Summary of Technologies Used For Continuous Liquid Level Measurement in Industrial Process Control

Non-contact radar liquid level transmitter
Courtesy Magnetrol

Automated liquid processing operations in many fields have requirements for accurate and reliable level measurement. The variety of media and application criteria demand continuous improvement in the technology, while still retaining niches for older style units utilizing methods that, through their years of reliable service, inspire confidence in operators.

Here is a synopsis of the available technologies for instruments providing continuous liquid level measurement. All are generally available in the form of transmitters with 4-20 mA output signals, and most are provided with additional outputs and communications. What is notably not covered here are level switches or level gauges that do not deliver a continuous output signal corresponding to liquid level.

Whether considering a new installation or upgrading an existing one, it can be a good exercise to review several technologies as possible candidates for a project. None of the technologies would likely be considered the best choice for all applications. Evaluating and selecting the best fit for a project can be facilitated by reaching out to a product application specialist, sharing your applications challenges and combining your process knowledge with their product expertise to develop an effective solution.

Displacer – A displacer is essentially a float and a spring that are characterized for a particular liquid and range of surface level movement. The displacer moves in response to liquid level, changing the location of a core connected to the displacer by a stem. The core is within a linear variable differential transformer. The electrical output of the transformer changes as the core moves.

Guided Wave Radar – A radar based technology that uses a waveguide extending into the liquid. The radar signal travels through the waveguide, basically a tube. The liquid surface level creates a dielectric condition that generates a reflection. Calculations and processing of the emitted and returned signals provide a measure of distance to the liquid surface. No moving parts.

Magnetostrictive – A method employing measurement of the transit time of an electric pulse along a wire extending down an enclosed tube oriented vertically in the media. A magnetic float on the exterior of the tube moves with the liquid surface. The float’s magnetic field produces the return signal to the sensor. Processing the time from emission to return provides a measure of distance to the liquid surface.

Pulse Burst Radar - A radar based technology employing emissions in precisely timed bursts. The emission is reflectex from the liquid surface and transit time from emission to return is used to determine distance to media surface.  Not adversely impacted by changes in media conductivity, density, pressure, temperature. No moving parts.

Frequency Modulated Continuous Wave Radar – Another radar based technology that employs a radar signal that sweeps linearly across a range of frequencies. Signal processing determines distance to media surface.  Not adversely impacted by changes in media conductivity, density, pressure, temperature. No moving parts.

RF Capacitance - As media rises and falls in the tank, the amount of capacitance developed between the sensing probe and the ground reference (usually the side metal sidewall) also rises and falls. This change in capacitance is converted into a proportional 4-20 mA output signal. Requires contact between the media and the sensor, as well as a good ground reference. No moving parts.

Ultrasonic Non-Contact – Ultrasonic emission from above the liquid is reflected off the surface. The transit time between emission and return are used to calculate the distance to the liquid surface. No contact with media and no moving parts.

Differential Pressure – Pressure sensor at the bottom of a vessel measures the pressure developed by the height of the liquid in the tank. No moving parts. A variation of this method is often called a bubbler, which essentially measures hydrostatic pressure exerted on  the gas in a tube extending into the contained liquid. It has the advantage of avoiding contact between the measuring instrument parts, with the exception of the dip tube, and the subject liquid.

Laser - Probably one of the latest arrivals on the liquid level measurement scene, laser emission and return detection is used with time interval measuring to accurately determine the distance from the sensor source to the liquid surface.

Load Cell - A load cell or strain gauge can be incorporated into the support structure of the liquid containing vessel. Changes in the liquid level in the vessel are detected as distortions to the structure and converted, using tank geometry and specific gravity of the liquid.

All of these technologies have their own set of attributes which may make them more suitable to a particular range of applications. Consulting with a product specialist will help determine which technologies are the best fit for your application.

Diaphragm Pressure Gauges for Industrial Process Measurement

Example of a diaphragm pressure gauge
Courtesy Wika

Diaphragm pressure gauges, like every device and instrument intended for use in industrial process measurement and control, have their own set of attributes making them an advantageous choice for some range of applications. Silvia Weber, product manager at Wika, a globally recognized leader in the field of pressure and temperature gauges, wrote an article for Process Worldwide (process-worldwide.com/) about diaphragm pressure gauges.

