The platform: the 4008 hydraulics by number, the blood-side modules, two CPUs and power
Identify the 4008 variant and options in front of you, follow the dialysate from the water inlet to the drain by the numbered parts of the flow diagram, and name the module or board behind each function on the blood side, in the monitor and in the power supply.
Which 4008 is this, and what is fitted
The technical manual covers four variants, and the first thing to settle on any call is which one you have. The 4008 E and 4008 B carry the UF monitor with segment displays and keys; the 4008 H and 4008 S have a screen and a status lamp driven by its own small board, the LP 923. Each variant has its own display board: LP 635 in the E, LP 649 in the B, LP 924 in the H and LP 922 in the S. A second split runs the other way: the E and H use the LP 639 power logic board, the B and S the LP 647. So an H and an S look alike from the front but differ in the power logic, and a B and an S share the power logic but not the display. Order boards by variant and by the part number on the board you remove, never from a photograph.
Options change the hydraulics and the test sequence, so list them before you start: single-needle blood pump, bibag dry bicarbonate cartridge, central delivery of concentrate (CDS), DIASAFE or DIASAFE plus dialysate filters, the HDF and online HDF options, ONLINEplus, and the blood pressure, temperature and volume modules (BPM, BTM, BVM). Machines with advanced hydraulics add a hydraulics processor board, the LP 941, with its own compressor, test valve and vent valves. The manufacturer's TSC report has a tick box for each option; filling it in honestly at the first visit gives you the configuration record you will need later.
Read both software versions at start-up. CPU 1 and CPU 2 must carry matching versions; the manual lists a mismatch as one cause of the 'CPU-II failed' error. The software level also decides the inspection interval, as lesson 3 explains. The 5008 that followed the 4008 is a different machine with a different architecture; do not carry part numbers, codes or limits from one to the other.
Water in: pressure, heat and degassing
Keep the flow diagram of the technical manual open while you read this; every part has a number, and the error texts use the same numbers. Water from the treatment plant enters through the inlet filter (63) and the water inlet valve (41), and a pressure reducing valve (61) brings it down to a steady working pressure so that plant pressure swings do not reach the hydraulics. It then enters the heater block (66). In the first chamber the incoming cold water passes the heat exchanger (77), where it picks up heat from the used dialysate on its way to the drain. That is why a fouled or leaking heat exchanger shows up both as a heating problem and in the pressure holding tests.
In the next chamber the heater rod (54) brings the fluid to the set temperature, controlled from temperature sensor 2. A float switch (5) in the third chamber opens and closes valve 41 to hold the level, and a vent tube lets air and, in hot rinse, steam escape so that the heater block never builds pressure. The concentrate pump (23) adds acid concentrate into this water once per balancing chamber cycle.
The degassing pump (29) then pulls the fluid through a fixed orifice (89). The restriction produces a strong negative pressure, around 0.8 bar below atmosphere according to the manual, and dissolved air comes out of solution in the degassing chamber (88a). The pump pushes fluid and released air into the primary air separator (88c), where the air rises and returns to atmosphere through the loading pressure valve (65), while degassed fluid leaves from the bottom. Valve 65 also sets the loading pressure that fills the balancing chamber. Degassing that is too weak sends micro-bubbles into the balancing chamber and the conductivity cell; you will see it as UF inaccuracy, unstable conductivity and frequent fill programs. Level 1 explained why degassing matters; on the 4008 you can measure it at test point D.
Concentrate, bicarbonate and the conductivity path
Acid concentrate and bicarbonate are drawn through suction tubes (94, 95) by two small membrane pumps: the concentrate pump (23) and the bicarbonate pump (25). Each tube has a filter (71, 72) at its tip. During cleaning programs the tubes are parked in the rinse chambers (90a, 90b), and reed contacts (10, 12) tell the machine whether each tube is parked or connected. That is the basis of the 'Wrong conc. supply' message that stops the T1 test: the machine sees a tube in the rinse chamber when it expects it in a container or on the central supply.
With the CDS option, concentrate arrives from a ring main through connectors 121 and 122 and valves 102 and 104, each watched by a pressure switch (123, 124); check valves (117, 118) and filters (119, 120) stop backflow into the supply. With bibag, the cartridge sits on its own connector and the machine adds a separate conductivity cell (132), temperature sensor (133) and pressure transducer (134) for that branch.
