Industry Terms Briefly Explained: Part 4 – Physical Properties and Other Definitions

In part four of our attempt to define and describe industry terms that have been known to be confusing or misunderstood, we focus on a random mixture of physical properties and other key phrases. Here we briefly explain the meaning of some terms associated with the engineering field.


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Density, or specific weight, is the weight (mass) per volume with the dimension kg/m3 or lbs/ft3. Somewhere hidden in the dimension is the gravitational acceleration (g) – which is an expression that indicates the intensity of a gravitational field in physics. Ever since man first walked on the moon, we came to the realization that weight depends on where you are. From then on, we have spoken formally about specific mass in kg/m3 and specific weight as N/m3. The difference between these two dimensions is the g, even though they are formal terms.


Specific Gravity

One term still in use today is specific gravity, which is the ratio of the density or mass of a substance to the density or mass of a reference substance for the same given volume. The reference substance is water in most cases. Specific gravity has no dimension, and, if it is less than 1, it means the substance floats in water.



Viscosity is the internal resistance to gradual deformation, or, in other words, to flow. It is a fluid’s thickness, which should not be mistaken for density. Density and viscosity are two different properties of a fluid. If a fluid flows, it has to be deformed, because the resistance that goes against the deformation is viscosity. There are two expressions for viscosity; the dynamic viscosity, and kinematic viscosity. The former has the dimension Centipoise (cPs), while the latter has the dimension Centistokes (cSt). The relationship between the two is the density of the fluid.


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The dynamic and kinematic viscosity of water is approximately 1 cPs respectively, and 1 cSt at room temperature. That makes the life of an engineer a bit easier, because knowing the specific gravity of the fluid (with water as the reference) allows one to calculate the dynamic viscosity from the kinematic viscosity (and vice versa), without worrying about the correct engineering units.


Flow Velocity

Flow velocity is typically the measurement result of an ultrasonic flowmeter and is expressed in e.g. m/s or ft/min. In order to determine the volume flow in m3/s or ft3/min, it is imperative to determine the inner area of the cross section in m2 or ft2 of the pipe or duct as accurately as possible. Most ultrasonic flowmeters will do the calculation from velocity to volume flow for you.


We hope that our term descriptions help eliminate some of the confusion and misunderstandings that are linked to their definitions. Our knowledge of terms stems from our experience and background in engineering. Visit our previous blog posts to read about the explanation of other terms such as the CE Marking, REACH, and ISO 9001. These terms relate to the measurement instruments we supply, such as the inline flow meter. For further details, visit our website or dial 1-888-358-2518.