UV 254nm is a water quality test parameter using ultraviolet (UV) light at the UV 254nm wavelength. The UV 254nm test parameter can serve as an indication of organics in water and wastewater since most organics are absorbed at the UV 254nm wavelength.

UV 254nm is measured in units of either UV absorbance (UVA) or UV percent transmittance (UVT). UVT is a measure of how much UV light is able to penetrate through a water sample. The less UV light that is transmitted through the water, the lower the UVT. UVA is a measure of the absorbance of UV light by matter in the water.

UVT is measured as a percentage. Pure water (DI water) will read 100 percent UVT, and completely opaque water will read 0 percent UVT. The UVT shares an inverse logarithmic relationship with the path length of the UV light through the water sample and the concentration of matter in the water sample. Because the UVT varies with path length and the concentration of matter, the UVT is usually expressed with the assumption that a 1 cm path length is used so that the UVT provides a measure of matter in the water independent of path length.

UVA is directly proportional to both the concentration of matter in the water and the path length of the UV light through the water sample. UVA is given by the following equation:

UVA = ε∙b∙c

where ε is the molar absorbtivity of the particular type of matter in the water sample, b is the path length of the water sample, and c is the concentration of matter in the water sample.

From the equation above, it can be deduced that if the path length is altered by a certain factor the UVA measurement will be altered by the same factor. Therefore, a UVA reading based on a 1cm path length can easily be converted to another reference path length if required.

Since UVA and UVT are essentially just different representations of the same physical phenomenon, they can be related by a simple mathematical expression as follows:

UVA = 2 – log₁₀ UVT

which indicates an inverse logarithmic relationship between UVA and UVT.

Click here to view a convenient UVT/UVA conversion chart.