Hanna-instruments HI 96710 Uživatelský manuál stáhnout pdf (Strana 6)

Precision is how closely repeated

measurements agree with each other. Precision

is usually expressed as standard deviation (SD).

Accuracy is defined as the nearness of a test

result to the true value.

Although good precision suggests good

accuracy, precise results can be inaccurate.

The figure explains these definitions.

In a laboratory using a standard solution of

7.0 pH and a representative lot of reagent,

an operator obtained with a single instrument

a standard deviation of 0.1 pH units.

In a laboratory using a standard solution of 1.00 mg/L free and total chlorine and a representative

lot of reagent, an operator obtained with a single instrument a standard deviation of 0.02 mg/L.

PRECISION AND ACCURACY

PRINCIPLE OF OPERATION

Absorption of Light is a typical phenomenon of interaction between electromagnetic radiation and

matter. When a light beam crosses a substance, some of the radiation may be absorbed by

atoms, molecules or crystal lattices.

If pure absorption occurs, the fraction of light absorbed depends both on the optical path length

through the matter and on the physical-chemical characteristics of the substance according to the

Lambert-Beer Law:

-log I/I

= ε

c d

A = ε

c d

Where:

-log I/I

= Absorbance (A)

= intensity of incident light beam

I = intensity of light beam after absorption

= molar extinction coefficient at wavelength λ

c = molar concentration of the substance

d = optical path through the substance

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Hanna-instruments HI 96710 Uživatelský manuál Strana 6