Endress+Hauser silometer FMC 671 Z - Level Measurement with Automatic Calibration Correction

8.4 Level measurement with automatic calibration correction
Operating mode 5 employs the limit switch on channel 2 to check the validity of the
calibration on channel 1. If a descrepancy is found, e.g. because of a change in
conductivity or dielectric constant for capacitance probes or in density for Deltapilot S,
the level displayed at V0H0 is corrected. If a Deltapilot S is being used, only a Liquiphant
can be used as limit switch.
The calibration at channel 1 is corrected every time the product passes the switching
point. A plausibility check prevents
• if the measured level is less than the installation height of the level limit probe
when this is covered
• if the measured level is greater than the installation height (V8H3) of the level
limit probe when this is uncovered.
Installation hintsInstall the switching probe where:
• it is frequently covered and uncovered
• as near as possible to 100% (ensures greatest accuracy)
- we recommend a height between 70…90%.
If, for level measurement with capacitance probes, the product is frequently changed,
the switching probe must be installed in the side of the vessel or a Liquiphant must be
used. For this arrangement, the properties may not change during a filling operation,
since there is no means of continuous compensation.
• It is imperative that the limit switch trips even with the smallest dielectric
constants (capacitance probe) and across the entire density range (vibration
sensor).
• To safely avoid overfilling, it may be necessary to install in separate overspill
protection system.
• This operating mode is not recommended for bulk solids.
• When using a capacitive limit level probe, where possible the calibration of
the limit switch should only be carried out with the probe free.
level
H2
H1
current
voltage
cap./press.Frequenz
capacit./
pressure
sens. [Hz/pF]offset [Hz]
electronic
insert
electronic
insert
V2
V1
level
analog
digital
range upper limit
V2
range lower limit
V1
volumelevel 1
frequency
set switching
level (units as V0H9)
K= -----------
frequency
2
1
offset
[pF]
sens.
[pF/cm]
k is recalculated every time
the level limit probe switches
and is stored at V8H7
correction factor
volume
measured
level
level
zero point shift
installed
BA063E46
Fig. 8.6:
Schematic diagram of
automatic calibration correction
Silometer FMC 671 Z/676 Z Chapter 8: Other Applications
47