Chemtrac HydroAct HA4, Руководство пользователя

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36 Chapter 3. Sensors
3.1.1.3 Theory of Operation
The UV254 Sensor Split-Sense Pro technology is designed to take sensor readings at 90° to each
other, where one reading is through the water sample under test and the other reading is through air.
The two sensor readings enable the instrument to compensate for lamp and sensor drift over time
due to instrument aging and temperature. Changes inside the cabinet due to changes in ambient air
temperature, intermittent exposure to sunlight, water temperature changes etc. are also compensated
for. In this way, the UV transmittance (UVT) and the UV absorbance (UVA) of the test water is calculated
without continuous need for calibration with DI water.
Turbidity interference is automatically compensated for within the UV254 Sensor by measuring
wavelength (550nm) where there is no absorbance from organics. Even though there is turbidity
compensation, all UV254 Sensor installations should be protected with a 50 micron filter. The following
diagram shows the Ortho-Beam technology.
3.1.1.4 Relating the UV254 to another parameter
Light at the 254nm wavelength is of particular interest since it is a wavelength that is very readily
absorbed by organic matter in the water. The UV254 absorbance parameter is proportional to the
concentration of organics in the water and it is a simple process to determine a linear correlation factor
that relates the UV254 parameter to other water quality parameters that provide a measure of organic
matter in the water. There are several such water quality parameters including total organic carbon
(TOC), chemical oxygen demand (COD), biological oxygen demand (BOD) and color (Hazen). All
these parameters have slightly different biases to different types of organic matter. UV254 can often
be linearly correlated to all of these parameters.
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The correlation factor will depend on the particular organic water quality parameter being correlated
to and will also depend on the matrix of organics being measured. This means that correlations are
site and application specific and so the correlation factor must be determined at the measurement site.
Most natural water sources such as raw water for drinking water and municipal waste water have a
good correlation between, for example TOC and UV254 absorption, and COD and UV254 absorption.
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It is possible for there to be no correlation between UV254 absorbance and other water quality parameters if that parameter
on a particular water source does not contain UV254 absorbing substances. An example of this would be a water supply
counting a lot of dissolved sugar, as sugar won’t absorb UV254 but will have a high Chemical Oxygen Demand (COD).