Friday, December 30, 2016

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

non-contact radar liquid level transmitter
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.


Wednesday, December 21, 2016

Diaphragm Pressure Gauges for Industrial Process Measurement

diaphragm pressure gauge 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.


Wednesday, December 14, 2016

Protect Valuable Pressure Gauges and Transmitters With a Pressure Limiting Valve

pressure limiting valve for gauge or transmitter protection
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.



Wednesday, December 7, 2016

Industrial Process Gauges - New Product Guide

industrial pressure gauge
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.