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...

Absolute Pressure

Absolute pressure is defined as the pressure measured with a pure vacuum as its reference point. That is zero gauge pressure (see gauge pressure defined below) will be approximately 1bar/14.7psi absolute.

Whilst not used often engineers and technicians must be aware that pressures and transmitters may sometimes be defined/specified in these units. It is a real trap that one can fall into assuming that the pressure is "gauge" or absolute when in fact it is the reverse. A...

Differential Pressure

Differential pressure is defined as the difference between two pressures.

Measurement commonly used throughout industry, especially in conjunction with flow metering devices such as Orifice Plates, Venturies, Pitot tubes etc.

Equal Column Liquid Manometer

This is one of the oldest methods of measuring differential pressure. In its most basic configuration it consists of two identical vertical tubes partly filled with a liquid. These tubes are connected together at the bottom. Pressure is measured by determining the change in height between the two tubes and is scaled according to the density of the liquid. Thus different fluids will give a change in the pressure range.

Typical fluids used are water and...

Flow Instrumentation and Measurement

Ultrasonic Flow Metering

A full page of Ultrasonic Flowmetering technical links - Thanks to Zedflo Australia.

V-Cone and Wet Gas Flow Metering

This is a swag of information on this innovative technology - Thanks to McCrometer.

Subsea Flow Measurement - Marcus Davis - Flow measurement in subsea production systems, modules and templates is a challenge for flow meters. The complexity of subsea production systems ranges from simple satellite wells with...

Gauge Pressure

Gauge pressure is defined as the pressure measured with local atmospheric pressure as its reference point. That is Pressure gauge equals Pressure absolute minus Atmospheric pressure.

Gauge pressure measurement is commonly used in Industry, if the data-sheet does not define "psia/MPaa" or "psig/MPag" then it usually pretty safe to assume that the measurement is being made in gauge.

Inclined Limb Manometer

This is another variation where the construction is based on a glass reservoir connected via its base to the glass tube which is set at an angle rising to the maximum level which can be in the cistern. Again it is basically a differential pressure measurement instrument.

Whilst use is diminishing today, it is still used in low pressure applications, usually about 20 inches wg maximum (4.981Kpa)

Typical example of Inclined Limb Manometer

Manometers come in many...

Level Measurement and Instruments

The Measurement of Level involves a Comprehensive Knowledge of many Level Measurement Techniques and the Associated Level Instrumentation. This page which is just full of free Information on Level Instrumentation is a Fantastic Technical Resource.

Go to Specific Subject: General Level Measurement Theory | Level Measurement Standards | Level Measurement Applications | Level Measurement by Industry | Accoustic Volume Measurement | Capacitance Level...

This email address is being protected from spambots. You need JavaScript enabled to view it.Level measurements info notes:

Basics.

Level is measured at the position of the interface between phases, where the phases are liquid/gas, solid/gas, or immiscible liquid/liquid. Level is simply a measure of height. It defines the position of the interface, that is, the surface where the two phases meet with respect to a reference point. This measurement is often converted to a volumetric or gravimetric quantity.

· Direct Level Measurement

Direct methods employ physical properties such as...

Pressure Instrumentation and Measurement

Theory

What is the Difference between Absolute, Gauge and Differential Pressure? From Instrumentation and Control Net.

The following excellent articles are from Emerson Process Management.

Pressure Best Practices -  Plants are continuously challenged to be more and more efficient - deliver more with less.  How can you reduce initial capital costs...

Profile Type Level Measurement

This type of level measurement profiles the level in a vessel measuring the various mediums, eg., sand, water, oil in a oil and gas production separator. The technique is very accurate and utilises principals such as nucleonics and capacitance.  The accuracy is however reflected in the cost in that the units are expensive,  however when the business case is examined on these devices they have the potential to make Oil and Gas facilities more efficient.  This is...

Risks Associated with the use of Mercury

Mercury is a poison hazard and thus contact has to be avoided. In the "old days" where Mercury manometers were used extensively special "mercury bays" were utilised to mitigate the hazard. Contact with Mercury can cause brain damage. I suppose you have heard the term "As mad as a hatter", well this was the result of the hatters utilising mercury to put the sheen on top hats. Mind you I have experienced a few mad Instrument Technicians who worked in...

Static Pressure

Static pressure is defined as the pressure of a fluid that is independent of kinetic energy . Pressure exerted by a gas at rest, or pressure measured when the relative velocity between a moving stream and a pressure measuring device is zero.

Sounds confusing? What it means is when the process fluid is "static" the pressure measured at that point is the static pressure. When fluid is in motion however the meaning is less clear. Lets look at an example of a vessel in which the...

Vacuum Pressure

Vacuum Pressure is defined as the pressure measured with local atmospheric pressure as its reference less the amount of vacuum being drawn.

Selecting Instruments for Vacuum Service - David W. Spitzer - I recently read an Internet posting by a well-respected instrumentation engineer about selecting instruments for vacuum service. His comments are informative in that he mentions that instruments can be damaged when operated under vacuum. Vessels can also be damaged. The plant...