocessor compares conductivity of CC1 with a selected value from the
operator; if the values are different then a correction signal is given to adjust the
delivery to DPB. The bicarbonate level of the dialysate solution must be in the
range of 20 – 40 mEq/L (mmol/L); this level reading is controlled through the
operating parameters 7 and 8.
15. Sample port: It provides a convenience access point for taking bicarbonate
samples and for venting excess pressure if the flow-path becomes blocked.
Venting helps preventing the tube from becoming disconnected inside the
machine. The bicarbonate sample port should be placed in the yellow coded.
16. Bicarbonate monitor conductivity cell (MC1): It maintains bicarbonate
conductivity at safe levels (if the level exceeds ± 0.33 mS/cm from set point then
the “BICARBONATE CONDUCTIVITY LOW/HIGH” alarm shows).
17. Bicarbonate monitor thermistor (T2): It adjusts the conductivity of MC1 against
changes in the dialysate temperature and generates redundant temperature alarms
(below 31.7º C or above 43.5º C unless the thermistor T4 indicates same alarm
condition). The “HEATER CONTROL DISABLED” alarm appears if T2 reads a
temperature higher than 43.5º C.
18. Acid/acetate diaphragm pump (DPA): It has two major functions:
a. Bicarbonate/acetate mode: The DPA pump delivers acid concentrate into
the flow-path during “BICARBONATE” procedure and acetate during
“Acetate” procedure. The proportion of acid/acetate to water is 34:1
(range of 34:1 to 44:1 according to the type of acetate used).
b. Clean mode: The DPA acts as a cleaning solution chemical pump when in
“CLEAN” mode. The operator should plug the acetate concentrate line
into the yellow chemical port in Figure 1. The port is internally connected
to a special container located at the back of the machine. The cleaning
agent (usually chloride solution) is drawn up through a 130 micron filter,
check valve, and flow switch FSA. FSA monitors pulses from pump to
pump to ensure that it does not run out of chemical.
19. Acid/acetate mixing chamber: The mixing acetate concentrate with water during
the “ACETATE” procedure, or acid and bicarbonate concentrate with water and
during the “BICARBONATE” procedure. It ensures that dialysate is thoroughly
mixed before conductivity is measured by final conductivity control cell CC2.
Polypropylene pellets are placed in the chamber to enhance mixing and lower
overall internal fluid volume to 175 mL. The mechanical float and ball-valve
helps removing excess air which accumulates in the chamber. The float acts
against a lever, and it closes the normally opened valve when the fluid level rises
while it opens the valve when the fluid level drops. Thus, the float prevents
accumulated air from flowing through the final conductivity control cell and
routes it to the inlet of vacuum pump (which is directly connected to the drain).
20. Final conductivity control cell (CC2): It is used by the servo-feedback control
loop to maintain proportioning of acetate concentrate at a ratio of 34:1 during
“ACETATE” procedure, and acid concentrate at nominal ratio between 34:1 and
44:1 during “BICARBONATE” procedure. The system consisting of CC2, T3,
DPA, and dialysate prep control microprocessor compares the conductivity
measured by CC2 and the value selected by the operator. If the values are
different, then the system sends a correction signal to adjust the delivery rate of
DPA. The volume of Na+ can vary from 130 mEq/L to 160 mEq/L.
21. Temperature trim / final conductivity control thermistor (T3): It has 2 primary
functions:
a. Temperature compensation: It adjusts the bicarbonate conductivity of CC1
and CC2 for changes in dialysate temperature. CC1 and CC2 close
together in the flow-path and operate with the fluid at the same
temperature to provide compensation for both control circuits.
b. Heater command trim: It maintains the final temperature of fluid going
back to the patient by adjusting the command sent to the thermistor T1.
The trim function helps compensating for the natural heat loss which
occurs as the fluid travels through the machine and for drops in
temperature when concentrate solutions are added. The temperature at T1
is normally 1 - 2º C higher or lower than the temperature of the dialyzer.
