Campbell Scientific CS547A-L Conductivity/Temperature Probe and A547A Interface - Manual (Page 18)

CS547A Conductivity and Temperature Probe and A547 Interface
6. Calibration
6.1 Conversion Factors
1 S (Siemens) = 1 mho = 1/ohm
Although mS·cm -1 and µS·cm -1 are the commonly used units of EC, the SI
base unit is S·m -1 . The result of the example programs is mS·cm -1
EC measurements can be used to estimate dissolved solids. For high accuracy,
calibration to the specific stream is required. However, for rough estimates,
values between 550 and 750 mg·l
-1 / mS·cm -1 are typical with the higher
values generally being associated with waters high in sulfate concentration
(USGS Water-Supply Paper #1473, p. 99). A common practice is to multiply
the EC in mS·cm
-1 by 500 to produce ppm or mg·l -1 .
6.2 Typical Ranges
Single distilled water will have an EC of at least 0.001 mS·cm -1 . ECs of
melted snow usually range from 0.002 to 0.042 mS·cm -1 . ECs of stream water
usually range from 0.05 to 50.0 mS·cm -1 , the higher value being close to the
EC of sea water (USGS Water-Supply Paper 1473, p. 102).
6.3 Factory Calibration
The CS547A is shipped with a cell constant calibrated in a 0.01 molal KCl
solution at 25.0°C ±0.05°C. The solution has an EC of 1.408 mS cm
-1 .
6.4 Field Calibration
The cell constant is a dimensional number expressed in units of cm -1 . The unit
cm -1 is slightly easier to understand when expressed as cm·cm -2 . Because it is
dimensional, the cell constant as determined at any one standard, will change
only if the physical dimensions inside the CS547A probe change. Error due to
thermal expansion and contraction is negligible. Corrosion and abrasion,
however, have the potential of causing significant errors.
A field calibration of the CS547A cell constant can be accomplished as
follows:
1. Make a 0.01 molal KCL solution by dissolving 0.7456 g of reagent grade
KCl in 1000 g of distilled water, or purchase a calibration solution.
2. Clean the probe thoroughly with the black nylon brush shipped with the
CS547A and a small amount of soapy water. Rinse thoroughly with
distilled water, dry thoroughly, and place in the KCl solution.
3. Connect the CS547A and A547 or probe and interface to the datalogger
using the wiring described in Section 4. Program the datalogger to make
the field calibration (see Section 6.4.1 if you have CRBasic datalogger or
Section 6.4.2 if you have an Edlog datalogger).
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