22 OIO 14en Edition 07/2017 Operating Instructions
Roller bearings
Function description
Roller bearings
The measuring unit is maintained without contact and with a low degree of friction in the housing of the
KRAL flowmeter by means of precision roller bearings. The bearing end at the pole wheel end is
realized as a fixed bearing. Depending on the respective size, the following bearings are used:
□ Single-row deep-groove ball bearings
□ Paired angular-contact ball bearings
□ Four-point contact bearings
The bearing point at the opposite end of the measuring unit is realized as a floating bearing with a
movable deep-groove ball bearing at all sizes.
Signal generation
A pole wheel 10, whose pole is sampled by a pick up, is affixed at the fixed-bearing end of the
measuring 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/Liter) and is specified on the rating plate as well as the enclosed calibration certificate.
Possible formats of the signals are:
□ PNP
□ Push-pull
□ NAMUR
Depending on the design of the pick up either cylindrical rotors with embedded magnets or toothed
metal disks with varying tooth density can be used (higher number of poles = higher K-factor). Various
types of pick ups can be used depending on the application (operating temperature, required
frequencies):
□ Inductive pick ups
□ Pick ups based on the Hall effect
□ Magnetic pick ups
The rating plate of the flowmeter lists a preferred direction of flow at which the pick up lies on the flow-off
end. This preferred direction is based on the aim to position the pick up of the low-pressure end for the
case that the flowmeter is blocked.
Linearization
The calibration certificate 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 fed into a suitable electronics unit across 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
during 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 larger
flow range without noteworthy errors.
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