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Technical Note (National Research Council of Canada. Division of Building Research), 1958-05-01
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The Measurement of Surface Moisture - Second Progress Report
Sereda, P. J.
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https://nrc-publications.canada.ca/eng/view/object/?id=44eaa445-0d9c-482e-b935-6be9fc247085 https://publications-cnrc.canada.ca/fra/voir/objet/?id=44eaa445-0d9c-482e-b935-6be9fc247085
DIVISION OF BUILDING RESEARCH
NATIONAL RESEARCH COUNCIL OF CANADA
'fE
C
1HIN ][ CAlL
NOTlE
No.
2,50
NOT FOR PUBLICATION FOR INTERNAL USE
PREPARED BY PoJ o Sereda CHECKED BY APPROVED BY NBH
PREPARED FOR ASTM Committee B-39 Subcommitte VII
Task Force DATE May 19,58
SUBJECT The Measurement of Surface Moisture -Second Progress Reporto
Work on this project continued during the last year and was an extension of the work outlined in the Progress Report published in the ASTM Bulletiny February 19,580 It iss therefore, assumed
that the reader is familiar with that report and matters which were then dealt with in detail will now be referred to only0
ｾ
This further study was directed towards collectingadditional records to establish the sensitivity and performance characteristics of the two types of sensing elements, namely, platinum and steel, and platinum'and zinco
It has been found that the two types of elements are
comparable in sensitivity for the same shunt resistance when first put into service and at temperatures above freezingo At
tempera-tures below freezing the measured potential of the platinum and steel cell decreases with decreasing temperature as was indicated in the first progress report, whereas a similar decrease in measured potential is not observed in the case
of
the platinum and zinc cello Under corroding conditions, it has been observed that the ｳ･ｮｳｩｾtivity of the plat num and zinc cell remains constant for over six months whereas the platinum and steel cell shows generally a
decrease in sensitivity after a couple of months operationo
Therefore, it is now apparent that the platinum and zinc cell is the more reliable oneo
When measurement of surface moisture was begun by this method it was thought that the area under the potential-versus-time curves would describe more accurately the corroding conditions than simply the period in hours during which the generation of any potential occurredo After stUdying the records obtained during
..
';
,
supposing that the area under the potential-versus-time curve was anymore representative of the conditions of wetness on the panel than simply the time during which any potential was reoorded o Subsequent records were collected in terms of the time-of-wetness in hours which is defined as the time when the potential as
generated by the oell 1s in excess of 001 voltso
Following the above ｣ｯｮ｣ｬｵｳｩｯｮｾ an attempt has been made
to
simplify the instrumentation required for the-measurement of the ｴｩｭ･ｾｯｦＭｷ･ｴｮ･ｳｳｯ Some progress has been made in this regard and a prototype unit is being testedo This involves a simple transistor amplifier operating with the sensing elements and controlling a simple clook motor which drives a ｣ｯｵｮｴ･ｲｾ The counter indioates the elapsed hours of wetness o Continuousrecords of the time-of=wetness have been collected. at Ottawa for a number of successive months beginning in September and going through the winter and springo These records include the ｭ･｡ｳｵｲ･ｾ
ment of the potential at various sensitivities and using both types of cells and show that the skyward and the groundward
surfaoes are exposed to different periods of wetness in any given time o These differences vary from month to month and for this reason it is not possible to indicate the extent without giving all the datao Similarlyg these data reveal that precipitation alone contributes only a part of the total time=of-wetnesso
This part varies from month to month over the range of 25 to
75
per cent of the total recorded time=of-wetness which also varies from month to monthoSince time=of=wetness was being obtained as a continuous record it was decided to include the measurement of other factors affecting ｣ｯｲｲｯｳｩｯｮｾ ｮ｡ｭ･ｬｹｾ temperature and S02 concentration in the ｡ｴｭｯｳｰｨ･ｲ･ｾ and to attempt to correlate these factors against the rate of corrosion of mild steelo
, ConsequentlySi two sites 800 yards apart were set up at
ｏｴｴ｡ｷｾｯ The temperature of the panels and time=of-wetness were recorded continuously at one of the sites whereas S02- concentration and weight loss owing to corrosion of mild steel was obtained from each site once a montha It was believed that the temperature and
time=of=wetness were the same for both sites o The S02 concentration was found to vary as much as three fold at the two ｳｩｾ･ｳ and corres-pondingly the weight loss due to corrosion of mild steel varied as much as two fold at the two siteso
The above data will be submitted to analyses to see if any correlation exists between the rate of corrosion and the three major factor.e, ｮ｡ｭ･ｬｹｾ time-of-wetness, S02 concentration and temperaturei
This progress report is submitted for the purpose of informing the Committee members what is being doneo The author is conscious of the fact that the results 'of the experiments are not included but this was not possible because of the lack of time and the fact that the experiments have not been completed o
