Burkert MS08, Руководство по эксплуатации

Страница: 18 / 84

Type MS08
Product description
18
3 Optical path 4 Mirror
5 Lenses 6 Light sources
The SAC254 Sensor essentially consists of four parts (See figure above):
▪ a defined light source that consists of two LEDs of different wavelengths
– wavelength LED 1: 254 nm
– wavelength LED 2: 530 nm (for turbidity correction)
▪ a lens system
▪
the optical path through the medium
▪ a detector with ambient light suppression
Both LEDs are connected one after the other in a special profile. The light emitted by the LEDs passes
through the medium on the way to the detector and is partially weakened by the medium. The detector
picks up the remaining light and thus determines its intensity I. The weakening of the light when passing
through the measurement medium is compared to the weakening caused by ultra-pure water. The meas-
urement in ultra-pure water provides the so-called basic intensity I
0
. Using equation 1 and equation 2, the
SAC254 Sensor determines the transmission T and the absorbance A of both of the above-mentioned
wavelengths.
T =
I
I
0
Fig. 13: Equation 1 - Calculation of transmission
A = −log
10
T
Fig. 14: Equation 2 - Calculation of absorbance
where:
T transmission in %
I current light intensity
I
0
basic light intensity in ultra-pure water
A absorption in AUs (AU = absorbance unit)
The light intensity of LEDs often varies with the temperature. Therefore, a temperature correction factor is
determined for each wavelength of the SAC254 Sensor and is used to calculate the measurement value.
Spectral Absorption Coefficient SAC
The SAC254 Sensor outputs the SAC of the wavelength of LED 1 at 254 nm (SAC254). Scattering of light on
particles in a solution is seen as turbidity by the observer. The SAC254 Sensor uses the absorbance of 530
nm (A530) for the turbidity correction of the absorption measurement of the wavelength emitted by LED 1
(A254). The SAC254 is calculated using the equation 3 In this equation, d is the optical path length in milli-
metres [mm]. Available path length for SAC254 Sensor: 50 mm.
(A − A ) ⋅ 1000
254 530
SAC
d
=
254
Fig. 15: Equation 3 - Calculation of the spectral absorption coefficient at nominal wavelength 254
where:
d length of the optical path in millimeters [mm]