Ultra Violet Transmittance and UV Units

In recent blog posts, we have looked at the importance of a UV unit, the UV dose a UV unit should deliver, and the screening of particles before they enter UV units.  Today, we will consider ultra violet transmittance and UV units.

The ultra violet transmittance (UVt) is a very important factor when sizing a UV unit.  The UVt is a measure of how clear the water is to UV light.  Equipment manufacturers assume a certain UVt when rating their UV units.  Commonly, they assume either 85% or 95% UVt.

If water has a UVt of 85%, it means that 85% of the light emitted by the UV bulb will be availale to sterilize bacteria while 15% of the light is being absorbed by other materials in the water.

The UVt can have a big impact on the performance of your UV unit.  Imagine that you selected a UV unit to deliver a dose of 40 mJ/cm2 as we discusse previously.  You set up a filter to screen out particles and turn your UV unit on.  Now imagine that the manufacturer assumed your water had a UVt of 95% but, in reality, it only has a UVt of 75%.  Your UV unit will not deliver anything like the UV dose you are expecting and, in fact, may not deliver a sufficient dose to sterilize bacteria at all!

Before installing a UV unit, always check the UVt of your water and make sure that the unit being installed is appropriate for water with your UVt.