In-situ SAC measurement requires no sample conditioning
December 2002Maintenance, Test & Measurement, Calibration
An in situ SAC (spectral absorption coefficient) measurement without any sample conditioning can be achieved with the new StamoSens measuring system from instrumentation specialist Endress+Hauser. The device measures the SAC value of aqueous solutions and suspensions.
The StamoSens measuring system offers an alternative to the costly and delayed COD (chemical oxygen demand) analyser measuring method. The COD analyser Standard Method - known as the dichromate COD test - involves a 2-hour reflux with concentrated sulphuric acid and potassium dichromate. Together with silver sulphate and mercuric sulphate catalysts, this method provides for the complete oxidation of all but a few organic compounds.
Yet, the high reagent costs and toxicity of the Standard Method has resulted in different oxidising agents being used (such as chromic acid or permanganate). The result is completely different measuring results for a single substance. If the composition of the sample is constant, the measurement will reveal the difference between the standard value and the measurement of the COD analyser. However, if two or more samples of wastewater need to be measured, the COD analysers will fail in comparison with the Standard Method.
"In contrast, no reagents are required for the SAC measurement method which is based on the self-UV-light absorption of organic materials. Light is transmitted through the aqueous sample, with UV and reference photo-receivers being used for evaluation. If organic ingredients are present, the UV light is attenuated," states Jacques Fouche, product manager, Endress+Hauser Analytics.
Because most of the COD is caused by organic molecules, there is a relationship between the UV absorbance and the COD and, if the sample matrix is fairly constant, there will be an accurate correlation to the COD.
"In addition, opto-electronic turbidity and contamination compensation make it possible for the StamoSens measuring device to work directly in the inlet or outlet of a sewage treatment plant without sample conditioning, while an interference-prone, mechanical cleaning unit (wiper) is not required," says Fouche. The sensor that measures the sample directly without the requirement of a sample preparation system, pumps, piping or measuring housing is not only faster than other measuring techniques, but also more stable due to the simplicity of the device.
The picture shows the StamoSens measuring device, which records the SAC (spectral absorption coefficient) value of aqueous solutions and suspensions.
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