UVT (ultraviolet transmittance) is a measure of the amount of UV light that is transmitted through a water sample. It is typically expressed as a percentage, with higher UVT values indicating that more UV light is being transmitted through the water.

UV254 is a measure of the absorbance of UV light by a water sample at a wavelength of 254 nanometers (nm). This is a common wavelength used in water treatment because many waterborne contaminants have a high absorbance at this wavelength.

This means that UVT is directly related to the clarity of the water, whereas UV254 is more sensitive to the presence of specific contaminants. As a result, UVT is often used to assess the overall quality of water, while UV254 is used to detect the presence of specific contaminants.