The article is included below and provides a comparison of the differences between Bourdon tube and diaphragm operating mechanisms, focusing on design and operational features of diaphragm pressure gauges and the range of application criteria for which they may be the best choice.

Pressure gauges are utilized in most operations where fluids are moved through a system. Gauges, though mechanical in operation, remain a mainstay of fluid operations because of their reliability, local display, ruggedness, and lack of reliance on electric power for operation. There are countless pressure gauge configurations to suit every application. Specifying the best gauge configuration for an application is accomplished by combining your process knowledge with the application expertise of a product specialist.

Applying Diaphragm pressure gauges in industrial settings from Miller Energy, Inc.

Protect Valuable Pressure Gauges and Transmitters With a Pressure Limiting Valve

Pressure limiting valve provides gauge
or transmitter protection from spikes
Courtesy Mid-West Instruments

Pressure gauges and transmitters, commonly found in fluid process control operations, are vulnerable to damage from transient spikes in system pressure that may range beyond the instrument's working range. These pressure spikes can impact instrument calibration, or even render the instrument or gauge inoperative. The cost of replacing gauges or transmitters is substantial enough to warrant the use of protective devices to prevent exposure to pressure spikes.

Mid-West Instruments manufactures a line of pressure limiting valves specifically intended for use with pressure gauges and transmitters. The Model 200 pressure limiting valve prevents instrument over-range and has an adjustable needle valve to dampen pulsation. The valve and be used with all types of instruments and pressure gauges, is suitable for mounting in any position, and is available in a range of materials for body and seals.

The document below provides more product detail, as well as installation and setup instructions. Providing a useful measure of protection for pressure gauges and transmitters is a simple operation. Reach out to product application specialists for help in formulating effective solutions.

Pressure limiting valves for transmitter and gauge protection from Miller Energy, Inc.

Industrial Process Gauges - New Product Guide

One of the many pressure gauge versions
employed throughout industry
Courtesy Ametek - U.S. Gauge

Even with the large growth in the use of electronic measurement instruments throughout the process control sphere, mechanical gauges and indicators remain an important part of process measurement and control operations.

A broad line of industrial gauges and diaphragm seals is available from U.S. Gauge. The company has consolidated its offering into a product guide that provides simple and quick reference to the various product series.

For pressure:

• Process Gauges
• Liquid Filled Gauges
• Test Gauges
• General Equipment Gauges
• Special Application Gauges

For temperature:

• Adjustable Bimetallic Thermometers
• Thermowells
• Industrial Bimetallic Thermometers
• Multi-Angle Industrial Thermometers
• Digital Thermometers
• Glass Tube Thermometers

The product guide also includes diaphragm seals and a range of electronic indicators, as well.

The guide illustrates gauges for every industrial application. Share your process measurement and control challenges with product application specialists, combining your process knowledge with their product application expertise to develop effective solutions.

Product guide for industrial gauges from Ametek U.S. Gauge from Miller Energy, Inc.

Basic Guide to Understanding Pressure

One style of absolute pressure transmitter
Courtesy Yokogawa

The impact of pressure on industrial processes would be difficult to understate. Pressure is an element of process control that can affect performance and safety. Understanding pressure concepts and how to effectively measure pressure within a process are key to any operator's success.

Yokogawa, a globally recognized leader in process measurement and control, has made available a handbook on pressure that covers a range of useful topics. The content starts with the very basic concepts and moves quickly to practical subjects related to process measurement and control.

The handbook will prove useful to readers at all levels of expertise. Share your process measurement challenges with application specialists, combining your process knowledge with their product application expertise to develop effective solutions.

Pressure Handbook for Industrial Process Measurement and Control from Miller Energy, Inc.

V-Cone® Flow Meter Conditions Flow For Accurate Measurement

Accurate measurement of fluid flow is a process requirement in many industrial operations. There are numerous methods employed in the measurement of fluid flow, of which the McCrometer V-Cone® is one entry with very particular advantages. Whether the application involves liquid, steam, or gas, this flow meter design, with its own flow conditioning built in, provides exceptional differential pressure flow measurements in a space saving format.

The video provides a clear illustration of how the V-Cone® design conditions fluid flow in order to provide better DP measurement performance. Share your flow measurement challenges with application specialists, combining your process expertise with their depth of product application knowledge to develop effective solutions.

