This automation industry quiz question comes from the ISA Certified Control Systems Technician (CCST) program. Certified Control System Technicians calibrate, document, troubleshoot, and repair/replace instrumentation for systems that measure and control level, temperature, pressure, flow, and other process variables. Click this link for more information about the CCST program.
To calculate a dimensionless number called specific gravity, you divide the density of a process fluid by the density of one of the following:
A) mercury or mercury vapors
B) oil or natural gas
C) water or air
D) any known liquid or gas
E) none of the above
Click Here to Reveal the Answer
Specific gravity (SG) is the ratio of the density of a solid or liquid to the density of water at 4°C. It also refers to the ratio of the density of a gas to the density of dry air at standard temperature and pressure. This specification is seldom used. Specific gravity is a dimensionless quantity, which is to say it has no units as density does.
The utility of SG is that it translates into any system units-SI, CGS, or American Engineering. Standard materials and properties tables use SG, leaving the conversion to density to the individual user.
Water has a specific gravity equal to 1. Materials with a specific gravity less than 1 are less dense than water and therefore will float in water. Substances with a specific gravity greater than 1 are denser than water and will sink.
The correct answer is C.
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