How to Select an Ultrasonic Level System

There is a wide selection of products and capabilities on the market today to measure level. To choose the best one for your plant, always consider your application needs against the many features available.

Analyzing Your Application

A good supplier will start by asking you all the details of your application before recommending a technology or product. Begin your checklist of application data with information about the material being measured– the type, temperature, and any related characteristics. Do you need to measure level, volume, and/or flow? If you are measuring level in a tank, determine the dimensions and shape of the tank, as well as details about process connections, internal obstructions, or other considerations.

When selecting a transducer, the most important criteria are the application’s measuring range and environment. Consider all the environmental conditions the sensor will experience so you can ensure the model you choose will be suitable and rated for those conditions. Is the transducer inside a building in a controlled climate, or outdoors in harsh weather conditions?

If the transducer is located in blazing sun or is subject to extreme temperature fluctuations, consider a sun shield and check that the transducer has a temperature compensation feature built in. Is it monitoring corrosive chemicals? Are there vapors or foam present? If so, a transducer with a special facing may be needed to ensure compatibility with the material, hazardous area approvals may be required, or a longer-range unit may be needed to compensate for signal attenuation. If the sensor will be installed in a constricted space, you will need a compact instrument.

Will the transducer be submerged in its application, such as in a flooded wet well? If so, you will need a submergence shield. It will maintain an air pocket in front of the transducer and maintain a high level reading during submergence.

Many applications require continuous monitoring to provide a constant readout of the material level at all times. In some cases, however, you may also want to install point level devices as high and low alarms.

What power sources are available in the plant? If your application will be integrated into a control system, how will the transceiver receive power?

Once you have assessed all the needs of the application, you can review manufacturer specifications to select the products and accessories that best serve your process needs.

Analyzing your needs in this way will help you choose the right functionality while also helping you scale the system to your application. While you don’t want to overspecify, be careful not to limit your options.

It’s a good idea to think about your future needs as well. Does the instrument have functions and features you could or may want to use in the future, such as pump efficiency monitoring or communications capabilities?

As more plants integrate their systems for central control, bus communication capabilities are in higher demand. When selecting ultrasonic systems, consider choosing those that offer digital communications capabilities with standard protocols such as HART or PROFIBUS. Even if your systems are not integrated today, consider if you may want this capability in the future.

Assessing Products

Using the application details you’ve gathered, assess the appropriateness of various ultrasonic products. Main categories to consider are accuracy, repeatability, reliability, features, functions, smarts, and modularity.

Many people confuse repeatability and reliability. Repeatability describes the ability of the device to distinguish the same level over and over again at different instances in time. Reliability describes the product’s ability to operate within specification without breaking down over a long period of time.

Manufacturers express their specs in different ways. If the meaning is fuzzy, don’t hesitate to ask for clarification. Remember that the quality of the measurement should always be the foremost priority.

When selecting a transceiver, make sure it is compatible with the transducer. You can select from rack, panel, or wall mount versions, depending on your plant configuration.

Above all, look for models that are easy to install and set up. Commissioning is always easier and more secure if the unit uses a handheld programmer.

Some instruments have a swivel head that makes it easy to line up with conduit − a nice feature, especially in tight spaces. Some models offer backlit displays for easy readability. This is very useful if you will be taking readings at the instrument location, but irrelevant if the instrument is connected to a central monitoring station.

Instrument Smarts

More emphasis today is placed on the instrument’s "smarts" or software for signal processing and communications.

We know that, to obtain the maximum signal, a transducer should be mounted directly over the material to be measured with the transducer face perpendicular to a flat target. The reality in a wet well includes inflows and outflows, turbulence, splashing, debris, and obstructions such as pipes, cables, braces, guide rails, pump cases, and ladders.

Ideally, you would want to mount the transducer clear of all these obstructions but that is very often not possible. Careful mounting and aiming to optimize beam angle can help. A better solution is a system with built-in signal-processing software that compensates for these situations.

Advanced Functions

When assessing transceiver functions, look beyond the individual application and determine your objectives for overall plant management. Make a list of your applications and determine how the data obtained from each fit into your overall plan. Are there simple applications where a standalone instrument is all you require?

Do you want all points integrated into a central system for monitoring and control? Do you need advanced functions to optimize certain aspects of your operation such as pump control, energy use, or chemical dosing? Do you require data logging and documentation of storms, overflows, or other events to meet regulatory reporting requirements?

Making these decisions will create a list of the advanced functions you require, and will help you design the right system for your plant.

Need Help?

Download, print, and out our level application datasheet (65K Adobe PDF) and fax it to Lesman at 630-595-2386. We'll help you figure out what technology will best suit your process.