Understanding Ultrasonic Concentration and Density Measurement Systems

Working with ultrasonic measurement systems can seem like a daunting task. However, designers can break down the task into manageable components, and work with providers to assemble a complete system. By separating the system into the sensor, the controller, and the software, one can understand how each works to provide fluid concentration and density measurements.


New measurement systems are often very similar to systems already customized to specific industries. Newcomers often find it easiest to study existing systems to understand how a custom system can fit into their own application. Some well-known systems are currently in use in breweries, soft drink and beverage production, crystallization processes, and print plants.

• Beverages – beverage systems require precise volumetric and temperature measurements. System components must allow for limited footprint space. They must also allow convenient operation. One concentration measurement involves specific gravity. For beer, this means a measure of the amount of fermentable and unfermentable components of the wort (the liquid from the mashing process) before the fermentation process. The specific gravity is an essential part of the recipe, and regulating this measurement is an important part of quality control. In soft drinks, a key concentration measurement is brix, which is the sugar content held in the aqueous solution. Agile measurement systems allow the concentration measurements to be taken at various points in the processing pipeline, and coordinated at the control center.

• Crystallization process – controlling crystal growth is an exacting process not unlike annealing metal, or curing a concrete slab. Like concrete, the crystallization process must be monitored and temperature controlled in order to yield a well-formed, stable structure. Ultrasonic systems allow the lab technician to determine the amount of saturation, supersaturation, and portion of crystal content, in order to control the process.

• Print Plants – ultrasonic systems control the blending process for the raw print materials, particularly the concentration of isopropyl alcohol (IPA). The system must ensure that the concentrations remain uniform throughout the process, that the amount of wasted alcohol is minimized, and that the IPA does not escape the system and pollute the environment.


Ultrasonic sensors take advantage of the property of fluids to conduct sound waves. Much like a toy boat is placed into a moving stream, a sound wave can be generated at one point of fluid travel and be measured some distance later in order to yield fluid speed. The speed of the ultrasonic wave not only depends on the speed of the fluid, but on the nature of the fluid itself. For this reason, a fluid with a different concentration will yield a different ultrasonic velocity. The velocity factors are well documented for many standard processes. Determining factors for new processes is a simple and straightforward matter. Sensors can yield highly accurate measurements using this technology. A representative accuracy is +/- 0.05 m/s.

Fluid Sampling

Sampling the fluids differs by application. Piped systems often take advantage of integrated inline sensor/detector combos. Other applications are suited for an immersion type sensor. Still others, particularly those involving caustic substances, can take advantage of the ability of an ultrasonic system to measure from outside the piping.


Controllers allow systems to operate in a stand-alone fashion. One controller can monitor one or more sensors to monitor measurements and check against measurement standards. Some controllers form a feedback system with the process. For instance, in the print plant example, a controller can reduce the IPA when it detects a growing concentration. Other controllers are set in monitor mode, and only alert the operator when concentrations exceed preset standards. Controllers will show an alert on their displays. They can also provide a current sink in order to activate a visible or audible alarm.


Controllers can be harnessed to a computer system for configuration and data analysis. Many can use standard computer networking connections such as TCP/IP. Some also offer a standard industrial digital connection such as Fieldbus.

Operators can use computers to connect to controllers in order to perform initial configuration. This will setup the ability of the controllers to communicate with the sensors, set the alarm limits, and specify the modes of operation. Technicians can also use the computer interface to sample concentration data, in order to analyze and log large amounts of concentration values over time. This enables companies to determine trends, identify potential problems, and document the level of quality control.

Designing a System

Providers will have sensors for almost any type of fluid concentration measurement application. Clients only need to supply the measurement values for their process, the sampling scheme for their system, and the logistics of the plant or laboratory. From that point, clients and providers can work together to finalize the specifications and assemble an appropriate system.