After the balancing chamber, the finished dialysate passes the main conductivity cell (7) with its built-in temperature sensor (3). The display is temperature-compensated to 25 degC. Sensor 3 does two jobs: it compensates the conductivity reading and it provides the displayed dialysate temperature. A second sensor (109) serves as a cross-check; at power-on the machine compares 109 with 3 and complains if they disagree. When conductivity or temperature leaves its limits, the bypass valve (26) opens and the dialyser valve (24) closes, so the dialysate goes to the drain through the secondary air separator and the balancing chamber instead of the dialyser.
Balancing chamber, flow pump and UF pump
The balancing chamber (68) is two chambers, each split by a flexible membrane into a fresh side and a used side: F1 and A1, F2 and A2. Eight solenoid valves (31 to 38) route fresh and used fluid, switching in groups of four so that one chamber fills with fresh dialysate while its used side empties to the drain, and the other chamber pushes fresh fluid to the dialyser while taking used fluid back. Because each membrane has a fixed travel, fresh volume out equals used volume in on every stroke. The manual gives about 30 ml per balancing chamber cycle, and lesson 3 shows how to check it.
The 4008 does not time the changeover. The flow pump (21), which pushes used dialysate into the chamber, sees a rise in motor current when a membrane reaches the end of its travel, and that current pulse triggers the valve changeover. Between strokes all valves are closed for a short dead time. Dialysate flow is set by the speed of the flow pump, at 300, 500 or 800 ml/min in dialysis. A relief valve (78) limits the flow pump pressure ahead of the chamber. Many balancing faults are really this pulse: a pump that never reaches a clean current rise, or a membrane that sticks, gives irregular changeovers.
Used dialysate returns from the dialyser through a filter (73), valve 24b and the blood leak detector (8) into the secondary air separator (88b). That chamber holds the pressure transducer (9), whose reading together with the venous pressure gives TMP, and a level sensor (6). If air collects, the air separation pump (97) removes it; if the level does not recover, the machine runs a fill program through valve 43. A fill program at the start of treatment is normal; repeated fill programs during treatment point to air entering somewhere, usually a leak on the suction side.
The UF pump (22) is a membrane pump of about 1 ml per stroke that takes fluid out of the closed loop between dialyser and balancing chamber. Since the balancing chamber returns exactly what it receives, every millilitre the UF pump removes is drawn from the patient through the membrane. The UF pump has a return spring, and a broken spring or a blocked filter (74) ahead of it is a classic UF fault. Downstream, used fluid leaves through valve 30, the heat exchanger and drain valve 87; the disinfection valve (84) admits disinfectant and carries electrodes that detect disinfectant where it should not be.
The blood-side modules
The blood side is a row of plug-in modules, each with its own controller and error display. The blood pump is a roller pump driven by a stepper motor, with its own processor and a dual-channel monitoring scheme; it carries the arterial (or single-needle) pressure transducer, a line diameter setting and semi-automatic line loading. Its control and display boards are the LP 624 and LP 748. The pump reports its own E-codes for things such as an uncalibrated arterial transducer, a Hall sensor rotation fault or a speed fault. The single-needle version is the same pump with single-needle control and a compliance chamber connection.
The heparin pump drives the syringe plunger with a stepper motor and a threaded spindle; a Hall sensor marks the end position and a slotted disc with an optical sensor watches the speed. A coding switch tells the pump which syringe make and size is fitted. It must match the syringe the unit actually uses, and the manual warns against changing it in operation. A wrong code means a wrong delivered heparin dose with no alarm.
The air detector module holds the ultrasonic air detector across the venous drip chamber, the venous line clamp, the venous pressure transducer and an optical detector below the chamber. The ultrasonic path is designed to fail safe up to the receiver: any component failure lowers the received signal and causes an alarm. After the receiver the signal splits into two independent paths, one stopping the blood pump and the other closing the clamp. The optical detector tells the machine whether the line carries blood or a clear fluid, and the machine uses it to decide whether it is in preparation or in treatment. That one sensor changes which alarms and tests apply, so a dirty or misaligned optical detector causes surprising behaviour. The level detector board for the air detector is the LP 450-2.
Monitor, two CPUs and power
Inside the monitor, CPU 1 on the LP 631 controls the machine and CPU 2 on the LP 632 supervises it: CPU 2 runs the protective checks, holds the bypass and the safe state, and runs most of the T1 test. They sit on the LP 630 motherboard with the LP 633 input board, which brings in the sensors, and the LP 634 output board, which drives valves, pumps and the UF pump timing. The LP 763 multi-interface board connects external systems, and the LP 742 interference filter sits at the mains input. Calibration values live in a non-volatile memory; a missing value shows as 'not calibrated' for that function and blocks the T1 test.
