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. Specific gravity is the heaviness of a substance compared to that of water, and it is expressed without units. In the metric system specific gravity is the same as in the English system. If something is 7.85 times as heavy as an equal volume of water (such as iron is) its specific gravity is 7.85. Its density is 7.85 grams per cubic centimeter, or 7.85 kilograms per liter, or 7.85 metric tons per cubic meter.

Specifications

The density range (mass per volume), accuracy, and response time characterize most of these instruments.

• Density range - The density range the instrument can measure. Mass per volume, usually specified in kilogram per cubic meter (kg/m^3). The density of water is 1000 kg/m^3 and snow density is usually measured as a ratio to this. So snow which is 100 kg/m^3 is specified as 100/1000, or 10 percent.
• Pressure range - The pressure range of the sample the density instrument can operate in.
• Temperature range - The temperature range of the sample the density instrument can operate in.

Simultaneous measurements and user interfaces are also important in choosing the proper density and specific gravity instruments.

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.

Technology Type

Density digital meters use the principle of either oscillating tubes or radioactive adsorption to determine density and specific gravity.

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.

In 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 and suspension methods.

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.

Display Options

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.

Applications

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

Standards

Density and specific gravity instruments must adhere to certain standards to ensure proper design and functionality. 

AASHTO-T 100 - This method covers determination of the specific gravity of soils by means of a pycnometer. When the soil is composed of particles larger than the 4.75-mm (No. 4) sieve, the method outlined in T 85 shall be followed.

ASTM C1039 - These test methods cover the determination of apparent porosity, apparent specific gravity, and bulk density of cores taken from graphite electrodes manufactured for use in electric arc furnaces.

ASTM C128 - This test method covers the determination of the average density of a quantity of fine aggregate particles (not including the volume of voids between the particles), the relative density (specific gravity), and the absorption of the fine aggregate.

Image credits:

Eagle Eye | Quantachrome Instruments | Micromeritics