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Capacitance Level Measurement Technology (Part 2 of 3) by David W Spitzer and Walt Boyes
Noting that the measured capacitance is dependent upon the size of the plates and the effective distance between the plates, variation in these parameters can cause measurement problems. The distance between the plates is held constant by the dimensional stability of the sensor. However, in nonconductive materials, dielectric constant variations can cause the effective distance between the plates to vary. In conductive materials, relatively low electrical conductivity of the material can reduce the effectiveness of the material side of the capacitor. This means that the level measurement can change when the electrical properties of the material change --- even when the level remains constant.
Conductive materials should maintain sufficient electrical conductivity such that the material effectively forms one plate of the capacitor. This is generally accomplished when the material exceeds a given electrical conductivity specified by the supplier. Knowing that the electrical conductivity of the material in a given application will exceed this value infers that the covered portion of the probe will form an effective capacitor plate and provide immunity to changing electrical conductivity in the application. Stated differently, measurement error caused by changing electrical conductivity is reduced when the material exceeds the electrical conductivity requirement of the sensor. Achieving this objective is practical in many applications involving conductive materials. Note that different sensors have different electrical conductivity requirement such that sensor selection can influence level measurement error.
For nonconductive materials, dielectric constant variations of the material cause the effective distance of the capacitor (and hence its capacitance) to vary. Therefore, measurement error caused by changing dielectric constant will be reduced when the material dielectric constant does not vary. Achieving this objective may not be practical in some applications because temperature and composition variations affect the dielectric constant of the material and result in significant level measurement error.
Excerpted from The Consumer Guide to Capacitance and Radar Level Gauges
Flowmeter Installation Considerations: Understanding Factors That May Affect Flowmeter Accuracyby David W Spitzer
Flowmeters are geometric devices. By this I mean that flowmeter operation is dependent upon the geometry of the installation. Measurement error can occur by relaxing or ignoring virtually any constraint of any aspect of the flowmeter geometry.
What if the flowmeter is installed backwards? The flowmeter may or may not work. If the flowmeter works, the measurement it provides is suspect and will likely not be accurate because most flowmeters present a different geometry to the flow stream when installed backwards. Bidirectional flowmeters such as certain turbine and magnetic flowmeters are exceptions in that they are designed to measure forward and reverse flow. However, it is usually recommended that these flowmeters be calibrated in both directions to meet their accuracy specifications.
What if the flowmeter is located immediately downstream of a control valve?The flowmeter will likely be inaccurate unless the flowmeter is immune to velocity profile effects.Finding a control valve located upstream of a flowmeter may indicate that fundamental problems exist and suggests investigation of the entire process, application and installation.
What if the flowmeter is not located in the center of the pipe? Again the flowmeter may exhibit measurement error due to the relaxation of a geometrical constraint. For example, the differential pressure produced by a concentric orifice plate that is installed with an offset from the center of the pipe may be different than if the orifice plate were installed within acceptable tolerances of center.
The flow measurement system includes the upstream straight run, flowmeter, downstream straight run, and their appurtenances. The flowmeter itself is only one component in the flow measurement system that, if installed in a proper flow measurement system, should measure accurately.
How well have you checked your flowmeter installations?
This article originally appeared in Flow Control magazine.
Orifice Plate Installation Factors by David W Spitzer
Orifice plates are often viewed as simple devices.Which of the following should be addressed during installation?
A. Center the orifice plate in the pipe
B. Remove burrs from the weld at the orifice flange
C. Make sure that the pipe is round upstream and downstream of the orifice plate
D. Measure the inside diameter of the pipe
E. Make sure that the inside diameter of the gaskets is the same as the inside diameter of the pipe
Examination of the standards for orifice plate installation may lead one to believe that the orifice plate is more than just a simple metal plate (with a hole) that is sandwiched between two flanges. Further, the wetted part of the flow measurement system is more than just the orifice plate itself --- it includes the geometry of the upstream straight run, orifice plate assembly, and downstream straight run.
Simply put, all of the answers contain items that can affect the geometry of the flowmeter system and therefore affect the flow measurement. The extent to which each item affects the flow measurement may be different, but standards and practices either state or imply that all of these items should be addressed during installation.
Additional Complicating Factors
Often the orifice plate, flanges and piping have already been installed before you can check these items. I was recently asked to analyze the installation and operation of some 800 psig steam flowmeters that were in service for over 50 years. In this case, I was limited in my options and had to "accept" the installation noting that no access was available.
This article originally appeared in Flow Control magazine.
ABOUT SPITZER AND BOYES, LLC
In addition to over 40 years of experience as an instrument user, consultant and expert witness, David W Spitzer has written over 10 books and 500 articles about flow measurement, level measurement, instrumentation and process control. David teaches his flow measurement seminars in both English and Portuguese.
Spitzer and Boyes, LLC provides engineering, technical writing, training seminars, strategic marketing consulting and expert witness services worldwide.
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