SPECIFICATION Porosimeter and methods of assessing porosityThis invention relates to porosimeters and to methods of assessing porosity.
Awidely accepted standard apparatus and method for testing porosity isto be seen in ASTM F 316-80.
The object ofthe test is to gain information about the maximum pore size and the distribution of pore sizes in a porous material under test. The material is saturated with test liquid, held in a holder and subjected to progressively increasing pressure of a test gas. The initial breakthrough of gas through the wetfilter is noted by a bubble point detector and thereafter the relationship between pressure applied andflowthrough the material is observed using a pressure gauge and a rotameter downstream of the material and directly impelled by the flow of gas.
This test method is highly dependant on the manual skill and dexterity and the intellectual ability of the operator requiring as it does the simultaneous operation of the inlet pressure regulator and observation of the pressure gauge and the flow rate. Thereafter these readings haveto be converted manually by drawing graphs and these are then interpreted.
Despite these drawbacks this method is one of the accepted methods oftesting porosity.
We have examined this and have in the present invention provided substantial improvements in various respects and in particularfromthe point of view of avoiding reliance upon manual skill.
In our apparatus and method we use a pressure gauge and flow meter only in an initial calibration which need not be done in the presence ofthe material to be tested. We further site our flow-rate sensor upstream ofthe material to be tested. In operation, we take both pressure and flow-rate readings directly from the respective sensors to an automatic recorder which draws mechanically the output in the form of a graph, or mayfeedthem to, for example, an integrating computer.
The siting oftheflow-rate sensor upstream of the material to be tested is of great importance since that sensorworks ata higher pressure on that side of the test material than it would on the downstream side and furthermore the positioning avoids any contamination ofthat sensor by the test liquid being sweptfrom the material. In the American Standard, aliquid trap is provided behind or downstream of thematerial but is nottotally successful in preventingaccess of liquid into the subsequent stages of the apparatus which therefore can affect the accuracy of the flow meter.
We further improve the process by standardising the test liquid. Those mentioned in the AmericanStandard are water, petroleum distillate, de-naturedalcohol or mineral oil. We find that various characteristics of volatility, surface tension or reactivity will not allow any one of those materialsto be used over a wide range of materialsto betested. We have selected a liquid which is ofthewidest possible applicability, having very low surface tension and vapour pressure and in particular very low reactivityforthefibrous materials which are likely to form the materials under test. This material is known as Fluorinert (RegisteredTrade Mark) which is recommended by its makersMinnesota Mining and Manufacturing as a cooling liquid for electronic components and devices.The preferred Fluorinert liquid is known as FC43 having a nominal boiling point of 1 74 C, a viscosity of 2.6 cs, a vapour pressure at 250C of 0.3 mmHg and a surface tension atthe same temperature of 16 dynes per cm.
Chemically, the liquid is a clear colourless perfluorcarbon fluid.
To relieve the operator further of the need for manual intervention we may provide a motorised drive for the inlet flow regulatorwherebyto achieve a linear increase of pressure applied to the sample at a pre-selected ratewhich could be determined by the nature ofthe test material butwill usually be independent of it, at least within a certain ranges of such pore sizes.
In the test method, it is preferred to take the wet curve first in a single sample holder and then repeat the run with the same sample in the same place to obtain the "dry" curve. In this way it is certain that an identical sample is giving the two curves (a comparison between which gives the necessary data) and that no contamination orthe like will enter the system as a result of its being opened up between the test. TheASTM assumesthatthe dry test will be taken first or else that two samples will be in the system.
The method ofthe invention may also include a calibration step performed before subjecting the sample to the test and which consists in running the gas through the system and first calibrating the pressure scale of the recorder against a pressure gauge coupled into the line and then disconnecting the pressure input to the recorder and calibrating the flow-rate scale against a flow meter coupled into the line. The calibration may include the steps of returning the pressure and flow back to zero and recalibrating the zero ofthe recorder, for as often as is necessary.
However, neither the pressure gauge nor the flow meterwill be used in normal operation during running of the tests.
In the accompanying drawings:Fig. 1 is a diag rammatic viewtaken from ASTM F316-80ofthetestapparatusas recommended and used until the present invention; andFig. 2 is a similar diagram ofthe apparatus according to the present invention; and Fig. 3 is a sketch perspective view of the apparatus as mounted for use.
