Calcium carbide/Speedy Test vs Gravimetric's……......................... Moisture Analysis done properly.
Russell Rafton
Director at DRYFIX PRESERVATION LTD Yorkshire's Leading Damp and Timber Specialists / Director Coleman Diagnostics
Calcium Carbide vs Gravimetric's…….Moisture analysis done properly.
During a recent survey I needed to take a sample from a wall to aid diagnosis and determine exactly what was happening to the wall beneath the current finish. As such, I removed a small section of the finish material and extracted a sample from the base of the wall via drilling.
The quickest and easiest way for me to establish the walls condition is to undertake a calcium carbide test also known as a speedy test. This process involves mixing a known weight of sample with a reagent in a sealed vessel.
The reagent reacts with the moisture in the sample and produces acetylene gas proportionate to the amount moisture in the sample material. The vessel is sealed and therefore the build-up of gas is measured thus, determining the moisture content. ?It’s an easy way to accurately determine the walls condition when compared to electronic measurements as it’s quick, simple and accurate.
The only problem is the disruption caused taking the sample, the waste material which is harmful and needs to be disposed of carefully and most importantly of all interpretation of the results! Yes, don't be fooled so easily.
If you’re not familiar with the calcium carbide test / speedy test you can read all about it in this article;
The problem is the Speedy test will only determine the Total Moisture Content (TMC) of the material sample, that’s a combination of ‘Free Moisture’ caused by rising, lateral, penetrative damp and leaks etc, and the ‘Hygroscopic Moisture’ content caused by contamination. This is moisture loading of the material from by airborne moisture / water vapour which is increased when the material becomes contaminated with hygroscopic (moisture absorbing) contaminates.
A calcium carbide meter/speedy test cannot differentiate between the two values and simply provides the Total Moisture Content value (that’s Free Moisture + Hygroscopic Moisture = Total Moisture).
The problem is the results can be misleading!
The results from my onsite calcium carbide test yielded a moisture level of 5.7%.
So, the question is, is the wall damp?...............
Well, The British Research Establishment Digest Paper 245 suggests “If the found moisture content (MC) is less than 5% at the base it is unlikely that the wall is severely affected by rising damp"
Our results were 5.8% MC so is the wall wet, or just a little damp?.....
Well, if you were only assessing the structure using the speedy test you’d be forgiven for coming to the conclusion that the wall is a little damp and I’ve seen many times walls diagnosed as 'damp' by surveyors using speedy meters and electronic meters when in fact they’re dry.
One of the most common enquiries at my office is for intrusive sampling using a calcium carbide test. This is normally requested by chartered surveyors or legal beagles as evidence for dispute cases. For the reasons mentioned above however, I always advise against this procedure as a calcium carbide test may not always provide accurate results.
So back to my survey. Well, this was in-fact a period building, an 18th century barn conversion which had recently been developed. The new homeowner just months into their occupation was now experiencing problems with damp staining throughout which was causing concern and inevitably meant friction and a possible claim on the developers.
In this instance I wanted to be sure that the advice I provided my clients with was 100% accurate, so I took another sample from the same wall which was taken from site and processed in our laboratory using the gravimetric analysis procedure.
?As stated in the BRE guidance document 245 “Rising damp in walls “Diagnosis and treatment†electrical moisture meters have a valuable role to play as preliminary surveying instruments and will identify areas where further investigation is required. To obtain more conclusive proof regarding the condition of the wall and source of any dampness the most satisfactory approach is to take samples of masonry and plaster for laboratory analysis.
The gravimetric analysis procedure is widely regarded as the most accurate method for determining the true moisture content of a sample and is the only method of an analysis which is capable of differentiating between 'Free Moisture' and 'Hygroscopic Moisture'. The process involves decomposing samples through various process’s and analysing mass.
If you’re not familiar with the gravimetric analysis procedure you can read about it in this article;
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Furthermore, if you’ve never taken samples for analysis here’s an article explaining all about moisture mapping and how and where to take samples:
Anyhow, after completing the gravimetric procedure the results of our analysis actually showed the wall was dry, very dry In-fact. The wall had a free moisture value of just 1.1%. The wall was however heavily laden with hygroscopic contaminates with a hygroscopic value of 4.1% thus, resulting in a Total Moisture Content of 5.2% (very close to the initial calcium carbide result).
Chemical salt analysis of the sample also yielded moderate amounts of Nitrate contamination, moderate Sulphate contamination and high levels of Chloride contamination.
So, now you've established the calcium carbide test and the gravimetric test are very similar when measuring ‘Total Moisture Content’ however, hopefully this has demonstrated just how confusing the results from a carbide meter can be, potentially misleading the surveyor into thinking the wall is damp, when it is in-fact dry!
I hope you found this article useful and informative and if you have any questions or queries about these subjects please don't hesitate to get in touch.
Happy surveying
Russ
If you would like to learn more about moisture mapping or damp diagnosis you may?find the following articles useful:
Rising Damp Explained
The great Damp Illusion -
Industry Perception Study -?
Russell Rafton - Dryfix Preservation Ltd Senior Surveyor
Dryfix Yorkshires Leading Damp & Timber Specialists
A 6 x Industry Award Winning Company with the Property Care Association.
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Advising construction professionals on waterproofing design, leaks and disputes.
1 å¹´Nice one Russ. thank you and thank you for processing some samples for me last month!!
Director: Lansdown Associates Chartered Surveyors and Property Consultants
1 年Fantastic article…. Another CPD ticked off
Damp and building defect investigations & repairs England
1 年unfortunately most people that use the speedy don’t really understand how to use it, so make up the diagnosis ??