D625 Users Manual
DENSITY EQUATIONS
A simple definition for Density (for a homogeneous object) is the mass of the object divided by the
volume of the object. Thus, common units of fluid density are grams per cubic centimeter,
kilograms per cubic meter, and pounds per cubic foot.
The hydrometer is probably the most common device used to measure fluid density in which it
measures the volume displaced by an object of known mass. The pycnometer is another
instrument used for measuring fluid density. However, these devices are mainly restricted for use
in laboratories. Another instrument used to determine the density of a liquid or a gas is the
oscillating U-tube digital density meter. The U-tube digital density meter is installed in a process
flow system where it continuously monitors “real-time” fluid density.
The oscillating U-tube is a method to determine the density of liquids based on an electronic
measurement of the frequency of oscillation. The density value is calculated by incorporating
frequency of oscillation, fluid temperature and pressure. The equations shown below are utilized
by the density processor or flow computer to determine the density of the fluid flowing through the
meter.
EQUATION 1: UNCORRECTED (PERIODIC) DENSITY EQUATION
D
U
= Density of the fluid based on the periodic time (frequency of oscillation) of the meter. It is
UNCORRECTED for Temperature and Pressure. Unit = grams per cubic centimeter [gr/cm
3
]
T = Periodic Time (frequency of Oscillation) of the density meter. Unit = microseconds [µs]
K
0
, K
1
, K
2
= Density Coefficients. These coefficients vary for each meter and are determined at
the factory during the meter calibration process. Refer to the Calibration Certificate included
with your meter.
EQUATION 2: TEMPERATURE CORRECTED DENSITY EQUATION
D
U
= Uncorrected Density, [gr/cm
3
]
D
T
= Temperature Corrected Density , [gr/cm
3
]
K
Tv
, K
T0
, K
T1
= Temperature Correction Density Coefficients – included with Calibration
Certificate.
T
F
= Live fluid temperature in degrees Fahrenheit [°F]
T
C
= Reference temperature in degrees Fahrenheit [°F]
EQUATION 3: PRESSURE CORRECTED DENSITY EQUATION
D
PT
= Pressure & Temperature corrected Density, [gr/cm
3
]
D
T
= Temperature Corrected Density , [gr/cm
3
]
K
Pv
, K
P0
, K
P1
= Pressure Correction Density Coefficients – included with Calibration Certificate.
P = Live pressure in Pounds per square inch [PSI
G
]
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