Reference data
54
Measuring principle
This equipment is based on the measuring principle that states that transmitted light is
reduced by the solids in a sample. Absorbance is obtained from a transmittance, and
concentration is converted from a relational expression between the Absorbance and
concentration. A near-infrared radiation LED is used as a light source. Silicon photocells are
used as a transmitted light photoreceiver and as a reference light photoreceiver. The light
source blinks at approx.10 Hz to eliminate the impact of ambient light.
First of all, the variations in the amount of light from a light source is corrected by computing
T/R, where T stands for the amount of transmitted light signals converted to a direct current by
synchronous detection of waves and where R stands for the amount of reference light signals.
T/R computed at the time of zero calibration is memorized as T
0
/R
0
, and T/R/T
0
/R
0
is
computed to obtain the transmittance T of a sample. Since transmittance decays in relation to
concentration in terms of indexes, a logarithm of the transmittance is computed and converted
to absorbance.
A = -logT
A: absorbance T: transmittance
For the decay of light by scattering (such as MLSS), absorbance is not completely
proportional to concentration. So, the relationship between absorbance and concentration is
given in a formula to measure a concentration of a sample.
The relational expression showing a MISS concentration and a MLSS absorbance has been
obtained by HORIBA Advanced Techno through sampling. Sensitivity may vary slightly
depending on the type of activated sludge. Span calibration is available as the capability of
adjusting a measured value to a analyzed value.
Moreover, for any usage other than the measurement of activated sludge, clay is memorized
as gray inorganic sludge and the relationship to white Kaolin is memorized to select a good
correlation to the analyzed value.
Sensor head
Reference-light
detector
LED
Transmitted-light
detector
