Hach DR/850 Procedures Manual page 438

TURBIDITY,
continued
Sampling and Storage
Accuracy Check
Method Precision
Collect samples in clean plastic or glass bottles. Analyze samples
as soon as possible. Store samples up to 48 hours by cooling to 4
°C (39 °F). Analyze the sample at the same temperature as it was
collected.
Standard Solution Method
The stored program has been calibrated using formazin, the
primary standard for turbidity. A 200 FAU formazin solution for
checking the accuracy of the test can be prepared using the
following procedure.
1. Pipet 5.00 mL of a 4000 NTU Formazin stock solution into a
100-mL volumetric flask.
2. Dilute to the mark with deionized water. Prepare this daily.
Convenient stabilized turbidity stock solution (200 NTU
™
StablCal Standard) is available from Hach.
Standard Adjust
To adjust the calibration curve using the reading obtained with
the
200 FAU formazin standard, press the
(using the arrow keys) to the STD setup option. Press
activate the standard adjust option. Then enter 200 to edit the
standard concentration to match that of the standard used. Press
to complete the adjustment. See Section 1, Standard
ENTER
Curve Adjustment for more information.
Precision
In a single laboratory, using a turbidity standard solution of 200
FAU with the instrument, a single operator obtained a standard
deviation of ±2 FAU.
Estimated Detection Limit
The estimated detection limit for program 95 is 21 FAU. For
more information on the estimated detection limit, see Section 1.
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