The heated water now passes through the concentrate pick-up tube connectors. If a concentrate connector is pulled out, the connector will seal and there will be a suction from the respective feeding pump to the tube’s stick.
In the concentrate connector port there is Hall effect element that gives information to the monitor if a connector is in or out of the concentrate connector port.
The concentrate/water mixture passes via the feeding pump A through a mixing chamber. The mixing chamber separates air (if any) and bypasses conductivity cell A, since air in the conductivity cell will disturb the measurement and leads to unstable conductivity regulation.
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Figure 3:14 AK 96 - Concentrate intake/mixing and conductivity regulation The temperature transducer in Conductivity cell A has nothing to do with temperature regulation; it is used for compensation of the conductivity measurement which by nature has a positive temperature coefficient of 1.7%/°C.
The B regulation circuit works in the same way with the B-supply coming from either the bottom of the BiCart or from the “B” pick-up tube.
As mentioned above, the temperature transducer in the “B”-cell is used both as the second measurement in the temperature regulation and for temperature compensation of the conductivity measurement in conductivity cell B.
A very important feature of the conductivity regulation is the supervision of concentrate pump speed. A parameter called relative pump speed is used. This expresses, in %, how much a pump’s actual speed deviates from the theoretical speed.
The theoretical speed is calculated from the Na+ /HCO3 -settings, from the preset list of the concentrate’s chemical components and the flow in channel 1. If the actual speed moves outside predefined limits, an attention alarm is issued, and the fluid monitor goes in Bypass. The default values for these limits are:
-10% 0 +10%
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The limits can be changed in Preset mode.
In order to optimize the dialysis treatment the concentrations of sodium and bicarbonate in the dialysis fluid can automatically be changed following a predetermined continuous decreasing or increasing linear graph. Profiling of sodium is controlled by the A-pump and profiling of the Bicarbonate is controlled by the B-pump.
The valve FLVA (Flush Valve) is used during the priming of the BiCart cartridge. Priming means filling up the cartridge with warm water and waiting for liquid bicarbonate to come out in the bottom. This is how it works:
When the priming is triggered – either by the fact that the cartridge is placed in the holder before the machine is started, or that the holder is opened during operation – both concentrate pumps stop. The machine runs, waiting for the measured conductivity (B) to drop below 2.0 mS/cm.
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The priming starts by opening the FLVA, connecting the flow pumps suction side to the bottom of the BiCart, and running the B-concentrate pump at full speed. The machine now waits for the B-conductivity to achieve more than 5.0 mS/cm. This will normally happen within 60 seconds, if it doesn’t an attention alarm appears. Otherwise, the FLVA is closed, the B-concentrate pump is stopped and the machine waits for the B-conductivity to drop below 2.0 mS/cm. The priming is completed.
After conductivity cell B the fluid passes a pH-cell. pH-monitoring is an optional safety feature with a low and a high limit, changeable from the front panel.
Safety feature in the sense that some disinfection agents can give the same conductivity but the pH value is totally of limit (differs a lot from pH 7).
Conductivity Control System, acetate Relative pump speed deviation
SRI(24)
The conductivity set point is calculated on basis off the sodium set value and the information in the concentrate preset.
The speed of the concentrate pump is constantly monitored and related to calculated speed (calculated from the conc. preset, the sodium setpoint, and the flow in ch1) to ensure that it stays within limits which are ±10 % (default). If these limits are exceeded an attention is issued:
INCORRECT CONCENTRATE Check concentrate
The relative pump speed – the deviation between the actual and calculated speed, in percent, can be logged, of course, and should ideally be zero.
On a machine, running with WRO-supply and high quality concentrates, it will normally be very close to zero.
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Conductivity Control System, bicarbonate
Cond. B2 Conc.pump B
Set point
The bicarbonate mode uses two feedback loops: one for the A-pump – the acid component, and one for the B-pump – the bicarbonate. Again, the set points are calculated on the basis of the sodium and bicarbonate set values, and the concentrate preset. Both pump speeds are monitored and related to the calculated speeds to ensure that pump speeds stay within limits which are ±10 % for the A-pump, and ±20 % for the B-pump (default). If the limits for the respective pumps are exceeded, attentions are issued:
INCORRECT ACIDIC CONCENTRATE Check concentrate fluid
INCORRECT BICARBONATE CONC.
Check concentrate fluid
Composition supervision
The pump speed of each feeding pump is supervised by the control system, which uses the conductivity values measured from conductivity cell A and B. The conductivity cell P measures the total conductivity of the fluid. The dialyzer is bypassed if the conductivity exceeds the alarm limits.
Despite these supervision functions there is a risk that the dialysis fluid has the correct conductivity but erroneous composition of the concentrates A and B during treatment in bicarbonate mode.
The composition of the dialysis fluid is therefore supervised during treatment in bicarbonate mode, in order to detect if conductivity cell A does not measure correctly. An erroneous dosage of concentrate A may otherwise result in bad fluid composition without causing a conductivity alarm.
The composition supervision can also detect if wrong concentrates are used at the same time as the conductivity is still between the alarm limits. This can for example happen if the powder in the BiCart does not dissolve properly.
The composition supervision is monitoring the pump speed ratio. This means that the actual pump speed is monitored. If the difference in the pump speed between the two pumps is more than the preset value (default 15%) an attention will be issued and the fluid will be bypassed.
INCORRECT DIALYSIS FLUID COMPOSITION
The value that triggers the attention can be logged as SRI(056). The value may vary between 850 and 1150 (default). The limits can be preset (2-22-3) as a % pump speed deviation. The default value is set to 15 %.
The variable is calculated as:
Pump speed pump A x Dose A Pump speed pump B x Dose B = 1
Where the value 1 is the ideal value. Dose is the dilution of the concentrate depending on the concentrate preset.
As one of the pumps deviate the value will change and if it varies more than 15% the attention is issued.
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