KRAL OMG Series User Manual

5 Function description
5.4 Signal generation
5.4 Signal generation
A pole wheel10 whose pole is sampled by a pick up is affixed at the fixed bearing end of the measur-
ing screw large7. This pick up generates a specific number of pulses per flow volume unit – depending
on the size and working point. This device-specific characteristic is called the K-factor (unit: Pulse/litre)
and is specified on the rating plate as well as the enclosed calibration certificate.
Possible formats of the signals are:
o PNP
o Push-pull
o NAMUR
Depending on the version of the pick up, cylindrical pole wheels with embedded magnets or toothed
metal disks with different tooth densities can be used (higher pole number = higher K-factor). Depend-
ing on the application (operating temperature, required frequencies) pick ups of different types can be
used:
o Inductive pick ups
o Pick ups based on the Hall effect
o Magnetic pick ups
A preferred flow direction for which the pick up is located on the downstream side is specified on the
rating plate of the flowmeter. This preferred direction results from the goal to position the pick up in
case of a blockage of the flowmeter on the low-pressure side.
5.5 Linearization
The calibration certificate of the flowmeter contains a mean K-factor that has been determined for the
flow range 10:1 and that can therefore be used across a wide flow range. However, the K-factor shows
slightly different values at different flow rates. These are also documented in the enclosed calibration
certificate. If highest measuring precision is required, it is therefore advisable, especially at strongly
varying flow rates, to take these different values into consideration by means of a "Linearization". The
K-factors are therefore stored in a suitable electronic unit by means of several interpolation values of
the flow rate. The K-factor relevant for the flow rate being measured is then determined by means of
linear interpolation between the two nearest interpolation values.
The viscosity dependence of the K-factors must also be taken into account. These are determined dur-
ing calibration at a viscosity of approximately 4.2 mm²/s. The influence of the flow rate on the K-factor
decreases at higher viscosities so that the mean K-factor can then also be used in a considerably lar-
ger flow range without noteworthy errors.
5.6 Temperature compensation
Flowmeters of the OMG series can be equipped additionally with temperature sensors for sizes
OMG-013 to OMG-100. For size OMG-140 the installation of a temperature sensor is recommended. If
the flowmeter is equipped with a temperature sensor, the current density of the flowing liquid can be
calculated from the measured values by using a density table stored in the electronic unit. This allows
standardised volume measurement in which the displayed values are converted to a reference temper-
ature that can be selected freely. This ensures that measuring errors caused by changes in the density
due to temperature variations are avoided.
5.7 Recognition of flow direction
Systems with a changing flow direction require the usage of a second pick up to recognize the change
in direction. In the case of systems with pressure pulsation a reversal of the flow direction can also oc-
cur briefly.
The flow direction can be determined by means of the additional phase-shifted signal and the incre-
mental encoder positions available in the KRALelectronic unit and then taken into account for the cal-
culation of the total values.
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OIO 14en-GB Edition 2020-01
Operating instructions

KRAL OMG Series Specifications

General

Brand	KRAL
Model	OMG Series
Category	Measuring Instruments
Language	English