Electronic Displacer Liquid Level Transmitter - How it Works, When to Use It

Electronic displacer liquid
level transmitter using spring
technology
Courtesy Magnetrol

An electronic displacer liquid level transmitter is intended for industrial applications requiring the continuous measurement of liquid level in a tank, vessel, or other containing space.

Magnetrol, a globally recognized leader in the design and production of level measurement instrumentation, describes the operating principle of their Digital E3 Modulevel® displacer level transmitter:

Electronic displacer level transmitter technology operates by detecting changes in buoyancy force caused by liquid level change. These forces act upon the spring supported displacer causing vertical motion of the core within a linear variable differential transformer.

The movement of the core within the LVDT generates an electrical signal which is further processed and serves as the output of the transmitter. The unit is designed to be externally mounted on a tank. Isolation valves are recommended.

The spring technology employed as a counterforce to the buoyancy of the displacer results in a stable signal that is not impacted greatly by vibration, agitation, or turbulence of the measured liquid.

The video below provides more detail, covering the features and advantages of this level measurement technology and the Magnetrol instrument. Share your level measurement challenges and requirements with a product application specialist. The combination of your process knowledge and their product application expertise will produce effective solutions.

Tank Blanketing Valve Function and Useful Features

Tank Blanketing Valve
Caschco - Valve Concepts

The filling of vapor space in a liquid containing tank with a gas is referred to as "tank blanketing", and sometimes "padding". Specialized valves are available, designed to simplify the incorporation of a tank blanketing function in an operation.

Often, the gas employed to fill the vapor space in a tank is nitrogen. The purpose of blanketing can vary, but generally involves preservation of the stored product or safety. In both cases, one goal is to keep oxygen levels in the vapor space sufficiently low to inhibit ignition of flammable products, or minimize oxidation and its impact on stored product quality. The inflow of blanketing gas can also be used to keep the tank under positive pressure relative to the surrounding space, considered to harbor contaminants which could otherwise leak into the tank.

What are some functions of a tank blanketing valve?

• Maintain positive pressure in the tank at a selected setpoint.
• Provide gas control at very low flow rates, or close bubble tight, when tank liquid level is static.
• Adjust gas flow to compensate for the maximum liquid draw down rate.
• Provide sufficient closure to prevent supply gas from excessively pressurizing tank.

Blanketing valves are used in conjunction with vents to provide a full range of control over the pressure and content of the vapor space within a tank. A single valve solution eases the design and component selection burden of amassing individual components and combining them into a working assembly. Some useful features of a blanketing valve include:

• Bubble tight shutoff to prevent wasting of purge gas.
• Self cleaning flow path design.
• Pressure balanced pilot, so supply pressure fluctuations do not impact the setpoint.
• Setpoint not appreciably affected by changes in temperature.
• Low maintenance requirements, including complete access to valve internals without removing the valve from the tank.

More detail, including a description of the elements required for proper valve sizing, is found in the document below. Share your fluid process measurement and control challenges with application specialists, combining your process experience and knowledge with their product application expertise to develop effective solutions.

Process Tank Blanketing Valve from Miller Energy, Inc.

Solenoid Valves - The Operational Basics

Industrial Solenoid Valves
Magnatrol

A solenoid is an electric output device that converts electrical energy input to a linear mechanical force.

At the basic level, a solenoid is an electromagnetic coil and a metallic rod or arm. Electrical current flow though the coil produces a magnetic field, the force of which will move the rod. The movable component of the solenoid is linked to, or part of, the operating mechanism of another device. This allows the switched electrical output of a controller to regulate mechanical movement in another device and cause a change in its operation. A common solenoid application is the operation of valves.

A plunger solenoid contains a movable ferrous rod, sometimes called a core, enclosed in a tube sealed to the valve body and extending through the center of the electromagnetic coil. When the solenoid is energized, the core moves to its equilibrium position in the magnetic field. The core is also a functional part of valve operation. It's repositioning causes a designed changed in the valve operating status (open or close). There are countless variants of solenoid operated valves exhibiting particular operating attributes designed for specific types of applications. In essence, though, they all rely on the electromechanical operating principle outlined here.

A solenoid valve is a combination of two functional units.

