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Appendix:
Conductivity Theory
XL20, 30, 50 and 60 meters
The salinity scale which ranges from 0 to 90 ppt is a measure of all salts, not just sodium chloride.
This scale was originally devised for seawater, and is based on seawater at 15 degrees
Centigrade has a conductivity equivalent to that of a potassium chloride solution of a known
concentration. This solution (0.44 molal) is defined as having a salinity of 35 ppt.
Note:
ppm = parts per million, ppt = parts per thousand, 1000 ppm = 1 ppt
The total dissolved solids scale approximates the ppm TDS in surface waters by multiplying the
conductivity of a sample by a factor, 0.66.
Some users prefer the use of resistivity units to describe their water, particularly where high purity
water is involved. The unit most often used to describe resistivity is megohm•cm., which is simply
the reciprocal of conductivity (µS/cm). The chart below shows the relationship between these
units.
Conductivity, µS/cm Resistivity, megohm. cm
0.056 18.0
0.1 10.0
1.0 1.0
2.5 0.4
10.0 0.1
Conductivity Measurement
accumet conductivity electrodes consist of glass or epoxy bodies in which platinum or platinized
sensing elements are fixed. Typically, each electrode has two such sensing elements and are
designated two-cell electrodes. The previous discussion has focused on this type of electrode.
Four cell electrodes are also available, and the theory and application of these are in a separate
section.
These sensors contact the solution whose conductivity value is sought. The exact cell constant of
the electrode must be determined prior to measuring the sample. In essence, this is
accomplished by fixing the nominal cell constant of the electrode into the meter, and recording
the observed conductivity value associated with a standard conductivity solution (usually a KCl
solution) with a precisely known value. The following calculation yields the actual cell constant.
k
= standard value of solution (µS) / observed value of solution (µS)
Fortunately, the accumet XL20, 30, 50 and 60 meter automatically does this calculation for you by
touching Standardize.
To produce an appropriate current signal for the meter it is important to choose an electrode with
an appropriate cell constant. The following table lists the optimum conductivity ranges for
electrodes with cell constants of 0.1, 1, and 10.
Cell Constant Optimum Conductivity Range, 2-cell
0.1
0.5 to 200 µS/cm
1.0
0.01 to 2 mS/cm
10.0
1 to 200 mS/cm
Prior to use the electrodes should be conditioned in distilled or deionized water for at least
10 minutes or in accordance with the manufacturer’s instructions.
Содержание XL 20
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