OBS-3A Turbidity and Temperature Monitoring System
7. Calibration
It is a good idea to use the
Retrieve and Save Coefficients to File
button
(under
Operations
on the
OBS-3A Configuration
screen) to save the original
coefficients before performing a calibration.
7.1 Turbidity
This section briefly describes the materials and equipment you will need and
the basic procedures for calibrating OBS sensors with AMCO Clear and
sediment. All sensors are factory calibrated with AMCO Clear and include a
calibration certificate expressed in NTUs. AMCO Clear is available from GFS
Chemicals Inc. (800-858-9682;
www.gfschemicals.com
). It is approximately
three times more expensive than formazin per NTU-liter, however: 1) it is
guaranteed to be stable for one year and has <1% initial lot-to-lot accuracy; 2)
AMCO particles are small and uniform in size and shape (0.31
±
0.1
μ
m versus
1.3
±
0.6
μ
m for formazin); and 3) it does not flocculate or settle so stirring is
not required. AMCO Clear must be made specifically for the OBS sensor.
Formazin can be purchased from the Hach Company (800-227-4224;
www.hach.com
); request the certificate of analysis when ordering it. Hach also
supplies premixed, StablCal which is like formazin except that it can be stored
for two years while maintaining ±5% of its nominal NTU value.
We strongly advise that the same turbidity standard be used throughout a study;
do not switch between AMCO and formazin standards. Also, AMCO is
premixed by the manufacturer and must be used in the container it was shipped
in.
Turbidity calibration is organized into steps for Preparation; recording values
with OBS-3A Utility; and Production of Standards. After completing the
preparations and starting HydroSense software, you must alternate between the
HydroSense and standard-production procedures.
7.1.1 Equipment and Materials
•
4000 NTU AMCO Clear, StablCal, or formazin
•
4 inch and 6 inch diameter black polyethylene containers. Concrete
sample containers (Cat. # TC-4, Deslauriers Inc., 800-743-4106;
www.deslinc.com
) work well for this.
•
2 L, Class A volumetric flask
43