22. pH probe: It measures the pH via an H ion-exchange method. It monitors the
solution flowing just after CC2 and generates an alarm when the pH exceeds
specified limits. It protects against the possibility of using wrong concentrate in
given mode, i.e. using acid concentrate instead of acetate concentrate in
“ACETATE” mode procedure. The pH probe has a high output impedance
(100 MΩ) and low DC output voltage (-60 mV / +pH unit) which is proportional
to pH of dialysate. Due to the high output impedance, the signal arriving from the
pH probe sent directly to dialysate prep CCA through coaxial cable.
23. pH probe fluid ground: It prevents small amounts of electrical (conductivity)
current present in dialysate solution from affecting the pH probe. It electrically
2
isolates the dialysate prep and UF
modules.
2
UF: Ultra Filtration
III) Dialysate Prep “ACDR
1. Recirculation Valve (V2): It is a normally closed 24 V DC solenoid valve that
opens to recirculate disinfectant solution upstream to inlet of the heat exchanger
during ACDR disinfectant mode. Hence, it ensures the hydraulic components at
inlet of machine are properly disinfected. GP1 provides the driving force. V2 also
opens during ACDR rinse procedures to flush out any chemicals in line and
minimize collection of precipitate or other debris.
2. Rinse valve (V7): It is a normally closed 24 V DC solenoid valve that opens only
during ACDR rinsing cycle. It provides a pathway from the main flow-path to
ACDR flow-path, allowing the machine to automatically rinse the concentrate
lines while they are connected to their chemical ports. GP1 pumps the heated
water via a rinse port through V7 into the concentrate line connected to chemical
ports, and then into ACDR flow-path to drain.
3. ACDR chemical lines and filters (130 micron): They connect lines of back of
machine (yellow for cleaning solutions and green for disinfection solutions) to
check valve and flow-switch to special chemical port in front (ACDR) panel of
machine.
4. ACDR chemical line check-valves: For each chemical line, they are located
between the 130 micron filter and T-connection going to flow switch (FSA or
FSB) and isolation regulator (PRV4 or PRV5). They prevent a backflow into the
chemical containers. The backflow could occur during ACDR rinse and would
dilute the chemical making it ineffective. The tubing pressure on the ACDR flowpath side is about 400 mmHg and on 130 micron filter, it is near atmospheric
pressure.
5. ACDR pressure regulators (PRV4, 5 & 6): They are adjustable back-pressure
regulators (4 and 5) used to stabilize flow switches in chemical rinse flow-path.
PRV4 isolates “CLEAN” chemical flow-path and directs the flow to DPB through
FSB during ACDR “DISINFECT” mode. Both regulators are calibrated to
pressure of about 400 mmHg. PRV6 is an adjustable back-pressure regulator used
to keep FS4 flow switch at a positive pressure of around 175 mmHg during
ACDR rinse cycles. On the other side of PRV6, the vacuum pressure from GP2
and drain system (vacuum pressure normally-600 mmHg).
6. ACDR chemical line and rinse flow switches (FSA, FSB, FS4, and FSC): FSA
and FSB are normally closed flow switches open when flow through them
exceeds 34 mL/min. FS4 is a normally closed flow switch that opens at
125 mL/min. FSA, FSB, and FS4 are monitored by both dialysate prep
microprocessors. The voltage across the switch contacts is about 0 V DC when
the switch is closed (no flow present) and about 4 V DC when it is opened (flow
3
ACDR: ACiD Rinse
3
” Section
present). FSA monitors yellow chemical port line during ACDR “CLEAN” mode
to ensure container doesn’t run out of chemical. Each switch pulses open within
0.5 mL stroke of pump. FS4 monitors fluid moving through ACDR flow-path to
drain during ACDR rinse cycle. The BiCart switch FSC is used during ACDR
rinse modes, when BiCart holder is closed, to ensure that all the bicarbonate
solution is rinsed from the system and that the BiCart flow-path is disinfected.
