Hanna Instruments HI 83749 - Manual (Page 6)

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 Secchi Disk is commonly used to
measure turbidity in lakes and other deep waters (mg/L SiO
2
). Both methods are visual and are not
considered very accurate. To obtain more accurate readings a nephelometer should be used as a
turbidity reading instrument.
The HI 83749 turbidimeter reports the measurements in NTU (Nephelometric Turbidity Units).
NTU units are equal to FTU units (Formazine Turbidity Units). The conversion table between these
measurement units is shown bellow:
U TJ U T F/ U T N Oi S
2
)L /g m (
U TJ 1 9 1 0 5. 2
U T F/ U T N 3 5 0. 0 1 3 1. 0
Oi S
2
)L /g m ( 4. 0 5. 7 1
MEASUREMENT UNITS
A light beam that passes through the sample is scattered in all directions. The intensity and pattern of
the scattered light is affected by many variables like wavelenght of the incident light, particle size,
shape, refractive index and color.
The optical system includes a tungsten filament lamp, a scattered light detector (90 0 ) and a transmitted
light detector (180 0 ). The microprocessor of the instrument calculates the NTU value from the signals
that reaches the two detectors.
The lower detection limit of a turbidimeter 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 83749 turbidimeter is designed to have very low stray light, providing
accurate results also for low turbidity samples. However, special care must be taken when measuring
low turbidities (see General Tips for an Accurate Measurement, page 10).
PRINCIPLE OF OPERATION
35
The LCD can be illuminated to allow the user to see the readings even in dark
environments.
To turn on or off the backlight, press the ON/OFF key.
The backlight will automatically shut-off after 25 seconds of non-use to save
the battery life.
The tag is housed in a rugged metal that can withstand harsh environments. However, it is better
to protect the tag from direct rain.
Place the tag near a sampling point. Fix it securely with the provided screws, in such a way that
the metallic iButton ® is easily accessible for reading the tag.
The number of tags that can be installed is practically unlimited. Additional tags can be ordered
(See Accessories, page...) HI 920005 - five tag holders with tags).
LCD BACKLIGHT
TAG INSTALLATION
The instrument tungsten lamp has a life longer than 100,000 measurements. In case of lamp failure,
the defective lamp can be easily replaced. When the lamp is broken, the instrument displays “no L”
error message.
To replace the lamp follow the next steps:
• Remove the battery lid.
• Unscrew the lamp connection using a screwdriver.
• Unlock the lamp and extract it by pulling it out
from the lamp holder handler.
• Place the new lamp in the right position and
push it until is securely locked.
• Insert the lamp leads into the connector and
tight them using a screwdriver.
Warning: After lamp replacement the meter needs
to be recalibrated.
LAMP REPLACEMENT