Mechanisms introducing air into the process
– Air gets mixed into the sludge for example when
material drops from a conveyor into a tank or vat. If
the tank level is low, the adding point is close to a
pump, or the throughflow time of the tank is short,
the air is not removed before the sludge flows out
again. The best way to bring material into a tank is
to lead the flow in under the surface.
– Strong stirring while the level in the tank is low may
also generate a whirl that sucks air into the material.
– Air may also be mixed in through a leaking joint on
the intake side of a pump.
– Dilution water is another source of air.
– Air may begin to collect inside a pipe bend that forms
the highest point of a line. When this air pocket gets
big enough, the flow through the pipe will eventually
drag it along. This kind of an air pocked may cause
a distinct peak in the total solids content measure-
ment.
– Foaming chemicals may generate air in the sludge.
• Chemicals
Chemicals in the sludge may weaken the measurement
signal. A maximum conductivity limit has therefore been
defined for each sensor type. If the process conductivity
is over this limit, the measurement signal is too weak
and there is noise and errors in the measurement result.
When the conductivity is very much above the limit, the
measurement signal will disappear altogether. Vari-
ations in chemical content may cause small errors in
measurement even within the conductivity specification.
These errors can be eliminated by using the Valmet
TS chemical compensation (see chapter 9).
• Temperature
Temperature affects the total solids content measure-
ment, and the temperature measurement of the Valmet
TS is used to compensate for this effect. If temperature
dependence is observed in the measurement, it can
be eliminated by the temperature compensation correc-
tion (see chapter 9).
• Antenna leak
An antenna leak causes the measurement signal to
slowly drift either up or down. If a leak is detected, re-
place the antenna and also its antenna cable, as the
cable may have got wet as well. A clear leak can be
observed by removing the antenna cover and checking
if water drips between the antenna and the connected
plugged to it.
• Antenna contamination
Antenna contamination causes the measurement result
to drift slowly upwards. If the antennas are periodically
cleaned, for example during a process wash, this error
will disappear. The antennas are made of polished
ceramic, and contamination may be caused by some
substance that adheres to the surface. Checking the
antennas for contamination requires a process stop,
as the device must be removed from the process.
• Sensor electronics failure
A typical electronics failure shows up as a sudden level
change in the measurement and a quick drop in the
signal level, when some component fails. A problem in
the reference channel always means that the sensor
electronics is defective. The self-diagnostics system
has an alarm limit for the reference channel signal, and
this ensures that obvious component failures are detec-
ted.
Sensor electronics faults may even cause a small
error or drift in the measurement result, if all of the
other causes listed above can be excluded. In such a
case, all self-diagnostic measurements may be within
the allowed limits and thus the diagnostics system
cannot detect the problem.
Locating the source of the problem may require
careful analysis of the device's internal trend and history
tables and their comparison to the error results, if the
data exists (e.g. laboratory follow-up). This requires
profound knowledge of the device and is typically per-
formed by Valmet's service personnel.
• Reference channel fault
See the information above for Sensor electronics fail-
ures.
Installation & Owner's manual, OUL00489 V2.3 EN
34
Valmet Total Solids Transmitter
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