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The Importance of Density.

Many applications or processes require an accurate knowledge of density. Examples include meter correction and mass flow determination.

However, there are many other applications where density is not immediately identified as the property of interest. For example, concentration measurement. When blending a 13M HNO3 solution it is the molarity of the solution that is important to the application. In such applications there may be several different ways in which to determine the molarity. Ultrasonic sensors can, from the velocity of sound, determine the concentration quite effectively but, like many properties, there is a non-linear relationship. This means that the maximum or minimum value for the velocity of sound does not correspond to the maximum or minimum concentration. This means that for any specific velocity of sound there may be two alternative values for the concentration. This is also true of density. Another parameter that could be used to determine the concentration is refractive index.

It is therefore important to know the precise operating conditions and the range of values expected to know which of the different parameters is the most appropriate to choose. Most important, other factors must be considered such as the effect of temperature.

In some other applications there is no such problem because, even if non-linear, there does not exist the possibility for ambiguous readings.

Density has a long history of application in fiscal measurements, particularly in the oil and gas industries. For many years flow measurement was made with positive displacement and inferential (turbine) type volumetric meters even though the interest is in mass flow. Density meters have been a valuable instrument to correct the volumetric measurement to a mass measurement. There is some interest in using coriolis mass flow meters for these applications but more recently there is the introduction of ultrasonic meters for fiscal measurement. These meters are inferential i.e. They infer the volumetric flow rate from the velocity of sound effects and the geometry of the meter section. Some even use the velocity of sound measurements for determining the density but not always with the accuracy needed for the mass flow determination. Again, density meters are a valuable instrument for mass flow determination because of their accuracy.

Razaghi Meyer can assist with a variety of analysers. In this section it is the vibrating element density meters that are considered.

As an introduction, visit this presentation by Don Griffiths: “About Digital Density Meters”

Density Technologies:

Vibrating Cylinder Portable density meter (DM 250)

Similar to the digital viscometer, it is the resonant frequency which varies with density.

Other resonant frequency sensors include tube and fork types.

LEMIS Process

Vibrating Cylinder  Gas density meter.

The resonant frequency varies with density.