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4.4 Interpreting the readings
In most practical situations where measurements of surface salt contamination
are required, it is unlikely that the precise ionic make-up of the contamination
will be known.
The Elcometer 130 SCM400 is calibrated against known concentrations of the
most common constituent of atmospheric contamination, namely sodium
chloride.
However, the specific resistivities of other contaminating salts do vary slightly
from that for sodium chloride and could give lower or higher readings.
To determine the accuracy of readings when the contamination does not
consist entirely of sodium chloride, test measurements have been made on the
instrument with salt mixtures which are known to be present on atmospherically
contaminated surfaces:
The results are compared with sodium chloride (Figure 4) in terms of Actual
Salt Level vs Salt Level Indicated on the Elcometer 130 SCM400.
This shows that the difference between readings for these salts and for sodium
chloride are acceptably small, particularly at levels of most interest, i.e. those
below 10 µg.cm
-2
.
Simulated Marine Contamination
BS 3900: Part F4 (Salt Spray) Simulated Urban Contamination
76.8% w/o sodium chloride
9.6% w/o magnesium sulphate
6.9% w/o magnesium chloride
3.2% w/o calcium chloride
2.1% w/o potassium chloride
0.8% w/o sodium bromide
0.6% w/o sodium bicarbonate
41% w/o ammonium sulphate
37% w/o sodium sulphate
12% w/o sodium chloride
10% w/o sodium nitrite
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