45
SECTION 4,
continued
1. From Salinity mode, press the
UP ARROW
. The instrument
will ask whether you would like to use the current
measurement as the salinity correction factor for DO
measurements.
2. Press
ENTER
to accept or
EXIT
to cancel. If you select
ENTER
,
Setup 7 in DO mode changes accordingly.
4.7
Substances that May Affect Measurement
When measuring very low conductivity levels (< 2 µS), protect
the sample from gasses such as ammonia or carbon dioxide.
These gases cause rapid changes in the conductivity when they
dissolve into water. To avoid this problem, measure conductivity
using the Low Ionic Strength Chamber.
Pretreat water that is likely to contain high amounts of hydroxides
(boiler water) with Gallic Acid Solution. Untreated samples may
result in falsely high values. To pretreat the sample:
1. Add four drops of Phenolphthalein Indicator Solution to
the sample.
2. Stirring constantly, add Gallic Acid Solution until the
pink/red color disappears. The solution will become colorless
if a small amount of hydroxides are present, or it may turn
brownish-yellow if large amounts of hydroxides are present.
Adding too much Gallic Acid can increase the conductivity,
so add the minimum amount to achieve the color change.
4.8
Common Conversion Factors
The sens
ion
156 meter converts conductivity readings to TDS
and salinity values at the touch of a key. Table 9 lists more
conversion factors that may be useful.
Table 9 Conversions
To Convert From
To
Use This Equation
mS/cm
µS/cm
mS/cm x 1000
µS/cm
mS/cm
µS/cm x 0.001
µS/cm
µmhos/cm
µS/cm x 1
mS/cm
mmhos/cm
mS/cm x 1
g/L TDS
mg/L TDS
g/L TDS x 1000
