For porous materials such as soil, for example, the method involves to saturate the sample with deionized water by using the capilary rise methodology. This procedure should be carried out for 24h or 48h. After the complete saturation of the sample you should evaluate the amount of water by the gravimetric method. The gravimetric water content, obtained by the relation between the water mass and the dry sample mass (48h in an oven), should be multiplied by the bulk density (g/cm3) to measure the volumetric water content. This last property has the same value of the porosity (cm3/cm3) of the porous material.
As explained in the previous contribution, the sample should be saturated with demonized water by capillary rise. If you really want to be sure not to entrap air bubbles, the imbibition should be performed in a desiccator where a vacuum should be maintained. But the tricky part is the measurement of the bulk volume of the porous material. if your sample has a geometrical shape (cylindre, cube, ..) you can calculate it easily. Other wise you need to measure the hydraulic weight Wh of the sample i.e. weight of the sample when completely immersed in water, and the weight of the water saturated sample Ww . The bulk volume is then calculated by the difference Ww-Wh
The total porosity is then (Ww-Wd)/(Ww-Wh), where Wd is the dry weight of the sample
The sample is hard and has dumbbell shape. Could you please explain more?
dumbbell shape isn't exactly the easiest to calculate the volume, therefore once your sample is saturated with water you keep it immersed and hang it underneath the balance. Many balance have a hook underneath to allow this kind of measurement.
See attached drawing
Do you absolutely need using water ? I'm measuring porosity with use of gaz. It is much faster and more accurate since you are never sure to saturate sample with water. Do not hesitate to contact me for more information.
If I need to determine the porosity of a hydrophobic polymer, Can I use this method with water?
I'm not sure water will work on hydrofobic material. But a methodology using gaz instead of water would work perfectly. If you have a sample, I could evaluate my method on my équipements.
as suggested by Fabrice, with hydrophobic material it is probably safer to work with a gaz method, like with Helium pycnometer to measure the real density of your material. But you will still need to measure the apparent density, which can be easy if you cut the sample in a geometrical shape for which the volume is easily calculated.
If the shape of the specimen is not perfect, It is possible to use a volumeter to determine the envelop volume of the specimen. I'm ready to perform the measurement for demo.
thanks for all of your sugestions
The samples are in a square form, but unfortunately, I don't have a Helium pycnometer, so I thought it would be easier for me to use water.
If you use a tension table, Richard Chambers you can use the capillary equation to estimate de radius of the biggest pore that remain filled with water after a water pressure head has been applied to the sample. Although in the capillary theory you calculate the radius of a capillar, at the scale of soil pores it is normally called "equivalent radius" a rough approximation of real pores. Normally the capillary equation can be approximated for most cases ..calculating r=0.149/h (h is the water pressure head applied in cm of water and r is the radius of the equivalent pore in centimetres.
if you know the real density of your material, you can just calculate porosity of your sample by measuring its bulk density (mass/volume). Porosity is derived as follows : porosity (%) = 100*(1 -( d bulk/dreal))
That is true if you only need a total porosity the easy way is to calculate it by the equation given by Claude. If you want to say pore size distribution then you should use mercury porosimeter or soil water retention curve and capillary equation
Hi Claude, could you help me by describing for me a simple procedure to determinate concrete porosity. by using tap water under normal pressure.
Thank you
Regards
Aissa
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