Bubble Point Testing and Micron Grade
The Micron Grade rating of a porous material is a general indication of the actual pore size since porous sintered metal materials have a wide pore size distribution. The Micron Grade Rating is a calculated value determined by a test method, known as the “Bubble Point Test”, which approximates the maximum pore size of a porous media. The bubble point test is not a highly accurate measurement of the over all pore size distribution, but serves as a reproducible, non-destructive test for ranking porous material grades by the largest pore size.
The maximum pore size of the porous media calculated from the bubble point test is not a direct indication of filtration performance, but the results can be correlated to more complex filter retention testing. The filter industry often refers to the diameter of the largest pore determined by the bubble point method as a ‘micron rating’. This terminology should not be confused with the diameter of a particle retained by a filter or the absolute filtration rating. The bubble point test method can also an excellent leak integrity test of a porous sintered metal assembly in order to determine if any larger pores were created during secondary operation processing such as welding or forming.
One of the most common pore size characterization methods is a ‘Bubble Point’ test which measures the pressure required to release the first bubble from the surface of a porous material submerged in a liquid and pressurized from one side. This Bubble Point test is an industry standard for estimating the maximum pore size of a material. A test piece is submerged in a wetting fluid such as isopropyl alcohol and a gas pressure until a bubble forms and breaks away from the porous surface. The liquid selected for this testing must have good wetting characteristics with relatively low viscosity and surface tension. Isopropal Alcohol is normally selected as the test fluid.
The Bubble Point Test Method is detailed in ASTM Standard E-128 and the International Organization for Standardization (ISO) test standard 4003. The bubble point test is based on Darcy's Law to correlate the measured bubble point pressure value to the calculated maximum pore size. This calculated maximum pore is then multiplied by a pore shape correction factor to estimate the filter rating, also known as the "Micron Grade" for rating our products.
Each industry can use a different shape correction factor, so there are differences in each manufacturer's specifications and bubble point ranges for assigning the "Micron Grade". Several different shape correction factors have been developed for calculating the maximum pore size values, depending on the type of porous material being tested since the nature of the pore changes for different materials. For example, a pore shape correction factor of 0.33 may be used for irregular shaped pores, whereas a correction factor of 0.25 might be used for more rounded pores.
|Chart of Bubble Point Pressure & Micron Grades|
|Traditional Bubble Point Pressure Mixed Units||Bubble Point Pressure in Inch H2O||Bubble Point Pressure in Inch Hg||Bubble Point Pressure in PSI||Bubble Point Pressure in mm Hg|
|0.05||10 - 13 " Hg||136 - 177||10.0 - 13.0||4.91 - 6.38||254 - 331|
|0.1||7.0 - 9.0 " Hg||95.2 - 122||7.0 - 9.0||3.44- 4.42||178 - 228|
|0.2||5.0 - 6.9" Hg||68.0 - 94.0||5.0 - 6.9||2.45 - 3.39||127 - 176|
|0.5||3.3 - 4.0 " Hg||40.8 - 53.0||3.3 - 4.0||1.62 - 1.96||76.2 - 99.0|
|1||2.2 - 2.7 " Hg||30.0 - 36.7||2.2 - 2.7||1.08 - 1.32||56.0 - 68.6|
|2||19 - 25" H2O||19.0 - 25.0||1.4 - 1.84||0.68 - 0.90||35.5 - 46.7|
|5||15 -18" H2O||15.0 - 18.0||1.1 - 1.32||0.54 - 0.65||28.0 - 33.6|
|10||8 -10.5" H2O||8.0 - 10.5||0.59 - 0.77||0.29 - 0.38||14.9 - 19.6|
|20||5.5 - 7.0" H2O||5.5 - 7.0||0.40 - 0.51||0.20 - 0.25||10.3 - 13.1|
|40||3.6 -4.6" H2O||3.6 - 4.6||0.26 - 0.34||0.13 - 0.17||6.7 - 8.6|
|60||2.5 -3.5" H2O||2.5 - 3.5||0.18 - 0.26||0.09 - 0.13||4.6 - 6.5|
|100||1.0 - 2.0" H2O||1.0 - 2.0||0.07 - 0.15||0.03 - 0.07||1.8 - 3.7|
|150||0.5 - 1.0" H2O||0.5 - 1.0||0.03 0.07||0.01 - 0.03||0.7 - 1.8|