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3 Measuring Principles
3.1 RBC,WBC and PLT Concentration
Detection
Detection is accomplished using the electronic impedance principle and occurs
in the orifice of the transducer.
The blood is diluted to 1:400 (WBC & HGB) and 1:40000 (RBC & PLT) through
a precise shear valve system. The shear valve (sampling valve, marked in the figure
below) “cuts” a very reproducible volume (25 µl) from the aspirated blood and
dilute with an equal precise volume of diluent (or lyzer) to achieve the final dilu-
tion rates.
Two separate measuring chambers and transducers are used, one for RBC/PLT
and one for WBC/HGB analysis. This excludes any possibility of cross contam-
ination between the lyzer and the RBC/PLT dilution.
A pressure is applied on top of the diluted sample and the diluted sample is
pressed through an orifice (aperture) of 80 µm diameter. Each side of the orifice
is equipped with a platinum electrode and an electrical current is applied between
the electrodes.
When a cell is drawn into a constant current, flowing from an electrode through
the orifice to a second electrode, the electrical conductivity changes. This gener-
ates an equivalent voltage pulse.
The amplitude of the pulse is directly proportional to the volume of the repre-
sented cell. The number of pulses corresponds to the number of cells detected.
Coincidence corrections are made within the software of the instrument, giving
the instruments a full linear range within the specifications.
1017.wmf
Sampling valve
1016en01
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