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Section 6. Calibration

6.1 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 nephelometric turbidity units (NTU).

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m versus 1.3 ±0.6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

(e.g. 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 OBS-3A Utility, you must alternate between the

OBS-3A Utility and standard-production procedures.

6.1.1 Equipment and Materials

x 4000 NTU AMCO Clear, StablCal, or formazin

x 4” and 6” diameter black polyethylene containers. Concrete sample

containers (Cat. # TC-4, Deslauriers Inc., 800-743-4106;

www.deslinc.com) work well for this.

x 2-liter, Class A volumetric flask

x 100 ml TD volumetric pipette

x 25 ml TD measuring pipette

x 2 gallons filtered distilled water (purified water from the super market

works fine)

x Slotted stainless steel stirring spoon

6-1

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