Page "Parts-per notation" Paragraph 5

Parts-per notations may be expressed in terms of any unit of the same measure.

For instance, the coefficient of thermal expansion of a certain brass alloy, α = 18. 7 ppm /° C, may be expressed as 18. 7 ( µm / m )/° C, or as 18. 7 ( µin / in )/° C ; the numeric value representing a relative proportion does not change with the adoption of a different unit of measure .< ref > In the particular case of coefficient of thermal expansion, the change to inches ( one of the U. S. customary units ) is typically also accompanied by a change to degrees Fahrenheit.

Since a Fahrenheit-sized interval of temperature is only < sup > 5 </ sup >⁄< sub > 9 </ sub > that of a Celsius-sized interval, the value is typically expressed as 10. 4 ( µin / in )/° F rather than 18. 7 ( µin / in )/° C .< sub > </ sub ></ ref > Similarly, a metering pump that injects a trace chemical into the main process line at the proportional flow rate Q < sub > p </ sub > = 125 ppm, is doing so at a rate that may be expressed in a variety of volumetric units, including 125 µL / L, 125 µgal / gal, 125 cm < sup > 3 </ sup >/ m < sup > 3 </ sup >, etc.