Understanding the System Principles
3-9
3.5 RBC/PLT Measurement
3.5.1 Volumetric Metering
An accurate cell count cannot be obtained unless the precise volume of diluted sample that
passes through the aperture during the count cycle is known. This analyzer uses a volumetric
metering unit to control the count cycle and to ensure that a precise volume of sample is
analyzed for the measurement.
The metering unit controlling the RBC/PLT count cycle consists of a metering tube with two
optical sensors mounted on it. This tube ensures that a precise amount of diluted sample is
measured during each count cycle. The exact amount is determined by the distance between
the two optical sensors. The rinse is used to create a meniscus in the metering tube. The
count cycle starts when the meniscus reaches the upper sensor and stops when the
meniscus reaches the lower sensor. The amount of time required for the meniscus to travel
from the upper sensor to the lower sensor is called the RBC Count Time and is measured in
seconds. At the end of the count cycle, the measured count time is compared to the
pre-defined reference count time (see Chapter 5.3 for details). If the former is less than or
greater than the latter by 2 seconds or more, the analyzer will report a RBC bubble or RBC
Clog error. Seeing the error message, refer to Chapter 11 Troubleshooting for solutions.
Lower sensor
Upper sensor
1 Empty when start 2 Liquid surface fall down through the
volumeritc tube
3 Counting start when the
liquid surface pass the upper
sensor.
4 Counting end when the liquid
surface pass the lower sensor
Lower sensor
Upper sensor
Lower sensor
Upper sensor
Lower sensor
Upper sensor
Figure3-5 Volumetric metering process
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