Process Density Measurement: Gas and Light Liquid Density Meters
In-line density measurement of gases and light liquids: Sarasota vibrating element density meter, HART 4–20 mA output and ATEX intrinsically safe options.
Why is density measured?
Density is the mass per unit volume of a fluid and one of the core parameters for product quality, composition and transfer measurement in the process industries. In gas and light liquid lines, density is an input to mass flow calculation; in natural gas, it contributes to specific gravity and calorific value calculations; and it indirectly shows composition changes in hydrocarbon mixtures. At custody transfer points, continuous and accurate density measurement increases the reliability of quantity determination.
Where is density measured?
- Natural gas and hydrocarbon lines: In mass and energy calculation together with a flow computer.
- Refinery and petrochemical: In quality monitoring of light liquid, LPG and product streams.
- Metering stations: In custody transfer and allocation control.
How is density measured?
The common method in pipeline density meters is the vibrating element principle. The fluid passes around or through an element vibrating at its resonant frequency; as the fluid density increases, the vibration frequency changes, and density is calculated from this frequency. Bypass-type instruments direct a small part of the flow to the measuring element; the value is read on a head-mounted smart display and transferred to the control system through a HART-compatible 4–20 mA output. Intrinsically safe approved versions are used in hazardous areas.
Combining density data with the differential pressure, pressure and temperature measurements in the flow computer makes it possible to calculate mass flow and energy content reliably.
Choosing a density meter
- Gas or liquid; pressure class and flange connection
- Temperature and pressure conditions; calibration range
- Hazardous area approval
- Flow computer and communication integration
Density measurement with Tetra
Tetra supplies the Thermo Scientific Sarasota FD900 density meter in Türkiye. It provides installation, commissioning, calibration and service together with flow computer and metering station solutions.
Frequently asked questions
What units is density expressed in?
In process measurement, density is generally given in kg/m³ or g/cm³. Relative density (to air) is also used for natural gas, and density at a reference temperature or API gravity for petroleum products. In commercial calculations, the temperature and pressure conditions of the value should be stated.
How is a vibrating element density meter calibrated?
These instruments are calibrated at the factory with fluids of known density, and coefficients defining the frequency-density relationship are loaded into the instrument. On site, verification is carried out with a reference fluid or gas of known density, or by comparison with a calculated reference value. If deviation is seen, the cleanliness of the measuring element and temperature conditions are checked first.
What errors occur in gas density measurement?
Liquid droplets, condensation or deposits on the measuring element change the vibration frequency and cause deviation. If the gas in the bypass line differs in temperature and pressure from the main line, the measured density does not represent main line conditions. Insulating the bypass line and keeping it short are therefore recommended.
Can a Coriolis flow meter be used for density measurement?
Coriolis flow meters measure density along with mass flow and are widely used, particularly on liquid lines. On low-density gas lines, separate vibrating element density meters may be preferred for precise density measurement. The choice should be based on fluid type, required accuracy and line size.
Products measuring this parameter
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