The only true way to determine if your zero standard is being contaminated is to have the zero solution
analyzed in a instrument like the Hach 2100AN bench top meter. This will tell you how much
contamination you have and what offset you need to apply. Most customers do not have access to this
type of laboratory equipment so there has to be a second option.
Enough testing has been done to show that, in cases where the equipment is properly cleaned and
serviced, the level of contamination of the zero turbidity standard is quite small. Typically the average
contaminant level ranges from 0.2 to 0.8 NTU. Knowing this, you can pick a number between these
points (0.5) and enter this offset using the sonde’s One-Point Calibration feature. This single point
calibration is only done after you have successfully completed a two- or three-point calibration.
Note: You cannot enter a value greater than zero for your first point, when performing a two- or three-
point calibration. The zero offset only works on the single-point calibration.
SETTING THE SINGLE-POINT OFFSET
ALL SONDES: Install a clean sensor guard on the sonde to be calibrated. If you are using a copper alloy
nti-fouling guard in your deployment, then it is critical that this guard be used for this calibration.
• With the guard installed place the sonde in a container of zero standard or filtered DI water. With
the guard installed the container can be glass or plastic and bottom color does not matter.
• Enter the Turbidity offset as 0.5 NTU.
• Operate the wiper and visually inspect the probe face for any air bubbles.
• Wait at least 60 seconds after the last wiper has stopped before you accept the 0.5 calibration
point.
• If chlorophyll or BGA sensors are installed, then it is recommended that you zero these sensors
at the same time.
FIELD PRECAUTIONS
Remember that any optical measurements can be interfered with by animals, debris, fouling, and air
bubbles. To optimize your measurements you must take certain precautions.
• If you construct your own mounting pipe make sure that it is built in such a way as to ensure
good flow even in fouling conditions. Drilled holes should at minimum 0.75″ in diameter on 2.0″
centers and cover from the bottom of the pipe to 6″ above the sensor guard. Remove all drill
debris and burrs from the holes.
• Slots can also be cut in the pipe for flow instead of drilling holes. Slots should be 1″ in diameter
and all debris and burrs must be removed. At least 4 slots are required 90° apart.
• Additional holes or slots should be drilled every 6″ above the sensor measurement area to ensure
flow through the pipe at all tides or other periods of high water.
• In locations where fouling is expected the PVC pipe must be painted inside and out with a good
(black) marine anti-fouling paint. It is especially important to paint the edges of all the drilled
holes so they do not foul and close. Apply two coats and make sure that the pipe has been
properly cleaned and prepped for the paint.
• Fireplace ch
imney brushes work well for cleaning deployment tubes.
• All pipes should have an open bottom so that if debris enters it can fall out.
•
Cleaning the pipe inside and out should be done on a monthly basis or sooner if conditions
warrant. Remember that if there is growth in the pipe then the turbidity and other sensor
measurements could be affected.
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