In displacement meters, the movable measuring walls are driven by the medium to be measured. The movement is detected and counted. Due to the design of displacement meters, measurement errors occur mainly due to leakage between the fixed and the moving components of the measurement setup. The gap losses are determined by the viscosity of the medium to be measured and by the physical behaviour of the flow. These losses become larger due to wear of the material. If the meter is overloaded, the wear occurs much faster.
Because there is a small clearance between the fixed and the movable measuring chamber wall, these meters are susceptible to contamination in the medium. This makes prior filtration, to a purity level that is acceptable for the meter indispensable.
Two displacement meters are described in more detail below. These include the annular piston meter and the oval wheel meter.
Oscillating piston meter
The design of the cylindrical piston meter is based on two hollow cylinders which are connected to each other via a central web (see Fig. 2-5). These hollow cylinders form the fixed measuring chamber walls. Inside the measuring chamber a slotted annular piston is fitted, which is supported centrally by a journal and guided radially via the slot. The annular piston moves along the inner and outer measuring chamber walls of the small or large hollow cylinder as well as along the central web. Meanwhile, the journal of the annular piston performs a pure circular motion.
The inlet (E) or outlet (A) openings are located either at the front of the measuring chamber or on the opposite sides. The two chambers with volumes V1 and V2, separated by the annular piston, are filled or emptied in such a way that there is no connection between the inlet and outlet openings at any time. The motion sequence can be described in the following phases. [BON-02]
- Phase: Liquid flowing into the measuring chamber pushes the piston counterclockwise out of its rest position. As soon as the inlet opening is even partially released, the liquid additionally presses on the outer surface of the annular piston, thus supporting the original direction of movement.
- Phase: As soon as the annular piston has reached the opposite point of the measuring chamber, the liquid pressure only acts on the outer piston surface. The volume V2 reaches its maximum in this position. The volume V1 of the outer chamber space is the same on both sides (inlet and outlet).
- Phase: The ratio of the volumes V1 / V2 becomes larger and larger as the rotation continues. As a result, the liquid to be measured in volume V2 is forced out through the outlet opening A.
- Phase: The liquid to be measured has been pushed so far beyond the outlet opening that the annular piston is in its initial position again.
The measuring principle is the same for measuring processes with forward and backward flow. This means that the same measuring accuracy can be achieved in both cases with the cylindrical piston meter. This depends mainly on the manufacturing accuracy of the piston and the measuring chamber. In addition, care must be taken to separate the air entrained in the fluid so that it is not also measured.
Oval wheel meter
This flow meter uses two oval gear wheels as movable measuring chamber wall, which interlock positively and rotate in opposite directions (see Figure 2-6). The oval wheels are moved by the pressure of the inflowing liquid. During the measurement, a separation of partial quantities takes place, whose volume is determined by the shape of the outer measuring chamber walls and the oval wheels. With each full revolution, the oval wheel meter delivers a precisely determined quantity of liquid. This quantity can be recorded by a counter as a function of the number of revolutions and at the same time displayed in the usual volume units as the total volume delivered.
The oval wheel meter is one of the high-speed meters among the liquid meters with movable partitions. It can measure high flow rates in relatively small installation spaces, whereby the instantaneous delivery is almost constant.
As a result of the pressure difference between the inlet and outlet side caused by the liquid, the oval wheels are driven. In order to keep pressure losses and frictional forces as low as possible, the oval wheels only touch each other along the line of engagement of the gearing. However, there is no contact with the measuring chamber wall or the end faces. This requires high dimensional accuracy in the manufacture of the wheels in order to keep the resulting gap losses as low as possible. The achievable measuring accuracy depends mainly on the gap losses.
Even with small meters, losses can be kept low due to the small clearance between the oval wheels and the measuring chamber walls, as well as the sealing between the wheels by several teeth. The following further advantages can be mentioned:
- high measuring accuracy with wide flow range
- a high degree of viscosity independence
- low pressure drops
- high starting torques
- long life span
A disadvantage, however, is the high sensitivity to dirt and the noisy running of these flowmeters.
Due to its advantages, the oval wheel meter is not only suitable as a quantity counter for liquids, but also as a transducer and signal transmitter for control and regulation tasks.
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