The power supply unit holds a power board and a heater board, producing the 24 V and 12 V rails and switching the heater, with a watchdog that can force the safe state. If a rail drifts out of range the machine reports a voltage failure and stops. A battery keeps the blood side and its monitoring running through a mains failure, the 'emergency operation' that the technical safety check tests by pulling the plug in dialysis mode. The manual names only the blood circuit and its monitoring for this mode, and the T1 battery step checks charge for 15 minutes of it; the course reads this as: the battery is not there to continue dialysis (check the operating instructions).
With the hydraulics, the electronics and the heater sharing one cabinet, the manual's precautions are practical: keep fluid away from open electronics, do not touch live parts such as the mains and heater connections, and connect or disconnect boards and modules only with the machine switched off. Lesson 5 covers isolation and ESD in detail.
| Part | What it does | How it fails | |
|---|---|---|---|
| 01 | Heater block (66) with heater rod (54) and heat exchanger (77) | Warms incoming water and recovers heat from used dialysate | Scale on the rod, slow heating, heat exchanger leak seen in pressure tests |
| 02 | Degassing pump (29), orifice (89), air separator (88c) | Removes dissolved air and sets the loading pressure via valve 65 | Weak vacuum, air in the balancing chamber, unstable conductivity |
| 03 | Balancing chamber (68) with valves 31 to 38 | Exchanges equal fresh and used volumes each cycle | Leaking valve or membrane, irregular changeover, pressure holding failures |
| 04 | Flow pump (21) and relief valve (78) | Drives used dialysate and gives the changeover current pulse | Weak pump or no clean current rise, wrong relief pressure |
| 05 | UF pump (22) with filter (74) | Removes the ultrafiltration volume, about 1 ml per stroke | Broken spring, blocked filter, stroke volume out of tolerance |
| 06 | Conductivity cell (7) with temperature sensor (3) | Monitors conductivity and temperature before the dialyser | Fouled electrodes, drift, constant reading, sensor disagreement with 109 |
| 07 | Secondary air separator (88b), sensor 6, transducer 9, pump 97 | Keeps air out of the balancing chamber and measures dialysate pressure | Worn pump segment or belt, level sensor fouling, frequent fill programs |
| 08 | Air detector module with LP 450-2, venous clamp and optical detector | Detects air and level, closes the clamp, tells blood from clear fluid | Clamp not occluding, misadjusted detector, dirty optical detector |
| 09 | LP 631 CPU 1, LP 632 CPU 2, LP 633 input, LP 634 output | Control, supervision, sensor inputs and actuator outputs | Software mismatch, connector faults, failed output stages |
| 10 | Power supply unit and backup battery | 24 V and 12 V rails, heater switching, emergency operation | Rail drift and voltage failure, aged battery failing the load test |
Typical scenario: the display board that did not fit
Typical scenario, not a recorded case. An engineer took a call about a dead screen on a machine the ward called 'the new 4008'. From a photo sent by the nurse he ordered a display board for the 4008 H. On site the machine turned out to be a 4008 S, which uses a different display board, and the screen fault was in fact a loose ribbon connector that had been disturbed when the monitor was cleaned. The board went back to stores and the machine was back in use the same afternoon after reseating, a power-on check, a T1 test and a check of the displays.
Identify the variant, options and software from the machine itself before ordering anything. On the 4008 the E, B, H and S differ in display and power logic boards, and the cheapest causes (connectors, cables, contamination) come before any board.
Water inside a pump motor on a haemodialysis machine (model not stated)
On a haemodialysis machine whose make and model were not given in the discussion, a pump motor lost its seal and water got inside. One engineer suggested opening the motor, cleaning rotor and stator and replacing the bearings and seals, since the motor looked like a standard type; the owner of the machine expected that the motor would have to be replaced as a whole. The outcome was not recorded. On a 4008 the same risk exists wherever the hydraulics sit close to motors and electronics: a slow drip from a coupling, a valve or a pump head can run down onto a motor or a connector long before anyone sees a puddle.
Treat any trace of fluid near motors and boards as a leak to be found and fixed at its source. A motor that has had water inside is replaced with the manufacturer's part, not dried and refitted, and the machine is electrically safety tested afterwards.
On-site checklist
- Variant (E, B, H or S), serial number and operating hours recorded
- Options listed: SN, bibag, CDS, DIASAFE, HDF, online HDF, ONLINEplus, BPM, BTM, BVM, advanced hydraulics
- CPU 1 and CPU 2 software versions read and recorded
- Flow diagram for this hydraulics version available, with part numbers
- Heparin syringe code checked against the syringes the unit uses
- Board and part numbers taken from the parts fitted, not from memory