6
Many methods were used to measure turbidity over the years. The Jackson Candle Turbidimeter was
used to measure turbidity as Jackson turbidity units (JTU). The method is visual and is not considered
very accurate. To obtain more accurate readings a nephelometer should be used as a turbidity reading
instrument.
The
HI 847492
haze meter reports the measurements in FTU (Formazin Turbidity Units). FTU units
are equal to NTU units (Nephelometeric Turbidity Units). The conversion table between these
measurement units is shown below:
U
T
F
/
U
N
F
/
U
T
N
C
B
E
C
B
S
A
M
L
E
H
U
T
N
1
U
N
F
1
U
T
F
1
1
5
2
.
0
5
2
.
7
1
0
1
C
B
E
1
4
1
9
6
0
4
C
B
S
A
1
8
5
0
.
0
4
1
0
.
0
1
9
7
5
.
0
M
L
E
H
1
1
.
0
5
2
0
.
0
5
2
7
.
1
1
MEASUREMENT UNITS
A light beam obtained from a High Emission 580 nm LED that passes through the sample is scattered
in all directions. The intensity and pattern of the scattered light is affected by many variables like
wavelength of the incident light, particle size, shape, refractive index and color. The light source LED
with a wavelength peaking at 580 nm ensuring that the interference caused by colored samples is
minimum. The optical system includes LED, a scattered light detector (90
0
) and a transmitted light
detector (180
0
). The microprocessor of the instrument calculates the FTU value from the signals that
reaches the two detectors.
The lower detection limit of a haze meter is determined by the so called “stray light”. Stray light is the
light detected by the sensors, that is not caused by light scattering from suspended particles.
The optical system of
HI 847492
haze meter is designed to have very low stray light, providing
accurate results also for low haze samples. However, special care must be taken when measuring low
haze samples (see General Tips for an Accurate Measurement, page 10).
PRINCIPLE OF OPERATION