7.3. 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.
7.3.1. 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.
7.3.2. 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.