Factory made standard cellular glass for thermal insulation is assumed to have 100% closed cells like described in the standard EN 13167. Indeed, the mu-value is not allowed to be lower than 40000. Measurements on materials with such a high mu-value easily take 3 months, which means that this method cannot be used for daily quality control.
On the other hand, the gas – pycnometer is a nice and simple instrument to measure the volume of the closed cells. If this volume is in line with the geometric volume, we know that the cells are closed and that the mu-value will be above 40000. Because this method takes a few minutes, it is the ultimate instrument for QC in factories where open (acoustic ) and closed cell material (thermal insulation) is produced.
The following paper describes nicely the principle of the measurement.
An over pressure in the sample chamber is released into the (calibrated) reference chamber. The pressure before and after release, together with the known volume of the reference chamber allows to calculate the volume of the sample chamber. If this volume is also known, we can calculate the volume of the sample. From this sample volume, we can calculate the amount of closed / open cells.
The instrument can be quite simple like shown here under:
For about 2000€, this instrument can be built and connected to a PC. It is not clear why the above commercial instrument costs about 16000€. For larger samples, a pycnometer with compressor in situ is also available on the market like shown in this leaflet.