Method of flow measurement
Flow measurement summarizes all measurement techniques used to measure the flow of gases or liquids. For the actual measurement, a so-called flow meter is used. There are many different methods of flow measurement - it must be decided for each application, which methodology is best. The most common methods measure the mass flow, the flow velocity, the volume flow or the differential pressure.
Depending on the application, the measuring techniques differ considerably. In addition to magnetic inductive methods, ultrasound, vane, oval and turbine meters, orifice plates or rotary piston meters, there are many other techniques that are used in flow measurement.
While classical impeller anemometers are sufficient for many measurements, vortex flowmeters must be used for highly flammable gases. Ultrasound is often used for flow measurement in mobile gauges, which means that no direct contact with the flowing matter is necessary. Negative, however, is the relative inaccuracy compared to other methods, which is why ultrasonic flowmeters are not suitable for all applications.
In the flow measurement of conventional air, the amount of air flowing through the measuring element is measured. As a rule, a distinction is made between the mass flow method and the volume flow method. Both methods are explained below.
Introduction to mass flow measurement
Imagine a cylinder of 1 liter capacity closed with a moving (weightless) piston. In it is 1 liter of air under ambient pressure, around 1 bar. That's 1.293 g, that's the mass.
If we now move the piston in the direction of the bottom to the middle of the cylinder, the trapped air only takes up the volume of ½ liter, the pressure has risen to 2 bar, but the mass is still 1.293 g. There is nothing added or gone. Mass flow should actually be logically measured in units such as g/h, mg/s or the like. However, most users think and work in volume units when measuring flow.
No problem, but you had to agree on standard conditions, under which mass is converted into volume. It was agreed to 0°C and 1.013 bar. The marking is made by the subscript letter n at the relevant volume unit.
Immediate mass flow measurement always converts to these standard conditions, unless you wish otherwise. So there are e.g. standard conditions based on 20 ° C instead of 0°C. If the difference is ignored, the result is an error of around 7%!
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Mass flow method
To measure the mass flow, one can use different methods. The measurement method we use works on the thermodynamic principle.
The working principle is based on the linear relationship between the flow rate and the energy required to keep the temperature constant along a flow through the measuring tube. As a sensor, a thermal mass flow meter with high accuracy and appropriate sensitivity is used.
The flow divider ensures proportional flow distribution, even under changing process conditions. Together with the temperature-stable sensor, the system offers a true mass flow measurement.
Unlike volumetric flowmeters, thermal mass flowmeters do not require additional temperature and pressure sensors to properly measure mass flow. Therefore, their installation is very simple and the possibility of automation is given in this measurement method.
Mass flow: the most accurate flow measurement
Other methods of flow measurement measure flow velocity, volume flow or differential pressure. However, they are only indirectly - namely via correction for pressure and temperature - able to make a statement about the mass flow. The immediate measurement of mass flow is therefore generally more accurate.
Volume flow method
The volume flow method allows for a wide measuring range with accurate measurement results and good reliability. The method measures the volume flow of air flowing through a defined area in a given time.
The principle is based on the basic elements of a leak tester according to the differential pressure method: The essential part of these measuring elements is a number of parallel flow channels, which the test air can flow through, in a certain speed range, absolutely free of turbulences. The pressure drop occurring when flowing through is measured at the beginning and at the end of the flow channels by means of pressure sensors. Crucial is the fact that in this laminar flow, the resulting pressure drop is proportional to the flow rate. This ratio makes it possible to view the differential pressure directly as the size of the leak rate.
The automation of the volume flow method is also given.
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