Last revised: February 26, 2025

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.

Density and Specific Gravity Instruments FAQs

How do density and specific gravity instruments differ in terms of their technology type?

Density and specific gravity instruments differ in terms of their technology type in several ways:

Oscillating Tube Technology

This method involves a hollow glass tube that vibrates at a certain frequency. When the tube is filled with a sample, the vibration frequency changes based on the mass of the sample. This frequency change is measured and converted into density. A built-in thermostat is often used to control temperature, as temperature variations can affect density measurements.

Radioactive Adsorption

Some density meters use radioactive adsorption to determine density and specific gravity. This method is not detailed in the documents, but it typically involves measuring the attenuation of radiation as it passes through a sample.

Microwave Phase Difference Measurement

This noninvasive method measures how a fluid's density affects the propagation of microwaves. By monitoring the phase difference between the original wave and the one that passed through the fluid, the density can be reliably measured. This technique is effective in applications involving suspended solids, slurries, and sludge.

Suspension Methods

These include measuring the density gradient and the Schlieren method. The density gradient is observed when two liquids of different densities are layered, resulting in a vertical density gradient over time. The Schlieren method involves immersing a liquid-filled tube in another liquid to observe horizontal flow, indicating equal densities.

Hydrometers

Hydrometers are calibrated glass floats used to measure the specific gravity or relative density of a material. They are used in various applications, such as measuring the percentage of salt in solutions or the percentage of sucrose in the sugar industry.

Pycnometer

This apparatus is used to determine the density of porous materials by measuring the mass and volume of a sample. It is a manual method typically used for liquids.

How does temperature affect density measurements?

Temperature can significantly affect density measurements in several ways:

Thermal Expansion

As temperature increases, most materials expand, leading to a decrease in density. Conversely, a decrease in temperature typically results in contraction and an increase in density. This is because density is defined as mass per unit volume, and changes in volume due to temperature will affect the density measurement.

Calibration and Measurement Accuracy

Instruments used for measuring density, such as oscillating tube density meters, often include a built-in thermostat to control temperature. This is crucial because temperature variations can alter the density of the sample being measured, potentially leading to inaccurate readings. Proper calibration at specific temperatures is necessary to ensure accurate measurements.

Specific Gravity Considerations

Specific gravity is the ratio of the density of a material to the density of water at a standard temperature. Therefore, temperature changes can affect both the sample and the reference (water), impacting the specific gravity measurement. Instruments like thermohydrometers, which combine a thermometer and a hydrometer, are used to account for temperature variations when measuring specific gravity.

What is the calibration process for density instruments?

The calibration process for density instruments is crucial to ensure accurate and reliable measurements. Here are some key points about the calibration process:

Technology Type Dependence

The calibration process for density and specific gravity instruments depends on the technology type used. For example, oscillating tube density meters and other types may require different calibration procedures.

Calibration Materials

Calibration often involves using standard materials such as nitrogen gas or water at specific temperatures and pressures. These materials are used to establish a baseline or reference point for the instrument.

Calibration Certificates

Some density meters may come with a calibration certificate, which provides assurance that the instrument has been calibrated according to specific standards and is ready for accurate measurements.

Pycnometer Calibration

For manual methods like using a pycnometer, calibration involves determining the volume of an unknown space by weighing the pycnometer empty and then filled with a liquid of known density. This process helps in calibrating sample cells used in various applications.

How do different technology types in density instruments cater to various industries?

Different technology types in density instruments cater to various industries by providing tailored solutions that meet specific measurement needs and environmental conditions. Here's how some of these technologies are applied across different industries:

Oscillating Tube Technology

Applications: This technology is widely used in industries where precise density measurements are critical, such as in the chemical and pharmaceutical industries. The built-in thermostat helps maintain temperature stability, ensuring accurate readings despite environmental changes.

Advantages: It provides high precision and is suitable for laboratory settings where controlled conditions are necessary.

Radioactive Adsorption

Applications: Although not detailed in the documents, this method is typically used in industries where non-contact measurement is essential, such as in the oil and gas industry.

