EXPLAINED: Moisture Content Analysis using the Gravimetric Process

Gravimetry is basically the quantitative technique which measures mass or a change in mass during an analytical process. For example, we’ve probably all unknowingly undertaken this ourselves when stepping on scales during a dieting regime or when in training. Effectively we’re measuring our own body mass and comparing this weight loss or gain to a prior measurement, this is basic gravimetry.

The gravimetric technique is used across many industries measuring mass for specialist applications using a variety of different process. The process involves separating an element of a compound so the mass change can be measured. This can be undertaken simplistically either at the start or end of a process or incrementally during the analytical process to determine the mass changes at intervals. Separating a compound for gravimetry can be undertaken using a number of methods such as:

Volatilisation gravimetry – Separation using thermal energy, often used in food industries

Particulate gravimetry – Separation using filtration or extraction, often used in water industries

Electro gravimetry – Separation by adsorption or ion exchange, often used in the metal and precious metal industries.

Our interest in determining the moisture content of masonry however is focused on volatilization & thermal gravimetry.

Volatilisation & Thermal Gravimetry

Using this method there are two generally accepted practices in which the mass of a sample can be determined using thermal gravimetry, these are the indirect and direct methods. 

The indirect method

This process involves decomposing a sample by heating up, using energy to remove the volatile part of the sample (in our scenario the volatile is water). With the sample weighed prior and after we can determine by the mass changes, the water content of the sample. In this circumstance, the water content is the difference between the samples initial mass and its final mass.

The Direct Method

Another technique to achieve the same outcome is by using the direct method. This method which is also a thermal process involves weighing and then heating the sample. Again, the volatile is removed however the volatile is now captured in an absorbent trap and weighed directly.

Both techniques provide the same result, however, the indirect measures the mass of the sample before and after to determine weight of the volatile which has been lost. The direct method captures and measures the volatile itself. As the indirect process is far easier and requires much less equipment, this is normally the favored approach.

Below is a graph which demonstrates a wet sample of mortar being decomposed as it's exposed to different temperatures over a few hours and weighted intermittently. As you can see, as the temperature increases, the weight of the sample decreases, this is due to volatile evaporation. Basically, the sample is becoming lighter in weight as water is evaporated. Note how as the temperature increases between 35 – 40 degrees how the weight loss decreases dramatically.

So how can Gravimerty be useful to damp analysis?

The gravimetric process is widely accepted as the most accurate process for establishing the true moisture content of a material sample and determining its moisture composition as described in BRE 245 ‘Rising damp in walls: Diagnosis and Treatment’.

With building materials gravimetry becomes a little more complex when we consider how the total moisture content of the material is made up. A materials total moisture content is the sum of its capillary moisture often referred to as free moisture introduced from issues such as rising damp, penetrating damp and water leaks. The other part is its hygroscopic moisture content which is the materials ability to absorb moisture from the air, often increased through contamination. No other process of analysis other than gravimetric testing has the ability to decompose a samples total moisture content and separate between hygroscopic and free moisture. As such gravimetric analysis is the most accurate and reliable.

Most building materials are hygroscopic and their moisture content can be influenced by the surrounding atmospheric conditions. This uptake or loss in moisture content will however be balanced with the environment, this is referred to as, at equilibrium. Building materials can also become contaminated with salts which can also be hygroscopic, allowing the material to take on additional moisture from the air. When masonry materials become heavily contaminated, they may never be truly dry as a proportion of their moisture will always be attributed to contaminates. Often in high humidity conditions these contaminates can be seriously problematic resulting in walls and plaster appearing damp / saturated even if the wall is dry and the only available source of moisture is airborne.

During the gravimetric process it is possible to determine not only its mass difference between a wet and dry state but also its hygroscopic uptake i.e. contaminates which contribute to its total moisture content by subjecting the sample to different environmental conditions and measuring its mass in stages. This is referred to as the materials hygroscopic moisture content.

