7.3.3. External electrical noise
If another instrument is near the analyzer can radiate electromagnetic signals in the 1 kHz -
100 kHz frequency region it can be picked up by the system (especially if they are very close
to each other, or the grounding is not quite perfect).
You can easily identify this noise source: by relocating the instrument noise (high PLT blank)
disappears. In this case you have to identify the possible noise source (switch mode power
supplies, computer monitors, since they are not shielded, centrifuges due to high switching
noise of rotor contacts, etc.), the power of the electromagnetic source, because if high power
is present, maybe relocation does not solve your problems, sometimes the electric power
supply makes the coupling, so UPS solves the problem.
Another source of coupling in external noise can be the reagent tanks and tubes. Especially
radio transmitters can cause problems of radiating so that even the reagents (diluent) guides
in the noise. A metal pack for the diluent tank, then a good earth grounding of this metal box
allows this coupling to disappear forever.
7.3.4. Internal noise sources
The most annoying but real cause is some sort of internal noise. The reason for this
phenomenon is that inside electrode - hot point - of the measuring circuit must be well
insulated from surrounding electronics, otherwise inside noise sources can take their effect.
7.3.4.1.Bad chamber insulation:
bad shielding of the chamber (floating shield couples signals to the chamber, and does
not prevent against them). Check grounding of shield, remove it and clean the surface
between the shield and the metal base.
bad reference electrode connection (floating ground reference). Repair is required.
bad sealing of aperture. Replacement of measuring tube is required.
broken measuring chamber starts to conduct through the gaps (ground path).
Replacement of chamber is required.
contaminated draining tube starts to conduct due to protein or lipid build-up. It is very
easy to identify this case. After replacing the drain tube of the measuring chamber
(mainly WBC), WBC histogram peak, or PLT becomes low soon. Normally a good
cleaner is required to dissolve lipid or protein build-up. Sometimes the cleaner is not
strong enough to keep this tube clean enough. Periodic washing using 1% hand warm
bleach solution helps.
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