Abacus
4
Service Manual
53
6.6. Possible Causes of Noise
Generally high count of any particle - even if you think it should be low, or near zero - can be
caused by NOISE, i.e. something interferes with measurement.
The most important thing in these cases to identify the source of NOISE, otherwise you cannot
protect the system against it.
NOISE can come from has several sources, and the different NOISE sources are added.
Sometimes we have to fight one of them, but sometimes more. Only one of them is enough to
make problem.
6.6.1.
Contaminated reagent
The most probable cause: real particles are in the reagent, and therefore the PLT blank is
continuously high (e.g. always 30-40). You can easily sort out this case by replacing diluent by
opening a new tank. PLT blank must go down is several blank measurements (below 10).
How can a good reagent become bad by time?
If the reagent tube was contaminated, and some bacteria begin to grow inside, once you put
an infected reagent tube into a new tank, by time it can become infected as well, i.e. the
background (PLT blank) becomes high. Wash the reagent tube - which is in connection with
the reagent - with 1% of bleach solution, then rinse with clean distilled water or diluent. It
can avoid the bacteria to grow inside.
If tank is open
– and cap is not installed or closed - external dust can make reagent dirty.
6.6.2.
Bad earth grounding
In this case external - ground referenced - noise can get into the system by ground coupling. If
system ground is not good enough, ground terminal can become a noise source as well, i.e.
external signals will be coupled into the system instead of protecting it.
If no earth ground is available, you can use a screw at the rear panel to connect a ground
potential to the case, so that noise immunity can be increased.
Measure voltage on ground terminal to make sure earth grounding is correct. AC voltage lower
than 1V is accepted in this case.
At some places - as a bad practice - electricians like to connect earth ground terminal to neutral
wire. Depending on the resistance of the neutral back wire (where it is really earthed), several
volts can appear, and this way any inductive noise will be coupled into the instrument. It is
better to create a real earth grounding and connecting it to the rear screw.
