Hach-Lange 2100 N Laboratory Turbidimeter User Manual - Manual (Page 29)

Ratio on or off
Ratio on provides very good linearity, calibration stability and a wide
measurement range. Ratio on helps correct for interference when color
is present in the sample that absorbs at the wavelength of incident light.
The manufacturer recommends that Ratio on be used for most
measurements. Ratio must be on to measure samples greater than
40 NTUs (268 nephelos or 9.8 EBCs).
Push RATIO to turn Ratio on or off. The Ratio light is on when Ratio is
on.
Notes:
• If the sample being measured is greater than 40 NTU (or equivalent)
and Ratio is off, the display will show 9s and the RATIO light will flash.
Push RATIO to turn Ratio on and remove the over-range condition.
• Measurements with Ratio on and measurements with Ratio off are
almost the same for turbidity measurements that are less than 40 NTU
if interferences caused by color or light absorbing particles are not
present.
Using the air purge system
The air purge system is used to keep condensation off the external
surface of the sample cell when cold samples are measured.
The air purge system pushes dry air through the optical compartment to
keep the outside the sample cell dry. The connection is made at the air
purge fitting on the back of the instrument
Figure 2 on page 8.
Use dry nitrogen or instrument grade air (ANSI MC 11.1, 1975) at no
greater than 138 kPa (20 psig). The manufacturer recommends an air
consumption rate of 3 to 10 SCFH (standard cubic feet/hour).
When the sample temperature is about or less than 2 °C (35 °F), use a
desiccant dryer and particle filter to make sure that the dew point of the
air purge is less than the sample temperature. The air dryer contains
silica gel desiccant that turns pink. Replace the desiccant when it turns
pink.
If only shop air is available, use a coalescing filter with an automatic
drain and a dryer and particle filter to get instrument quality air. Use a
coalescing filter that typically operates for greater than 2000 hours.
Replace the particle filter when the air dryer is replaced.
Figure 7 and Figure 8 show the methods for connecting the two types of
air supply to the instrument.
Note: The dryer and filter are not necessary if dry nitrogen is used.
Figure 7 Instrument quality air
1 Particle filter (Balston DFU 9933-
05-BQ or equivalent)
3 Pressure regulator
2 Air dryer (Balston DAU 9933-
05-101 or equivalent)
4 Instrument air
Figure 8 Standard shop air
1 Particle filter 5 Filter (Balston 100-12-BX or
equivalent)
2 Air dryer 6 Auto drain (Balston 20-105 or
equivalent)
3 Coalescing filter/regulator (0–
30 psig)
7 Filter housing (Balston
FR-920-30 or equivalent)
4 Shop air
English 27