Variable area flow meter
A variable area flow meter (also known as a rotameter ) is a measuring device for determining the volume flow of fluids , ie liquids or gases , in pipelines.
A variable area flow meter consists of a conical tube through which the fluid flows from bottom to top. In this cone there is a vertically movable float. This has a flow resistance in the flowing fluid , that is, a force acts on it in the direction of the flow, which is dependent on the flow velocity, the float shape and the properties of the medium ( density , viscosity ). At the same time, the float experiences a static buoyancy that counteracts the weight .
The height of the float in the float flow meter depends on the mass flow. For fluids and gases with known densities, the dependency on the volume flow is reduced. The flow resistance increases as the volume flow increases. The float rises and the area between the float and the glass tube increases. As a result, the float sinks again until the sum of the forces acting on it equals zero, the following applies: = + . The height as a measure of the flow can be read off a scale attached to the device.
Designs
Variable area flowmeters are offered in a variety of designs. While simple plastic or glass cones with balls were originally used, devices with metal cones are also widespread today.
Basic structure
In its simplest version, a variable area flow meter consists of the following components: float, measuring cone, flow scale, which are fastened in the pipeline with connection fittings, flanges or screw connections and sealed with sealing elements. The float movement is limited by stops and the measuring tube is surrounded by a protective housing.
Glass measuring tube
With glass measuring tubes, the scale indicating the flow rate is attached directly to the measuring tube. The measuring point of the calibration is usually indicated below the scale on the float symbol with a line (e.g. sphere with line above, in the middle or below). There is also the unit of the measured flow rate.
All-metal design
The float position in the all-metal flow meter is transmitted to a mechanical or electrical display via a magnet system. Appropriate transmitters allow the transmission of measured values.
The height position of the float as a measure of the current flow is then displayed by means of transmission systems outside the measuring tube. With the help of transducers with an analog, electrical output signal (4–20 mA), the values can also be transmitted remotely for flow control purposes or for display and registration using bus protocols such as HART or Profibus . Limit values can also be signaled.
Interpretation and conversion
Variable area flowmeters generally have a measuring range width of 1:10. The accuracy for variable area flowmeters is defined according to the VDI / VDE guideline 3513 Bl. 2 by different accuracy classes. Each error range is assigned to an accuracy class. The guideline also describes the calculation method for floats. The procedure allows the conversion for the design of a device or the adaptation to new or changed operating data.
The leading manufacturers of variable area flowmeters offer software for conversion.
Applications
The simple and functional design enables use in a wide variety of industries, such as: apparatus engineering, laboratory applications, water treatment systems, monitoring of cooling circuits, chemistry and industrial furnace construction.
Advantages :
- Variable area flowmeters are relatively easy and inexpensive to manufacture.
- A variable area flow meter does not require an external power supply.
- Local display without auxiliary power
- Independent of electrical conductivity
- Measurement of the smallest gas and liquid volume flows
- High repeat accuracy
Cons :
- Variable area flowmeters must be calibrated for the corresponding fluid.
- The scale must be converted according to VDE / VDI 3513 for deviating measured material data.
- Only for vertical installation with a flow from bottom to top, which is now regulated with a spring resistor.
history
The term variable area flow meter was coined in 1908 by the market launch of defined rotating floats and by the industrial production of the Rota company from Wehr (Baden) , located in the triangle (CH / D / F). Karl Kueppers from Aachen is named as the inventor for the German patent number 215225 from 1908, which relates to a gas meter with a float. Experts still speak of so-called rotameters or rotameters when variable area flowmeters are required. "Rotameter" is a registered trademark of Rota Yokogawa GmbH & Co. KG.