Fluidquip Australia

UV Intensity and UV Dose

UV dose is measured in millijoules seconds per cm2 (mJ/cm2) and is calculated using the following parameters:

UVIntensity(I)-measured in milliwatts per cm2 (mW/cm2); UVTransmittance(UVT)(%); • Exposuretime(t)(seconds).

The following is an example of a simplistic, generalised formula for the calculation of an average UV dose. Please note that this a generalised formula and is based on a non-validated UV intensity sensor mounted at the surface of a UV chamber. It cannot be applied to validated UV systems: 

UV dose = (I/UVT) x t


I = UV Intensity
UVT = UV Transmissivity (a measure of how much UV light can penetrate through the water being treated)
t = time of UV exposure



The important thing to understand from this relationship is that UV Intensity and UV dose are two different things.
UV Intensity measures the “amount” of UV energy actually penetrating through the water being treated. UV dose is the amount of UV energy penetrating the water, multiplied by the amount of time the water is exposed to this energy. It is the UV dose that determines the log reduction of a pathogen.

UV dose is usually quoted as either the “average” dose or Reduction Equivalent Dose (RED). The average dose implies that some of the water being treated will receive the prescribed dose, some will receive more than the prescribed dose, but, importantly, some water will receive less than the prescribed dose. If some water receives less than the prescribed dose, then the prescribed log reduction may not be achieved. This concern has led to the adoption of the RED concept. In essence, RED suggests that all the water passing through the UV system will receive at least the prescribed dose, thereby ensuring the prescribed log reduction targets are achieved. RED is the concept on which UV systems are validated. 


Graham Smith. Fluidquip Australia Pty Ltd. 


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