# FACTOR OF SAFETY

The ratio of ultimate strength to allowable stress. It is used to ensure that a structural member is not subjected to excessive loads that would cause it to fail or not be able to perform the job it was designed for.

## Related Terms

### DIELECTRIC STRENGTH

A measure of the ability of an insulating material to withstand electric stress (voltage) without failure. Fluids with high dielectric strength (usually expressed in volts or kilovolts) are good electrical insulators. (ASTM Designation D 877.)

### HALF-POWER POINTS

Power ratios used to define the angular width of a radar beam. One convention defines beam width as the angular width between points at which the field strength is 71 percent of its maximum value. Expressed in terms of power ratio, this convention defines beam width as the angular width between half-power points. A second convention defines beam width as the angular width between points at which the field strength is 50 percent of its maximum value. Expressed in terms of power ratio, the latter convention defines beam width as the angular width between quarter- power points.

### FILTRATION (BETA) RATIO

The ratio of the number of particles greater than a given size in the influent fluid to the number of particles greater than the same size in the effluent fluid.

The ratio of actual work output of a heat engine to the ideal output.

### BETA-RATIO (ß-RATIO)

The ratio of the number of particles greater than a given size in the influent fluid to the number of particles greater than the same size in the effluent fluid, under specified test conditions (see 'Multi-Pass Test').

### BULK MODULUS (OF ELASTICITY)

A ratio of normal stress to a change in volume. A term used in determining the compressibility of a fluid. Data for petroleum products can be found in the International Critical Tables.

### AIR-FUEL RATIO

The ratio weight, or volume, of air to fuel.

### SENSIBLE HEAT FACTOR

The ratio of sensible heat to total heat.

### POISE

A measure of viscosity (absolute viscosity) numerically equal to the force required to move a plane surface of one square centimeter per second when the surfaces are separated by a layer of fluid one centimeter in thickness. It is the ratio of the shearing stress to the shear rate of a fluid and is expressed in dyne seconds per square centimeter (DYNE SEC/CM2); 1 centipoise equals .01 poise.

### POROSITY

The ratio of pore volume to total volume of a filter medium expressed as a percent.