7. ACDR chemical ports (yellow and green): The yellow port is for “CLEAN” port
and the green port is for “DISINFECTION” port (mechanically sized so that they
can only be connected to correct the concentrate line). The yellow port is
connected with the brown acetate line connector, and the green port will mate
with the orange bicarbonate line connector. The spring-loaded piston with the Oring seals the port when the male connector is removed. Each port is internally
connected through a flow switch, check-valve, and 130 micron filter to a
dedicated chemical container at the back of the machine so that the operator does
not come into contact with chemicals when moving concentrate lines during
ACDR. The yellow port is typically connected to a container filled with 5.25%
NaHCl2 (sodium hypochlorite/bleach). The green port is connected to a container
filled with disinfectant such as 37% formaldehyde. In ACDR, the operator may
connect either acetate or bicarbonate concentrate line to the designated chemical
port. The other port remains connected to the rinse port.
a. When filling a system with disinfectant chemical, the solution is drawn in
by DPB through a 130 micron filter, check-valve, FSB, green chemical
port, BiCart module, FSC, another 130 micron filter, pump itself and
finally pulsed into the main flow-path just before bicarbonate mixing
chamber.
b. When “DISINFECTANT” rinse mode is in progress, fresh rinse water is
provided by GP1 through the acetate rinse port and into V7, through the
bicarbonate concentrate line into the green chemical port, through FSB,
through PRV5, FS4 and PRV6, and finally to the drain through GP2.
c. When filling the system with cleaning chemical, the solution is drawn in
by DPA through a 130 micron filter, check-valve, FSA, yellow chemical
port, pump itself, where it is pulsed into flow-path just before acid/acetate
mixing chamber.
d. When “CLEAN” rinse mode is in progress, fresh rinse water is provided
by GP1 through bicarbonate rinse port into V7, through acid and acetate
concentrate lines into yellow chemical port, through FSA, PRV4, FS4 and
PRV6 and then finally drain through GP2.
IV) Dialysate Prep “Vacuum/Drain” Section
1. Vacuum pump (GP2) and check-valve: GP2 provides a suction force for 2
systems: drain system and vacuum system. It is identical to GP1, but it is driven
with a constant voltage of 18.5 ± 0.5 V DC and is usually operated in reverse
configuration (reverse polarity and hydraulically connected for reverse flow so it
helps GP2 withstand large amounts of air coming from the drain and vacuum
lines without decoupling). The drain system begins at the output of PRV1 in
ultrafiltration module and proceeds through waste side of heat exchanger. It
continues though GP2 (optionally preceded by 158 micron filter), “Waste
Handling Option” or WHO and its venturi (pressure vent), and out of the machine
through a drain hose. The optional addition of a 158 micron filter (upstream from
pump) traps small particles of plastic or other debris that may come through the
main flow-path, BiCart module or vacuum system. It helps protect GP2 and
prevents loss of vacuum and flow (if present, it must be cleaned every 100 hours
of operation. A check-valve placed in parallel with GP2 provides a flow path
around the pump head when the pump is not running; it is used during certain
passive flow ACDR modes.
2. Vacuum regulator (VR1) and check-valve: The vacuum system composes of GP2,
VR1, hydraulic connections to tops of heater/deaeration chamber, bicarbonate and
acid/acetate mixing chambers, and air separator. VR1 is a bleed-in, needle type,
regulator that allows air into the drain system. The amount of air allowed in
controls the suction pressure at the inlet of GP2. The vacuum system is calibrated
to a pressure of 450 – 650 mmHg (depending upon the altitude). The check-valve
before VR1 prevents the air from flowing back into the chambers when the
machine is turned off or when GP2 is shut off during ACDR. Once power which
is given to GP2 is off, negative pressure exists in the vacuum line and could draw
up unwanted fluid (if it reaches GP1, a false “AIR SEPERATION HIGH” alarm
is raised).
3. “Waste Handling Option” (WHO): It is optional and provides convenient inlet to
drain system for operator. It handles excess blood, saline, and other liquid waste
from blood lines and dialyzer. The operator can dispose of waste by connecting a
tube to the WHO inlet port on the front panel and let the gentle suction of venturi
system pull the waste liquid to drain. The WHO is designed around the venturi
principle. Through the T-connection, the flow is directed both to venturi and up
the WHO rinse arm on the front panel of the machine. When the rinse arm is
closed, the flow continues through the WHO receiver port, down the suction line,
through two check-valves, into the suction port of venturi, and down to drain.
When the rinse arm is open, the flow is stopped by internal check-valve and
receiver port is opened to atmosphere. The suction line lightly draws air and/or
water fluids through the receiver continuously, until it is closed. The check-valves
in the receiver/suction line are redundant, ensuring that no flow from GP2 can
come out of WHO assembly through front panel. The WHO should be bleach
cleaned routinely as it comes into direct contact with patient fluids and blood.
