Chemtrac HydroAct HA4, User Manual

Page: 37 / 97

3.1 UV254 Sensor 37
The way the UV254 Sensor provides an output for a correlated parameter is by applying an offset
and calibration to the UV254 measurement. This is calculated by setting up a “virtual channel” in the
instrument. This channel is then related to the original UV254 measurement by way of a formula in the
form:
y = mx + c
where,
y correlated water parameter (eg. TOC, COD)
x UV254 absorbance value
m calibration slope
c calibration offset
In order to calculate m and c , sufficient data must be collected over time to generate a graph similar
to the graph shown. A spreadsheet program can then fit a straight line to the data and provide the
required values to be entered into the UV254 Sensor configuration. In order to derive a correlation
between TOC, DOC, BOD, or COD and UVA, data will need to be gathered using both parameters
over a period of time.
1. It is recommended to record the UVA numbers daily/weekly/monthly (depending on how frequently
the water quality changes, the more often data is collected the more accurate the correlation will
be.
2. Compare these UVA results to your other organic test parameter values (TOC, DOC, BOD, or
COD) over the same period of time and graph the results to determine the relationship between
the two parameters.
3. Keep in mind that the relationship will be different for every water source, i.e. the same correlation
from one site could not be used on another site.
4. Once the correlation has been determined, the relationship can then be entered into the UV254
Sensor to output the value of your correlated parameter such as TOC or COD.
To set up the virtual channels see section 3.1.4.8.