Density and Specific Gravity Instruments

Density and specific gravity instruments are meters used to determine the density and specific gravity of a mixture that may be solid, gas, or liquid.  The density range (mass per volume), accuracy, and response time characterize most of these instruments.  Simultaneous measurements and user interfaces are also important in choosing the proper density and specific gravity instruments.

Density digital meters that use the principle of either oscillating tubes or radioactive adsorption to determine density and specific gravity are the most common types of density and specific gravity instruments.  An oscillating tube is a hollow glass tube that vibrates at a certain frequency.  The vibration frequency changes when the tube is filled with a sample.  The higher the mass of the sample, the lower the vibration frequency.  This frequency is measured and converted into density. A built-in thermostat controls the temperature (no water bath required).  A thermostat is often necessary since the density of the sample could be changed by temperature variations.

Radioactive adsorption, the use of gamma rays or x-rays to determine density, is helpful in applications such as piping or mining where intrusion into the system may be costly.  Energy is emitted by a source that passes through the pipe walls and the process material.  The process material adsorbs the energy.  The amount of energy reaching the detector varies with each material.  Electronics convert this energy reading to a density measurement.  This method is particularly effective in process applications that involve extremely high flow rates, high pressure, and high / low temperatures.

Another noninvasive method used by density and specific gravity instruments includes microwave phase difference measurement. Microwave phase difference exploits the way a fluid's density affects the propagation of microwaves when they pass through it.  This allows a reliable measurement of the fluid's density by monitoring the difference in microwave phase between the original wave and the one that passed through the measured fluid.  Measuring fluid density by observing a wave's phase difference is unaffected by flow velocity and/or is not affected by the contamination and/or bubbles.  This technology is effective in various applications where determining the consistency of suspended solids, slurries, and sludge is necessary such, as in the pulp and paper and wastewater industries.

Suspension methods include measuring the density gradient and the Schlieren method.  Density gradient is measured when two liquids of different densities are layered in a glass tube so that over time, diffusion results in a vertical density gradient.  The Schlieren method involves immersing a liquid-filled tube in another liquid,  the liquid will only flow horizontally from the tube if the densities of the two liquids are equal.

Density and specific gravity instruments provide various readouts are available based upon the user’s needs.  Analog and digital displays are available on portable instruments.  Display parameters may include Brix, Plato, % alcohol, API gravity, percent solids, percent mass, and percent volume may be available.  Other options may include computer interfaces and software for programming customized concentration or specific gravity tables, data analysis, and/or control.

Density and specific gravity instrument systems range from laboratory applications where autosamplers and cleaning components are integraged to tank management systems for remote monitoring to flow transfer and control applications.

Calibration of density and specific gravity instruments is dependent upon the technology type used.  Nitrogen gas or water at specific temperatures and pressures may be required prior to using the instrument.  Some density meters may include a calibration certificate.

• Density Range
  Density range is the range of densities that a density or specific gravity instrument can measure. Density is expressed as mass per unit volume, and usually specified in kilograms per cubic meter...
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