4. Sample port / pressure vent (clear): The drain relief pressure vent is a clear
colored assembly. It is located in the drain line, at outlet of vacuum pump after
the WHO flow venturi, and vents at a pressure of about 425 mmHg. It protects
against accidental blockages of drain line and is not normally used as a sample
port.
V) “Ultra-filtration” Module
1. Balance chamber assembly (including valves and Hall sensors): The ultrafiltration
section of the flow-path is designed around a volumetric, closed-loop principle.
The first primary component of the section is the balance chamber assembly. The
assembly uses 2 synchronized chambers to ensure that the flow to the dialyzer,
and the patient, is continuous and that the fluid volume in loop is constant. The
balance chamber assembly consists of two identical 125 mL chamber units. When
viewed from the rear of the machine, balance chamber 1 becomes on the left
while balance chamber 2 becomes on the right. Each chamber includes:
a. Flexible diaphragm with magnet encapsulated in the center.
b. Two Hall-effect sensors, one mounted on each side of the chamber at the
center. HL1 and HR1 are on chamber 1and HL2 and HR2 are on chamber
2.
c. Four normally closed 24 V DC solenoid valves, one inlet and one outlet on
each side. VB2, 3, 6 and 7 are on chamber 1 and VB0, 1, 4 and 5 are on
chamber 2.
The diaphragm divides the chamber into two halves: one half of the chamber is
used to move fresh dialysate while the other half moves waste fluid. With the
diaphragm installed, the usable internal volume of the chamber is about 115 mL.
The magnet in the center of the diaphragm provides magnetic field that will tell
the processor how close the diaphragm is to the wall of the chamber. Viewing the
balance chambers from back of the machine, and visualizing the diaphragm
magnet, the north pole is to the left and the south is to the right. Hall sensors
detect the magnetic field of the diaphragm as it moves inside the chamber. The
left Hall sensors HL1 and HL2 have a voltage range of 3 – 6 V DC; the right Hall
sensors HR1 and HR2 have a voltage range of 9 – 6 V DC. The output near 6 V
means that the magnet is close to the chamber wall. An amplifier conditions this
voltage to improve linearity. Its output is then processed by an A/D converter and
sent to ultrafiltration control processor. From this voltage processor develops trip
points for Hall sensors. Trip points are used for changing valve positions and to
keep diaphragms from hitting the chamber walls. The valves control the direction
of the fluid through balance chambers. They are electrically connected together in
pairs. Each pair, VB0 & 1, VB2 & 3, VB4 & 5, and VB6 & 7, has its own driver
transistor. During normal operation, the valves are opened and closed, four at a
time. More specifically, two valves on each chamber are opened while the other
two (on both chambers) are closed. Therefore, the fluids enter one side of the
chamber while the other side is being emptied. Note that the pairs of valves that
open together are on the opposite sides of the chamber which helps flush fresh
fluid across the whole surface of the diaphragm. The operation of the balance
chambers is built around a “fill” and “cycle” sequence. “Fill” refers to fresh
dialysate fluid moving the diaphragm towards the left Hall sensor, and “cycle”
refers to waste fluid moving the diaphragm back towards the right Hall sensor.
Both movements together complete a full cycle or circuit of balance chamber
diaphragm. When calibrated, each chamber moves about 100 to 105 mL of fluid
per “fill” and “cycle” sequence. The processor monitors the fill and cycle times
for each chamber and uses this information to control the flow pump GP3. The
calculations for the fill time of chamber 1 are displayed on the status screen as the
flow rate of machine. The chamber is “on-line” when fresh dialysate fluid is being
pushed from the chamber towards the patient while it is “off-line” when the right
half of the chamber is refilling with fresh dialysate fluid. At the end of the flow in
each direction, the processor signals valves to switch and chambers reverse their
functions (if VB0, 1, 2 and 3 are open then VB4, 5, 6 and 7 are closed). When
commands are sent, all 8 valves reverse their positions, which consequently leads
to having one chamber always filling with waste fluid (on-line mode) and pushing
fresh dialysate towards the dialyzer while the other is filling with fresh dialysate
(off-line mode) and pushing the waste to the drain.
