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Section 6. Calibration

There is no way to cancel a calibration after the

Compute button is clicked.

WARNING

Review the quality assurance checks discussed later in this section. Look at the

plot of residuals, which show the differences between the standard and

computed NTU values. The average residual magnitude should be less than

1% of the calibration range. For example, a calibration range of 2000 NTU

(maximum minus minimum NTU values) should produce an average residual

less than 20 NTU. Residuals higher than 1% of the calibration range indicate

that errors were made in the procedure. By inspecting the plot of residuals, you

can decide which values to repeat, edit, or delete. Repeat values using the

procedures described above. To edit a value,

6. Click the Edit button; select the number of the calibration value you want

to change from the data table; enter the revised calibration value; and click

OK.

7. The data table and plot will be updated. Use the Delete button to remove a

calibration value by number from the data table.

8. Once satisfied with the calibration, click the Compute button and make a

final quality check.

9. Use the Done / Exit button to return to the OBS-3A Utility.

6.1.4 Making Turbidity Standards

1. For the zero NTU calibration point you will need a black 20”X14”X16”

container filled with clean tap water. A Rubbermaid plastic storage box

makes a suitable container.

2. For calibrations from 50-250 NTU use a 6” diameter container with 2

liters of filtered water or a premixed standard. Hold the sensor in the

container so that the beam looks down and across the diameter to prevent

the beam from reflecting off the wall.

3. A 4” diameter container can be used for solutions of 250 NTUs. Add one

liter of filtered water and the necessary amount of formazin. See TABLE

6.1-1 or the formula below for the preparation of standards.

4. Stir the formazin solution with the spoon and position the OBS-3A in the

standard.

5. Go to Step 2 of OBS-3A Utility Steps (Section 6.1.3).

The formula for preparing turbidity standards other than shown in TABLE

6.1-1 is:



stkdw

stk

stkstd



stdstk

dwstd

stk

6-3

1 2 ... 34 35 36 37 38 39 40 41 42 43 44 ... 57 58

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