Rotameter

A variable-area flow meter is a measuring instrument for determining the volume flow of fluids, ie, liquids or gases in pipes.

A variable-area flowmeter consists of a conical tube that is passed through from the bottom up by the fluid. In this cone, there is a movable float. This has a flow resistance in the flowing fluid, that is, it affects him a force in the direction of the flow, which depends on the flow rate, the floating body shape and the fluid properties (density, viscosity) is. At the same time, the floating body experiences a buoyancy which acts against the weight force.

The height of the float in the float flowmeter is dependent on the mass flow. For fluids and gases with known densities reduces dependence on the flow rate. With increasing flow rate, the flow resistance increases. The float rises and the area between the float and the glass tube is larger. Thus, the float drops again until the sum of the forces acting on it is zero, we have: = . The level position as a measure of the flow rate can be read from a scale attached to the equipment.

Designs

Variable area flowmeters are offered in a variety of designs. While originally simple plastic or glass cones came with balls used today, devices with metal cones are widely used.

Basic structure

In its simplest form, there is a variable-area flowmeters from the components: float, measuring cone, Durchflussskale that with fittings: be attached fittings or flanges or screw in the pipe and sealed with sealing elements. The movement of the float is limited by stops and the measuring tube surrounded by a protective casing.

Glass measuring tube

In glass measuring tubes The flowrate range is mounted directly on the measuring tube. The measuring point of the calibration is specified usually displayed below the scale on the icon of the float with an underscore (eg ball with line above, center and below ). There is also the unit of the measured flow rate.

All-metal construction

The float position in all-metal flow meter is transmitted by a magnetic system, a mechanical or electrical indicator. Corresponding transmitters allow the transmission of measurement values.

The height of the float as a measure of the instantaneous flow rate is then displayed by means of transmission systems outside the measuring tube. With the help of transmitters with analog electrical output signal ( 4-20 mA), the values ​​can also be used for flow control or for display and registration using bus protocols such as HART or Profibus be transferred remotely. In addition, limits can be signaled.

Interpretation and translation

Variable area flow meters generally have a wide range of 1:10. The accuracy for variable-area flowmeter is 3513 Bl 2 defined according to VDI / VDE Directive by different accuracy classes. Each fault zone has an accuracy class. The policy also describes the calculation method for the float. The method allows the conversion for the design of a device or adaptation to new or modified operating data.

The leading manufacturers of variable-area flowmeters offer software for the conversion.

Applications

The simple and functional design enables the use in various industries, such as: Apparatus, laboratory applications, water treatment plants, monitoring of cooling circuits, chemical and industrial furnaces.

Advantages:

• Variable area flowmeters are relatively easy and cheap to make.
• A variable-area flowmeter requires no external power supply.
• Local display without auxiliary power
• Independently of electrical conductivity
• Measurement of very small flow rates of gas and liquid
• High repeatability

Cons:

• Variable area flowmeters shall be calibrated for the corresponding fluid.
• For other Messtoffdaten the scale according to VDE / VDI 3513 must be converted.
• Only for vertical installation with flow from bottom to top, which is, however, now controlled by a spring pressure.

History

The concept of variable-area flowmeter was built in 1908 by the launch of defined rotating floats and by the industrial production of the company Rota from Wehr (Baden), located in the border triangle (CH / D / F), embossed. For the German patent No. 215225 of 1908, which relates to a gas meter with a floater, is named as inventor Karl Kueppers in Aachen. Today, still speaks the professional world of so-called rotameters or rotameters, if variable-area flowmeters are required.