2. Dump valve (V3) and yellow restrictor: V3 is a normally open 24 V DC solenoid
valve connected to the balance chambers fresh dialysate inlet line through a
yellow restrictor. It provides an alternate path around balance chambers when
pressure relief or special flow is required. It opens also for certain alarm
conditions. V3 is controlled by the ultrafiltration control processor and is usually
open for a short time (< 100 ms) at the end of each cycle (longer periods during
ACDR periods). One balance chamber is always filling with fresh fluid while the
other is filling with waste. When the right side of the chamber fills first, and the
Hall sensor trip points have been reached, V3 is opened to release any excess
pressure produced by the flow from GP1. If the other chamber finished filling its
left side first, V3 is not used. Rather, the flow pump GP3 will be used to maintain
proper pressure in the system. V3 only opens to “dump” excess fresh dialysate to
drain if the right chamber fills before the left chamber, in any given cycle.
Restated, the dump valve V3 and flow pump GP3 are used to synchronize the
chambers, hence preventing the hydraulic lines from separating. The dump valve
bypasses fresh dialysate fluid around the balance chambers through PRV1. The
yellow restrictor in line with V3 provides resistance to flow so that the pressure
through the balance chamber system stays constant when V3 opens and closes.
3. Balance chamber pressure regulators (PRV1 – PRV2): PRV1 and PRV2 are
identical, manually adjustable, back-pressure regulators. These regulators perform
three major functions:
a. Provide back-pressure through diaphragms so that the inlet pressure to
each balance chamber remains essentially constant.
b. Provide stabilizing pressure so that the pressure within the system remains
relatively constant and any internal drift is minimized.
c. Isolate the balance chambers from elements downstream (vacuum system
or dialyzer) that could affect the flow through the chambers.
PRV1 is calibrated to a specific pressure and is kept there. PRV2 is used to keep
the pressure of the waste dialysate basically the same as the pressure of the fresh
dialysate. It is calibrated to compensate for differences in system components and
minimize drift during normal balance chamber operation.
4. Ultrafiltration pressure regulator (PRV3): It is a manually adjustable, back-
pressure regulator. It is set to crack pressure of around 1000 mmHg and isolates
the ultrafiltration diaphragm pump DP3 from vacuum state. This high pressure
setting is necessary due to positive inlet pressure at DP3 (about 500 mmHg) and
negative pressure (> -500 mmHg) of vacuum system. PRV3 prevents fluid from
leaking out of the volumetric loop through DP3 to drain.
5. Ultrafiltration diaphragm pump (DP3): It is designed around a volumetric, closed
loop principle (flow-path ultrafiltration). It is the second primary component of
this section. Its single function is to remove the fluid from the closed loop system.
In a volumetric system, there is a constant volume of fluid and a general ambient
pressure within the loop. When fluid is removed from the loop, negative pressure
is created. The pressure will continue to increase as more fluid is removed or until
fluid inlet is provided. In this system, the dialyzer is the normal inlet for the fluid
to enter the loop. The fluid drawn from the patient (passing through the dialyzer)
is called ultrafiltrate. To protect against pumping the wrong ultrafiltration rate, a
metering system turns off whenever the blood pump is off. It also shuts down
whenever the flow pump GP3 is turned off or when there is a specific alarm
affecting the ultrafiltration system. When the ultrafiltration system off, no fluid
moves from the blood side of the dialyzer to the dialysate side.
6. Final conductivity monitor cell (MC2): It assures that the conductivity of the
dialysate flowing to the dialyzer is maintained at a safe level. The output
displayed on the status screen as the conductivity of the system. It generates a
“CONDUCTIVITY LOW” and “CONDUCTIVITY HIGH” alarms if the final
conductivity varies by more than ± 0.66 mS/cm from the set point.
7. Final temperature monitor thermistor (T4): It assures that the temperature of the
dialysate flowing to the dialyzer is maintained at safe level. Also it is used to
compensate the conductivity measuring circuit for changes in the temperature of
dialysate. The output of T4 is displayed on the status screen as the system
temperature. The “TEMPERATURE LOW” alarm is generated when the
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