• The solenoid (electromagnet) described above.
• The valve body containing one or more openings, called ports, for inlet and outlet, and the valve interior operating components.

Flow through an orifice is controlled by the movement of the rod or core. The core is enclosed in a tube sealed to the valve body, providing a leak tight assembly. A controller energizing or de-energizing the coil will cause the valve to change operating state between open and closed, regulating fluid flow. There are almost countless variants of solenoid operated valves, specifically tailored for applications throughout industrial, commercial, and institutional operations.

The document provided below illustrates a portion of the broad array of solenoid valves available for industrial control applications. There are also some good cutaway illustrations showing the internal operating valve parts. Share your valve requirements and challenges with an application specialist. Combining your process application knowledge with their product expertise will produce effective solutions.

Industrial Solenoid Valves, Bronze and Stainless Steel from Miller Energy, Inc.

Succeeding: Engineer as Peacemaker

Make Allies, Not Adversaries

Let's take a step away from the technical, but still focus on an important aspect of our work.

As engineers involved in process measurement and control, we are accustomed to everybody else looking to us for answers and solutions. We are the people that make things work. Occasionally the pressure and stress can get a little intense and strip away some of our civility in our dealings with those around us. You may have bitter experience with this as either victim or perpetrator. It never ends well. With a private and candid self-assessment about how we view and interact with other stakeholders in our projects, we may be able to scale down some of our stress and better focus on the reality of the task at hand. Consider the points below. Comment and add a few points of your own.

You are an expert, but so are they.

Accept that, just as you have specialized knowledge that others do not, they have specialized knowledge or insight you may lack. Encourage the sharing of knowledge with those you interface with on a project. Try to be proactive and ask gently probing questions to ascertain the comprehension level of others involved in the project in various roles. Their increased understanding of key project technical concepts will promote more effective communication throughout the duration of the project. It can also help to avoid missteps in your own progress. Good people appreciate the time you take to provide basic explanation of concepts they may not fully understand, but need to know. Make valuable allies of the other project stakeholders by freely contributing your expertise. It is an investment that costs you little, but may pay immense dividends at some future time.

Everybody else's job usually looks easier than it really is.

All jobs have their own special challenges and responsibilities that generate stress. Accept the notion that you probably do not fully comprehend the burdens on those around you. Your portion of the project is certainly critical, but no more so than that of anybody else. Everybody needs to perform or nobody succeeds. Try not to view your project tasks as compartmentalized, but rather as part of the combined joint effort of all stakeholders. Help out others whenever you can. Again, make allies.  

Everybody is somebody's customer.

Whomever you deliver your work product to is your customer. The people delivering their work to you should view you as their customer. Make your customers happy by adjusting aspects of your procedures to better satisfy their needs. In a more technical sense, your modified process output becomes an improved input to their process. Small changes in your delivery may produce comparatively large returns in customer satisfaction. Allies.

Do not embarrass or demean others...especially in public settings.

Embarrassment breeds anger, a desire for revenge, and other bad and unproductive things. Avoid words and deeds that will make a coworker or stakeholder look bad in front of others. If there is a problem, if there is a mistake, try to deal with it discreetly whenever possible. Giving a someone a chance to repair a mistake before it becomes public builds value in your relationship. Certainly, there can be instances where more is at stake than someone's pride. Use good judgement to recognize when you can privately give someone an opportunity to amend a situation without causing harm.

Reach a common understanding of project scope and technical details

Your organization's management or your company's client, whatever the case may be, will likely have project expectations which will be clearly understood in their mind, but perhaps not fully described to all those tasked with specific performance. It is also possible, even probable, these same stakeholders will have misconceptions or a lack of technical knowledge about certain facets of the project. Omissions from the project specs and gaps in the common understanding of technical aspects related to the work requirements can easily turn a fairly straight forward task into a wildfire of organizational mayhem. The way in which these situations are handled must be diplomatic. Injured egos can do more damage to project harmony and progress than the facts ever will. The delivery method for the facts will likely be more crucial than the facts themselves.

It's not about being right. It's about being successful.

At our company we recognize customers are more than merely people that buy things from us. They are people to whom we contribute our time and talent to help achieve their success,... which inevitably will lead to ours. Never hesitate to let us know how we are doing, or how we can help.

Miller Energy - Industrial Instrumentation and Process Control Solutions

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