Figure 12-2. Mercury porometer. 



porosity of a sample, is to divide the dry weight of the specimen by an assumed 

 matrix density to find the grain volume. Matrix densities that are in reasonable 

 agreement with true densities are 2.65 for sandstones, 2.72 for limestones, and 

 2.86 for dolomites. 



(2) Measurement of Grain Volume By Gas Displacement 



The extracted and dried sample is placed in a steel chamber that is filled 

 with gas to a specific pressure. The gas in the chamber is then expanded into 

 a burette at atmospheric pressure and measured — or, as an alternative, into an 

 expansion chamber and the new pressure in the system measured. The volume 

 of gas, or system pressure, obtained by expansion from the sample chamber 

 containing the specimen is compared to the same measurement with the sample 

 chamber empty. The volume of gas displaced, or the increase in system pressure, 

 due to the sample is a measure of grain volume. For rapidity of measurement, 

 calibration curves are prepared. Instruments (fig. 12-3) operating on this 

 principle are available commercially, or can be constructed at relatively low cost. 



(3) Measurement of Grain Volume By Pycnometer 



The sample need not be extracted or dried for this procedure. A specimen 

 is broken from the core and the bulk volume determined by any of the procedures 



236 



