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UVT / UV254nm / SAC - What is UV Absorption / Transmission?


UV absorption & transmissivity measurement is used in the testing of water to predict water quality and the amount of dissolved organic carbon. The UV254nm optical measurement can be supplied for either transmission (UVT) or absorption (UVA) measurement.

UV254nm transmissivity measurement is used to control UV disinfection processes to account for absorbance by dissolved organics and suspended solids. To ensure efficient sterilisation, the treatment process lamp intensity may be regulated by a control instrument measuring UVT.

UV254nm absorption is typically used as a general indicator of organic loading and water quality and is also sometimes referred to as SAC (Spectral Absorption Coefficient). However, since each chemical species absorbs the UV spectrum differently, the test must be applied carefully. Where the sample matrix is highly variable or contains components which do not absorb at the 254nm frequency, the alternative UV- persulfate TOC analyser methodology may be more accurate, as it is not affected by chemical species. 

Protoc 300 TOC
Protoc 300 TOC
WTW-trescon uno
WTW Trescon Uno
Protoc 300 TOC
Realtech Analysers
Realtech P200
Realtech P200



Instrumentation is available for flow through analysis or alternative in-situ probe measurement. For UVA measurement, suspended solids also absorb transmitted light and interfere with the measurement. Consequently it important to ensure that the sample is either pre-filtered to remove the suspended solids or the instrument is capable of applying suspended solids correction if the method is to trend DOC (dissolved organic carbon). Different path lengths can be used to accommodate different application / measuring range requirements thus providing various levels of sensitivity. Automatic cleaning (either chemical or air blast) may be needed to maintain reliable measurement especially in waste-water applications.

Limitations Of Test

Only organic chemicals containing a double bond within their structure strongly absorb UV254nm, single bonded chemicals do not. Every organic chemical absorbs different amounts of UV254nm. The technique is not typically suitable for applications where the contaminant is unknown or highly variable in it's chemical form. The technique can be applied to a known process or to simply alarm where the contaminant is known is to strongly absorb at this frequency.

Alternative On-line Methodology

Total Organic Carbon - TOC can be used to report the dissolved organic compounds in less than 3 minutes. The test is not compound sensitive and does not suffer interference affects from suspended solids. By determining the TOC or Total Organic Carbon component of a sample stream, a correlation may be set up with other test such as COD and BOD. Peak organic loads may therefore be determined in real-time allowing for improved measurement and control.

More Information

On line continuous measurement of Total Organic Carbon for water and waste-water applications.