4690 Series
Turbidity systems
10 Fault Finding
36
IM/4690–EN Rev. A
10.2 Unstable or Erratic Readings
There are a number of possible causes of unstable or erratic
readings. Check for air bubbles in the sample. This may be due
to degassing of the sample, caused by either a drop in sample
pressure or a rise in temperature. Frequent cleaning of the
optical windows helps to prevent the build-up of bubbles. If
bubbles are observed it is recommended that the optional
de-bubbler unit is installed – see Section 3.4, page 10.
Where the noise level gradually gets worse over a period of time,
this usually indicates that solids are building up in the flowcell.
Increasing the flowrate through the flowcell may reduce this
build up. Ultimately the flowcell needs to be cleaned manually.
10.3 Intermittent Short Term Spikes
in Turbidity Readings
This is usually caused by bubbles passing through the light path
within the flowcell. The bubbles are a result of degassing.
Degassing is not an instantaneous process and it is possible
that it is taking place after the sample has gone through the
de-bubbler. The bubbles start to form in the sample pipework
and the flowcell. As they grow gradually, the bubbles finally
release and flow through the light path, causing a spike in the
turbidity reading.
The analyzer can be programmed for Bubble Rejection to
remove short-term spikes from the readings – see page 28. For
optimum operation, operate the wiper unit frequently to prevent
bubbles building up on the windows.