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    A Practical Overview of Level Continuous Measurement Technologies

    Introduction

    There are multiple technologies available on the market to measure level. Each and every technology works, when applied appropriately. This paper will discuss the strengths and weaknesses of RF Admittance, Displacers / Floats, Capacitance, Ultrasonic, Radar, Nuclear, Differential Pressure, and Bubbler level measurement technologies. Level measurement for liquids, granulars, slurries and interfaces can be accomplished with several different level technologies. There are over 20 different technologies being offered on the market today. But how do you know which technology to specify for your application? Obviously all technologies have (or had) their place, otherwise companies would not be in business today offering them. The truth is that every technology for level measurement works, when used in the specific circumstances where they have a high probability of success. This paper will discuss the most popular level measurement technologies used in industry, outline their advantages and disadvantages, and tell you where they are best applied. The Application Box Rating Included with the description of each technology is an application box rating for the four categories of level

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  • measurement (Liquids, Granulars, Slurries and Interfaces). This gives you an at-a-glance overview of where the technology in question is best used .

    LiquidsA green box indicates that this technology is suitable for this category of level measurement.

    Granular

    A red box indicates that this technology is not practical for this category of level measurement. This does not mean that people have not used this technology for measurement, but just that it is not practical. This could mean that there is excessive maintenance, excessive error, unreliable outputs, or it just simply doesnt work in this application based on the characteristics of the level measurement.

    Solids

    A yellow box indicates that this technology can sometimes be applied to this application. But be careful. There are usually special considerations or additional maintenance needed.

    Example: The Application Box Rating below indicates that the technology in question works on liquids. It needs special considerations or additional maintenance when used on slurries, and it is not practical for granulars or interfaces.

    Liquids Granulars Slurries InterfacesEXAMPLE ONLY

    Differential Pressure

    Liquids Granulars Slurries Interfaces

    Theory

    Perhaps the most frequently used device for the measurement of level is a differential pressure transmitter. Using DP for level is really an inferential measurement. A DP is used to transmit the head pressure that the diaphragm senses due to the height of the material in the vessel multiplied by a density variable.

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  • Advantages

    The primary benefit of DPs is that it can be externally installed or retrofitted to an existing vessel. It can also be isolated safely from the process using block valves for maintenance and testing. There are certain measurements such as total level in separator vessels that due to wide variations in material composition of the upper phase DP is the only viable if not ideal option.

    Disadvantages

    D/P transmitters are subject to errors due to changes in liquid density. Density variations are caused by temperature changes or change of product. These variations must always be compensated for if accurate measurements are to be made. DPs are mainly intended for clean liquids and require two vessel penetrations. One is near the bottom of the vessel where leak paths are the cause of the majority of problems. D/Ps should not be used with liquids that solidify as their concentrations increase. An example is paper pulp stock.

    Practical

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