Hach-Lange FP 360 sc User Manual - Manual (Page 20)

20
Operation
4.5 Calibration
4.5.1 Factory calibration
The calibration curve zero point and slope are preset. Retrospective calibration of these
basic settings is generally not required outside of the inspection intervals.
Do regular zero point checks to make sure that impurities or faults are being detected
(refer to section
4.5.3 on page 21 ).
If the zero signal increases due to the measuring medium components or the installation
conditions, you can compensate for this influence via an offset correction. Carry out a lab
analysis of the sample to do this. If there is no PAH/oil contamination in the measuring
medium, enter the measurement value shown by the device as the offset (refer to section

4.5.4.2 on page 22 ).
4.5.2 Process calibration/adjustment
The sensor is pre-calibrated with various concentrations of a special calibration standard
in ultra-pure water. These ideal measurement conditions rarely occur in reality. The
measurement values shown are qualitative trend indicators if no adjustment is made to the
on-site measurement conditions.
If you require quantitatively correct measurement values, either a slope adjustment or a
multi-point calibration needs to be carried out. Both of these operations must be carried
out on site using lab analysis data. Basic prerequisites for quantitative measurements are
precise knowledge about the oil type in occurrence and constant measurement conditions,
e.g. in cool water in a heat exchanger. If several oils are present in varying quantities, it is
generally not possible to carry out a quantitative measurement.
If the measurement conditions change, you must check the accuracy of the results again
through lab analyses and make any adjustments where necessary.
The measurement conditions can change in terms of
• Composition of PAH or oil impurities
• Distribution of impurities in water
• Temp
• Measuring medium composition
• Measurement sensor and measurement window
4.5.2.1 Determination of factors and adjustment of slope
To adjust the slope:
1. At the sensor installation site, draw a lab sample of the measuring medium and
promptly analyze the sample for PAH and oil content.
2. Make a note of the measurement value shown on the controller at the time of the
sample being taken. Make sure the correct unit is shown for the measurement value,
e.g. as ppm oil.
3. Repeat steps 1 and 2 several times.
4. Use the sample value and the value shown on the controller at the time of sampling
to calculate a factor.
5. Find an average value from the factors.