GRX-Q - Flowed bodies

This method is used to measure the flow of liquids and gases. After the differential pressure method, the flow meter is one of the most frequently used methods and is characterized by a good price-performance ratio. It is easy to install and is very robust.

The flow measurement is realized with the aid of a resistance body in a conical measuring tube or a conical resistance body in an orifice plate. The measuring principle is based on a fluid flowing vertically from bottom to top around the resistance body. Under constant conditions, an equilibrium of forces is created between the fluid and the flow around the resistor body (see Fig. 2-12 - free-cut system).

The change in the flow velocity of the fluid results in a change in height (lift height h) of the float, which is triggered by the dependence of the force exerted by the fluid. The relationship between the force and the flow velocity is based on the law of resistance.

The weight and buoyancy of the floating body remain constant. The drag coefficient changes due to the conical tube with the height of the body and the Re number. The resulting equilibrium equation can be described as follows:

from which the equilibrium condition is:

In a measurement setup, the variables VK, ρK, ρM, g and AK are constant. This means that a change in the flow velocity vM only changes the drag coefficient cW. Thus, the relationship between the lift height h and the flow velocity vM is included in the drag coefficient cW. This means that the drag coefficient is a function of h and the Re number.

This means that it is not possible to calculate the drag coefficient. The values must be determined empirically by calibrating the measuring arrangement. This determination can be carried out with the help of VDI/VDE 3513. The guideline helps the user to create a flow scale in conjunction with the characteristic values of the fluid and the measuring device. The guideline can also be used to transfer an existing scale for the flow to a different medium.

The variable area flowmeters can be used with almost any gaseous or liquid medium. These measuring devices, which have a glass cone with the scale printed on it for direct reading, are very widely used. For opaque gases or liquids, variable area flowmeters made entirely of metal are suitable.

This measuring method is insensitive to dirt and contamination, provided that the particle size of the contamination does not exceed the width of the annular gap between the float and the conical measuring tube. Variable area flowmeters do not require a straight inlet section and can be used directly downstream of pipe bends or valves. The pressure losses of the devices are low and can be kept at a very low level with appropriate dimensioning. The accuracy of the sensors is specified as 2%, but it can be reduced to as little as 1% after extensive calibration.

Spring disc flow meters work on a similar principle to that of the rotameter. Instead of the weight of the resistance body, the counterforce of a measuring spring creates a balance with the flowing fluid. [BON-02]

The measuring spring is rigidly connected on one side to the inlet of the conical measuring tube and on the other side to a freely movable spring disc (see Fig. 2-13). The spring is dimensioned so that even at maximum deflection, it remains well below the elastic limit. This ensures a long service life for the spring. The measuring spring with the spring washer are concentrically guided on a rod inside the conical measuring tube, on which they can be moved almost without friction.

The spring is deflected by the change in the flow velocity of the surrounding fluid until a state of equilibrium is restored. This means that the position of the spring disc is the measure for the respective flow.

The volume flow q_v is calculated from the mean flow velocity v_M and the cross-section of the annular gap A_R

Using equations (Eq. 2-15) and (Eq. 2-16)

the volume flow q_v is obtained after equating and solving.