The images below show the difference between two separate samples of masonry. Both have been dried and removed from a scientific oven. Each sample has been left on a balance for a period of hours to measure its moisture uptake as the samples reach equilibrium with the environment (which in this circumstance was 20c @ 70% RH). 

Sample 1 is dry with no contaminates. Sample 2 was also dried, although, is contaminated with hygroscopic salt. You will note the increase in mass in the second sample (contaminated) is far greater than that of the first. This is due to the hygroscopic effect.

Sample A - Dried sample with no contaminates.

The first image shows the sample weight kiln dried directly from the oven at 16.783g

The second weight is taken after the sample has been exposed to an environment of 70Rh @ 20 degrees. Note the weight change is minimal to 16.786g a total weight change of 0.003 grams which is equivalent to an increase in mass of 0.017%

Sample B - Dried sample with contaminates.

The first image shows the sample weight kiln dried directly from the oven at 15.522g

The second weight is taken after the sample has been exposed to an environment of 70Rh @ 20 degrees. Note the weight change has increased to 15.558g a total weight change of 0.036 grams which is an equivalent mass increase of 0.213%

During the gravimetric process after initially weighing the samples in their potentially wet condition, by exposing the samples to a stable environment of 75% RH (Relative Humidity) at 20 degrees, there will be sufficient water activity for the hygroscopic salts to retain water. This is basically engaging the contaminates as highlighted in the example above. The mass of the sample can then be measured again, and an equation used to determine its percentage hygroscopic moisture content in relation to it’s dry condition.

The 75%RH recommendation was chosen by the BRE (British Research Establishment) as a bench mark for gravimetric analysis on masonry however, from my research and knowledge there appears to be no real scientific answer as to why 75% RH, it’s just a bench mark. To quantify a samples hygroscopic uptake there must be sufficient atmospheric water to engage the contaminates and the environment to which samples are subjected must be stable and constant. 75% RH is a relatively stable and easy environment to maintain at room temperature using salt solutions. In addition, there is sufficient water activity to engage the salts, therefore I suspect this was the main consideration for the bench mark.

It is possible to expose the samples to higher RH levels using different solutions however, they’re not always as stable and easy to maintain. The RH exposure however is irrelevant, it's about creating a stable environment wet enough for hygroscopic contaminates to uptake airborne moisture. Samples exposed to conditions above 75%RH the mathematical equation would still result in the same percentage of hygroscopic moisture from the sample, just different values with uptake or loss in mass? 

So now you understand the gravimetric process how is this knowledge of benefit to a damp surveyor?

Like with any method of analysis one sample on its own isn’t much use however, when a wall is profiled vertically like how you would use an electronic moisture meter we’re collecting lots more information allowing us to determine exactly how ‘wet’ a wall is and what contribution to this wetness is owed to hygroscopic contaminates. In addition, by profiling we can also determine quantitively where the wettest areas of the wall are and exactly where the contaminates are deposited.

The image below shows a cross section diagram of a wall we recently profiled using the gravimetric method.

As you can see the moisture content of the structure varies dramatically between floor and ceiling level. The profile of damp migrates vertically from the base of the wall diminishing between the heights of 1.8m – 2.0m with moisture levels varying between 12.1% - 4.2%. There is a significant spike in the hygroscopic moisture content of the structure between the heights of 0.6m (8.5%) – 1m (7.9%) -, this is where the salt contaminates are deposited within the wall. 

Obviously, this is a pretty wet structure although having this information within our knowledge bank allows us to quantitatively understand how wet and contaminated the structure is, so we can advise our clients regarding action required accordingly.

Many thanks for taking the time to read my article and i hope you found it useful. If you have any questions regarding the gravimetric process or other diagnosis techniques, please don’t hesitate to get in touch.

Russell Rafton – Dryfix Preservation Ltd - Director / Senior Surveyor

Dryfix Preservation, Yorkshires Leading Damp & Timber Specialists.