8.2
Level measurement with reference probe
Fig. 8.4 shows two arrangements for level measurement with a reference probe. Under
defined conditions, a continuous level measurement on channel 1 can be compensated
for the effects of changing products or electrical properties by using a reference probe
at channel 2.
Requirements
Prerequisites for the product to ensure to correct function are:
•
The product is homogenous throughout
•
No build-up, no crystallization.
The measuring and reference probes can be built into the tank as shown in Fig. 8.4. The
active part of the reference probe must always be below the minimum level which can
be measured by the measuring probe. Thus any change in capacitance can be attributed
to a change in product properties rather than a change in level. This places strict
requirements on the probe design:
•
Probe material, rod diameter, type and thickness of insulation must be
identical.
•
Both probes should be fitted with a perforated ground tube or grid which is
exactly the same length as the insulated rod.
•
In the case of vertical installation, a reference probe must be chosen with an
inactive length screening equal to (or slightly longer than) the length of the
measuring probe.
•
The same electronic insert and insert range must be used in both probes.
Minimum level
To ensure that the calculated correction factor is correct, the reference probe must always
be covered with product. For vertically installed reference probes in particular, the
Silometer provides an additional safeguard in the minimum level entry at V8H3. When
the level drops below the value entered, the reference function is temporarily disabled
until the value is exceeded again. In the meantime, the system measures with the last
correction factor.
①
Measuring probe
a = active length
Out put 1
level
Output 2
level
②
Reference probe
Relays, default:
Relay 1 = level
Relay 2 = reference
BA064E44
Channel 2
Channel 1
Channel 2
Channel 1
Fig. 8.4:
Level measurement with
reference probe.
Top: Reference probe installed
laterally below the measuring
probe
Bottom: Reference probe
installed vertically with active
length below the end of the
measuring probe
Silometer FMC 672 Z/677 Z
Chapter 8: Other Applications
44
Содержание Silometer FMC 672 Z
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