H72.0.01.6C-02
Operating Manual GMH 3431
page 7 of 14
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6 Principles of the measurements
6.1 Basics about conductivity
Definition of conductivity
: The ability of a material to conduct electric current:
γ
•
l
:
length of the material
A
:
diameter
R
:
measured resistance
Unit
γ
, common for liquids: and
The conductivity is the reciprocal value of the resistivity.
(The conductance is the reciprocal value of the measured resistance R)
6.2 Conductivity measurement
The conductivity measurement is a rather uncomplicated measurement. The standard electrodes are stable
for a long time if used correctly and can be adjusted by an integrated Cal-function.
Measuring ranges: 0.0 - 200.0 µS/cm
0 - 2000 µS/cm
0.00 - 20.0 mS/cm 0.0 - 200.0 mS/cm
If the range selection is set to „
Auto Range
“, the range with the best resolution is automatically selected. In
this case, the output value of the interface will always be the measured value with the highest possible
resolution (e.g. display value: 187.6 mS/cm
interface output: 187600.0 µS/cm).
6.3 Resistivity measurement
The resistivity is the reciprocal value of the conductivity and the device displays it in kOhm
•
cm.
Measuring ranges:
0.000 - 2.000 kOhm*cm
0.00 – 20.00 kOhm*cm
0.0 - 100.0 kOhm*cm
If the range selection is set to „
Auto Range
“, the range with the best resolution is automatically selected. In
this case, the output value of the interface will always be the measured value with the highest possible
resolution (e.g. display value: 18.76 kOhm*cm
interface output: 18.760 kOhm*cm).
6.4 TDS measurement
At the TDS (
t
otal
d
issolved
s
olids) measurement the filtrate dry residue is determined by means of the
conductivity and a conversion factor (C.tdS). Well suited for easy concentration measurements of e.g. salt
solutions. The determined value is displayed in mg/l.
Measuring ranges:
0.0 - 200.0 mg/l
0 – 2000 mg/l
If the range selection is set to „
Auto Range
“, the range with the best resolution is automatically selected. In
this case, the output value of the interface will always be the measured value with the highest possible
resolution (e.g. display value: 1876 mg/l
interface output: 1876.0 mg/l).
Displayed value TDS = conductivity [in µs/cm, nLF-temp. comp. at 25°C]
•
C.tdS (input at menu)
Approximately:
C.tdS
0.50
Monovalent salts with 2 ion types (NaCl, KCl, etc.)
0.50
Natural waters / surface waters, drinking water
0.65 - 0.70
e.g. salt concentration of aqueous fertilizer solutions
Attention: This are only approximate values – good for estimations, but no precise measurement.
For precise measurements the conversion value has to be determined for the corresponding solution for the
relevant concentration range.
This may be done by comparison with known reference solutions or by actually evaporating a certain amount
of solution with determined conductivity and subsequent weighing of the dry residue.
6.5 Salinity measurement
At the salinity measurement “SAL” the salinity (salt content) of seawater is determined (based on:
International Oceanographic Tables; IOT). Standard seawater has a salinity of 35 ‰ (35 g salt per 1 kg
seawater).
Commonly the measured value is displayed dimensionless in ‰ (g/kg).
Additionally the term “PSU” (Practical Salinity Unit) is sometimes used, the displayed value is the same.
