Some Useful Conversions, Equations, and Constants

(prepared by Professor J. M. Cimbala)

Conversion Factors within the English (British Gravitational) System (often very confusing)

These are the definitions of mass units slug and lbm:

Use these ratios (or their inverses) when converting. All three ratios are dimensionless and equal to 1:

Some textbooks use the archaic g_{c}, which is really just the inverse of the middle ratio above:

Note: It is *not* proper to say that one lbm *equals* one lbf, but it *is* proper to say that one lbm *weighs* one lbf. Example of how to use these conversion ratios properly - What is the weight of one lbm? Solution:

Useful conversions (in terms of dimensionless ratios) for energy, power and volume flow rate:

Angular Velocity

or

Example: If N = 3600. rpm, the angular velocity can be calculated as

__Ideal Gas Equations__

Note: The ideal gas constant, R, here is the *specific* ideal gas constant,
not the *universal* gas constant, R_{u}.
To convert from one to the other,

where M is the molecular weight of the gas.

Thermodynamicists like to use specific volume,

whereas fluid mechanicians like to use density, which is simply the inverse of specific volume, i.e.

.

Here are some ideal gas relationships:

Ideal gas constants for air in both SI and BG units:

Temperature Conversions from Relative to Absolute

English (BG) system:

Metric (SI) system: