Have you ever wondered what those have to do with each other? Probably more than you think. More than 200 years ago, early experiments already helped to lay the foundation of how to determine sound velocity, and therefore for the development of modern measurement technology.
Early cornerstones of modern measurement techniques
In the 17th century, the velocity of sound in air was already known due to travel time measurements with cannons. For this purpose, the cannon was positioned in a far away, but known distance. The passing time between the muzzle flash and the bang of the cannon was measured. At that time, the approximate value obtained for the speed of sound in air was 300 m/s. The physicist Ernst Chladni conducted research on the speed of sound with the help of an organ pipe and was eventually the first one to determine the velocity of sound in solids and different gases. Then in 1826, Jacques Charles François Sturm was able to conduct the first precise sound velocity measurements in water. From this point on, sound velocity gained importance and is today used as an additional variable for successfully conducting measurements of liquid concentrations in modern density and concentration meters.
How sound velocity improves measurements
One of the earliest methods to determine the concentration of liquids is density measurement. It was considered the most reliable method. But traditional measuring devices were not yet perfected: On the one hand, hydrometers provided a fast, but inaccurate method. Pycnometers, however, were accurate, but difficult to operate and time-consuming. In the 1960s, Anton Paar developed automatic density meters applying the oscillating U-tube method, which made measurements easier, faster and more accurate. But there still is one limitation: Density measurements can solely be conducted for concentration measurements when the sample is a two-component solution.
However, many industrial raw materials, products and auxiliaries are three-component liquids, such as in soft drinks or alcoholic beverages, lacquer, paints, nitrating agents or infusions. A ternary solution consists of two components and a solvent, mostly water. In this case, one variable, for example density, is not enough to characterize the sample’s composition. A second variable is required to determine the concentration of ternary mixtures and sound velocity is a great parameter for doing so.
Technology for measuring sound velocity
An ultrasonic pulse is emitted by a piezo transmitter at regular intervals. It is detected and amplified by the receiver after traveling an exactly known distance which is filled with the sample under investigation. The time the sound impulse takes to travel is measured at a constant sample temperature. The propagation speed is specific to the particular substance and is dependent on concentration and temperature.
By combining density and sound velocity measurements, an even more suitable technique for concentration measurements could be developed. Having sound velocity as a second variable, the purity of two-component mixtures can be tested with double safety (one concentration is derived from sound velocity and also density) and two concentrations of a three-component solution can be determined.
Modern measuring devices
The DSA 5000 M combines density and sound velocity measurements and simultaneously determines two different physical properties within one sample. The instrument is equipped with a density and a sound velocity cell, thus combining the oscillating U-tube method with a highly accurate measurement of sound velocity. Both cells are temperature controlled by a built-in Peltier thermostat. The DSA 5000 M even eradicates a disadvantage of other common oscillating U-tube devices: It eliminates the influences of sample viscosity on the measured density automatically. Combined density and sound velocity measurements are widely applicable in diverse sectors of chemical, petroleum, pharmaceutical, food or beverage industries. They are used for monitoring production processes or for the quality control of incoming goods and final products.
Sound velocity is indeed an important part of Anton Paar’s measurement techniques and thanks to early groundbreaking experiments with velocity of sound, modern technology today is able to accomplish stunning results.