Inlet and outlet sections are relevant factors in any flow measurement, including the magnetic-inductive measurement method. Here, the focus is primarily on 0 D, meaning no inlet and outlet section. What this means, which factors influence the measurement, and what Siemens offers in this regard are explained in the following three chapters.
- What influences the accuracy of a measurement?
- What does Siemens offer?
- What does 0 D mean in practice?
What influences the accuracy of a measurement
To understand what can influence the accuracy of a magnetic-inductive flow measurement, it is important to understand how this measurement principle works. As shown in the figure, a magnetic field is generated within the measuring tube. When an electrical conductor, such as water, is moved perpendicular to the flow direction of the field, a voltage is induced at the ends of the conductor. Thus, the electrode signal is directly proportional to the velocity of the liquid. Based on the flow velocity and the cross-sectional area of the pipe, the volumetric flow rate can then be determined.It is especially important that the medium to be measured flows perpendicularly through the magnetic field. If this is not the case, measurement inaccuracies can occur. The following installations can disturb the flow profile and thus cause measurement inaccuracies:
- 90° elbows, pipe bends
- Branch pieces
- Simple 90° T-pieces
- Diffusers
- Nozzles/reducers
- Slides, shut-off valves
- Control valves
- Thermometer wells
What does Siemens offer?
Like its competitors, Siemens tries to positively influence the flow profile through the design of the measuring tube. The goal is always to balance pressure drop and flow performance. With the SITRANS MAG5100W and MAG8000, the measuring tube is slightly conical on both sides. Due to a maximum angle of 7.5 degrees, the resulting pressure loss is equivalent to three meters of pipeline and is thus very low.The advantages gained through this design are certified by the FORCE certification body according to OIMLR49-2. A clear procedure for testing and evaluation using the following three profile elements is defined. These represent the most demanding flow profile conditions:
- Vortex generator Dextrosal
- Vortex generator Sinistrorsal
- Flow disruptor
The following results are confirmed:
What does 0 D mean in practice?
As shown in the results above, an inlet section of 0 D is achievable with corresponding accuracies. In general, if one wants to fully utilize the metrological potential regarding the accuracy of a measurement, inlet and outlet sections must be considered. Siemens states the following:Inlet and outlet sections play a significant role, especially with larger nominal diameters. For smaller nominal diameters, the inlet and outlet section is often already integrated into the construction length.
Example: MAG5100W DN25:
Inlet section: 5 × 25 = 125
Outlet section: 3 × 25 = 75
Total inlet and outlet section: 200
Construction length: 200
Furthermore, a distinction must be made between "normal" and type-approved MIDs according to MI-001. The MAG5100W with MAG6000 CT has approval according to Directive 2014/32/EU, Annex III Water Measuring Instruments (MI-001) in nominal diameters DN50 to DN1200 with a Class II approval. The following tolerances are allowed:
This allows calibrated MIDs, in this case the MAG5100W sensor in combination with the MAG6000CT transmitter, to be operated with a 0D inlet and outlet section.