TM8400B-L-00
1 November 2004
3. OPERATING DATA
3.1. MEASUREMENT THEORY
The instrument permits measurement of the electrical conductivity ratio of saline samples at a
controlled temperature. Once measured, the conductivity ratio can be used to determine salinity
through the use of a conversion table or the application of a mathematical equation.
Low pressurized air forces the saline sample from the sample bottle and through the sampling
element which is called a conductivity cell. The sample passes as a continuous flow through the
conductivity cell. Electrodes implanted in the cell initiate signals that are proportional to the
sample's conductivity. Using an internal preset electrical reference to produce an error signal, the
instrument provides a numerical readout, which corresponds in magnitude and direction to the
error signal. The display reading provides a valid measurement value when the internal
reference has been preset, or standardized, against a known external reference. Standardization,
using IAPSO recognized sea-water standards, is accomplished by operating the instrument as for
a routine conductivity ratio measurement with the standard sea-water and making adjustments as
necessary to the instrument so that the reading corresponds with the known conductivity ratio for
that particular standard seawater.
In practice the instrument is calibrated so that the displayed reading is a 2 to 1 conductivity ratio
of the measured sample to standard sea-water. The resolution of the instrument is such that the
last digit is approximately equivalent to 0.000 2 PSU at 35 PSU. At this level of accuracy,
fluctuation of the last digit by up to five counts (±0.001 PSU is of minor significance to the total
reading.
In preparation for measurement of the conductivity ratio the following two functions are initiated
by the operator:
(a) Temperature selection - to enable the sample temperature to stabilize at a precise preset
level prior to the conductivity ratio measurement;
(b) A flushing action - to enable systematic purging of the conductivity cell, thereby ensuring
that no residue from the previous sample remains. Residue from the previous sample
would degrade sample purity. Not more than 100 ml is required to measure any sample,
starting from fresh water in the conductivity cell and including flushing volume. Only
50 ml is required if the difference in salinity from the previous sample is not greater than
3 PSU.
Due care must be exercised in preparation and handling of samples in order to ensure accurate
results. For instance, a single drop of rainwater in a sample bottle, or mere hand contact
introducing salts from skin moisture into the sample water, may be enough to cause measurable
errors in salinity.
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