Description Of The Hydraulic Unit - Fresenius Medical Care 4008 E Technical Manual

1.3.1

Description of the hydraulic unit

As soon as the inlet valve (41) opens, the water flows through the pressure reducing valve (61)
into the chamber (66a) of the heater block and across the heat exchanger (77) into the heater rod
chamber (66b).
The concentrate pump (23) admixes concentrate to the inflowing water per balancing chamber
phase.
The vent tubing prevents pressure from building up in chambers b and c. In the hot rinse mode,
the developing vapor can escape through the vent tubing.
While it is rising, the fluid is warmed up to the preset temperature by the heater (54). The heater
is controlled by the temperature sensor (2).
From chamber b, the dialysate flows into the chamber (66c). Incorporated in this chamber is a
float switch (5), which controls the solenoid valve (41), thus ensuring the correct fluid level.
The degassing pump (29) draws in the dialysate via the degassing orifice (89). This generates a
negative pressure of 0.8 bar.
In the lines and the following chamber (88a), the dialysate is degassed to a level which is
sufficient for hemodialysis.
Via the degassing pump (29), dialysate and released air are directed tangentially into the primary
air separator (88c), where air bubbles and the airless dialysate are separated. The air accumu-
lates at the top of the chamber (88c). Then, together with the recirculation flow and via the loading
pressure valve (65) as well as the chamber (66c), the air escapes into the atmosphere.
Chamber 88c is provided with a separating disc, which serves to prevent bicarbonate, if added,
from being recirculated via the heater rod chamber (66b).
At the bottom of chamber 88c, the degassed dialysate is pressed out and into the balancing
chamber (68) by means of the loading pressure.
Together with the eight solenoid valves (31 to 38), the balancing chamber (68) constitutes the
balancing system. Each of the two sections of the balancing chamber comprises two compart-
ments separated by an elastic membrane each. Hence, there are two chambers with four spaces:
– F1 and F2: fresh fluid
– A1 and A2: waste fluid (used)
As soon as one of the chambers (A1 or A2) is filled with dialysate, the solenoid valves are
reversed in groups of four. The valves are reversed by the electronic evaluation of the current rise
pulse of the drive motor of the pump (21), which receives this pulse upon membrane abutment.
Within the filling phase, F1 or F2 is filled with fresh dialysate by means of the loading pressure. In
order to obtain a continuous flow, a second chamber is switched parallel to the first chamber. The
second chamber is operated at an inverse sequence.
Each time the chamber is changed over (maximum deflection of the membrane), all valves are
closed for approx. 100 ms (dead time).
From the balancing chamber, the dialysate flows through the conductivity cell (7) with integrated
temperature sensor (3). The measured conductivity values are indicated on the monitor in ms/cm,
related to 25 °C.
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Fresenius Medical Care 4008 4/08.03 (TM)