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.

Specialty Valves for Biotech and Pharma Applications

Stainless steel ball valve for sanitary applications
Courtesy Habonim USA

Sanitary fluid process operations require the use of valves with unique characteristics that make them suitable for use. Materials of construction, FDA approved materials for seals, clean draining, and no retainage of the process fluid are some of the requirements for sanitary valves. Pharma and other bioprocess industries, including food and beverage, will also have stringent requirements for sterilizing or cleaning in place.

One manufacturer, Habonim, a globally recognized manufacturer of high quality ball valves, offers a complete line of ball valves specifically designed for sanitary process applications.

The TuBore™ valve series is available in line sizes from 1/4" to 6" with clamped or welded connections and available with manual operator, pneumatic or electric actuation.

The datasheet included below provides more detail and specifications. Share your sanitary fluid process control requirements and challenges with product specialists. Combining your process knowledge with their product application expertise will produce effective solutions.

Sanitary Ball Valves for Biotech and Pharma Applications 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.