HYDROSENSE HS2410
Revision 5.0
P a g e 8 | 27
2.3 INTERFERENCES
AND AFFECTS TO ACCURACY
The UV fluorescence technique monitors the intensity of light emitted from the passing stream at a
selected wavelength band.
This technique can be quite selective by eliminating the light affect of compounds in the water that do
not share the same fluorescence characteristics of hydrocarbons.
1. When chemical compounds in the water are excited with light energy, only certain compounds will
emit the light back out of the water at a higher wavelength than excited with. These are referred to
as fluorescing compounds. The HydroSense does not respond to most chemicals because it only
responds to fluorescing compounds, of which aromatic hydrocarbons are included.
2. The light used to excite the compounds is filtered to 254 nm +/-. Of all the fluorescing compounds
only certain ones will respond to this wavelength. Some respond to higher and some to lower
wavelengths. This filter narrows the HydroSense response to only those that fluoresce from 254 nm
+/-.
3. This limited number of compounds that do fluoresce from 254 nm light may emit light at any number
of wavelengths such as 290nm, 310 nm, 350 nm 480nm, etc. Aromatic hydrocarbons happen to
fluoresce at approximately 350 nm. By filtering the light sensor from all light except 350 nm +/-, only
compounds that emit light at 350 nm +/- are indicated at the receiver.
4. Oil and Grease in water may be made up of hundreds or thousands of different hydrocarbon
compound structures. The aromatic compounds are fluorescing compounds. The proportion of
aromatics within the total hydrocarbons is generally consistent in a product or process. The
aromatics are therefore used as a tag to correlate the monitor to total hydrocarbons in water.
Changing Oil Types and Sources
Different oils have a different make-up of compounds and the fluorescing strength may vary between oil
types. For instance, diesel fuel may fluoresce much stronger than transformer oil. If the HydroSense is
calibrated using 100 PPM of diesel, 100 PPM of transformer oil may only give a display reading of 50
PPM.
Crude oil may vary from one well to another, lubricating oils from different manufacturers may vary in
their make-up; oils may be dissolved or free, and so on.
The calibration is therefore site selective and should be done using actual process water or with
samples of oil that are to be targeted by the monitor.
The calibrated accuracy relies on the oil type and conditions being consistent. The HydroSense will
respond positively to aromatic hydrocarbons but the display accuracy may be affected by variations in
the types and sources of these hydrocarbons.
Other Chemicals in the Water
The light sensor is selective to compounds in the water that emit light at 350 nm when excited from 254
nm light. If there is a background chemical in the water that fluoresces at these wavelengths, the
HydroSense will respond to them.
If this background chemical concentration is consistent, this interference will be zeroed out during
calibration. Calibration is recommended using process water so that any background interferences are
zeroed out.