Advantages: It allows for the measurement of density without direct contact with the sample, which is beneficial in hazardous or high-pressure environments.

Microwave Phase Difference Measurement

Applications: This method is effective in industries dealing with fluids containing suspended solids, slurries, and sludge, such as the pulp and paper and wastewater industries.

Advantages: It provides reliable measurements unaffected by flow velocity, contamination, or bubbles, making it ideal for challenging fluid conditions.

Suspension Methods

Applications: These methods are used in research and development settings where understanding the density gradient or equal density conditions is necessary.

Advantages: They offer a visual and experimental approach to density measurement, useful in educational and experimental setups.

Hydrometers

Applications: Hydrometers are used across various industries, including food and beverage, petroleum, and chemical industries, to measure specific gravity or relative density.

Advantages: They are simple, cost-effective, and provide quick measurements for quality control and process monitoring.

Pycnometer

Applications: This method is used in laboratories for precise density measurements of liquids and porous materials.

Advantages: It offers high accuracy and is suitable for applications where manual measurement is feasible and necessary.

How does the calibration process differ for various density measurement technologies?

The calibration process for various density measurement technologies can differ significantly based on the specific technology used. Here's a breakdown of how calibration might vary across different types of density instruments:

Oscillating Tube Technology

Calibration Materials: This technology often requires calibration using standard materials such as nitrogen gas or water at specific temperatures and pressures. These materials help establish a baseline for accurate density measurements.

Calibration Certificates: Instruments using this technology may come with a calibration certificate, ensuring that they have been calibrated according to specific standards.

Radioactive Adsorption

While specific calibration details for this method are not provided, it typically involves ensuring that the instrument is correctly set up to measure the attenuation of radiation through a sample. Calibration would likely focus on ensuring the accuracy of the radiation source and detectors.

Microwave Phase Difference Measurement

This method involves calibrating the instrument to accurately measure the phase difference of microwaves passing through a fluid. Calibration would ensure that the instrument can reliably detect changes in phase difference due to variations in fluid density.

Suspension Methods

Calibration for these methods might involve ensuring that the experimental setup accurately reflects the density gradient or equal density conditions. This could involve using known density liquids to verify the accuracy of the measurements.

Hydrometers

Calibration for hydrometers involves ensuring that the scale is accurate for the specific type of liquid being measured. This might include verifying the hydrometer's readings against known standards for specific gravity or relative density.

Pycnometer

Calibration involves determining the volume of the pycnometer by weighing it empty and then filled with a liquid of known density. This process helps in calibrating the pycnometer for accurate density measurements of liquids or porous materials.

What are the specific applications of hydrometers in different industries?

Hydrometers have specific applications across various industries due to their ability to measure the specific gravity or relative density of liquids. Here are some of the key applications:

Food and Beverage Industry

Brix Hydrometers: Used to measure the percentage of sucrose by weight in solutions, which is crucial for quality control in the sugar industry and for determining the sweetness of beverages.

Petroleum Industry

API Hydrometers: These hydrometers use a standard scale defined by the American Petroleum Institute (API) to measure the specific gravity of petroleum products. This is important for determining the quality and grade of crude oil and its derivatives.

Chemical Industry

Baumé Hydrometers: Used for liquids denser than water, such as syrups and certain chemical solutions, to determine their specific gravity.

Alcohol Production

Tralle and Proof Hydrometers: These instruments display the percentage of ethyl alcohol by weight in water, which is essential for the production and quality control of alcoholic beverages.

Salt Solutions

Salt Hydrometers: Used to measure the percentage of salt in solution by weight, which is important in industries where salt concentration needs to be monitored, such as in food processing and water treatment.

General Laboratory Use

Thermohydrometers: These combine a thermometer and a hydrometer to enable the accurate measurement of both temperature and specific gravity of a sample, which is useful in various laboratory settings for precise analysis.

Density and Specific Gravity Instruments Media Gallery

References

GlobalSpec—Hydrometers Information

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