Fig. 1 shows a pressure source 1 with a pressure regulator 2 operated by a manual control 3. The pressureoutputfromthe regulator is seen on a gauge 4 and passes to a sample holder5 between the two parts of which a sample is mounted, the sample having been moistened by any one of water, petroleum distillate, de-natured alcohol our mineral oil, all of a specified characteristics.Following the passage of gas through the sample is an oil trap 6 and the duct is initially coupled up afterthatto a so-called bubble point detector 7, wherein output if any being caused tobubble through liquid orto a rotameter8which is directyimpelled bytheflowofliquid.Theinitial breakthrough of gas through the sample is noted by observation ofthe bubble pointdetectorwhich sets a zero for a graph which isthen drawn by manual correlation ofthe pressure gauge against the flow rate downstream ofthe sample as measured by the rotameter 8.
In the present invention in contrast, as seen in Figs. 2 and 3, a pressure regulator 20 such as a Schrader regulator preceded by a filter 21 between it and a source of compressed gas 22, usually compressed air is driven from a synchronous motor (not shown) through belting orother drive train anta ratchet clutch 23. It can be returned to zero by manual intervention on the part 24 ofthe clutch nearer to the regulator20. Next in the line 25 leadingfromthe regulator are two pressure gauges 26,27 which can be brought into communication with the line through respective valves. The pressure gauges are used for calibration only and they are sensitive in different ranges of pressure, only one being used for any given calibration. Next in the line are transducers 28,29 for pressure and flow-rate respectively.The pressure transducer may be of any known type but we have used one ofthe stressed metal film (also known as bonded foil strain gauge) type which gives an output to an XY recorder 30 having a pen 31 drivable overgraph paper32 in either or both of the X Y directions inaccordance with respective inputs.The other input isderived from aflow-ratetransducer29 and we preferto use a thermal mass flow meter since it is devoid ofmoving parts. Itcan be seen that these devices aresensing both pressure and flow-rate in the ductingupstream of the sample 31 which is mounted in a standard holder 32 and which optionally may be followed bythe standard liquidtrap 33 and buy a two-way valve34which can divert flow either to a standard bubble point detector 35 orto standard flow meters 36, 37. Which of these two is connected to the line is selected by respective valves.The flow meters 36,37 which are of the typewhich is impelled by the flow of gas are used only in calibration of the device.
The bubble point detector is however used to manually note the initial point ofthe breakthrough of the gas at which time a tick or similar mark is made manually on the graph paper in the recorder 30 as a safeguard for extra accuracy in determining the origin of the curve which will be obtained.
In operation, calibration is first carried out without a sample in the holder. Gas is run through the system at a pressure similar to the maximum which will be expected to be used in the following test. One or other ofthe pressure gauges 26,27 is selected and the outputfrom pressuretransducer28only is fed to the recorder 30. The correlation between the position of the pen andthe reading on the pressure gauges made and if necessary thezeros are then calibrated, with recalibration atthe high pressure and soon.Similarly, calibration of the flow-rate is carried out by taking the output from flow-rate transducer 29 only to the recorder and comparing the other reading ofthe pen with the reading achieved by one ofthe two directimpelled flow meters 36 or37 which may be appropriate to the expected maximum flow during the test.
When calibration has been achieved the pressuregauges and theflow meters are switched outofthe system.
The valve 34 is switched oversothatthe bubble detector is in the circuit, a standard sample which may be for example a filter paper, a sintered micro4ilter, a blotting paper, a geological material or any othermaterial of which it is wished to knowthe porosity, is saturated in Fluorinert FC43 and placed in the holder.
The automatic drive 23 is coupled to the pressure regulator so there is an automatic and predetermined increase in the pressure applied to the sample and the test is run, achieving a wet curve graph. The testis continued until that line becomes substantially straight, showing substantially complete drying ofthe sample. The pressure is then returned to zero and the run repeated with the samplestill in the holderto obtain the dry curvewhich issubstantiallyastraight line. Thetwo curves are plotted on the same piece of graph paper 32 automatically by the recorder 30 and are then removed for thenecessary interpretation.
Itis apparentthat oncethe outputsignals have been reduced to an electrical form as they are here, the resultscouldbetakenouttomeanssuch asa computer which would interpret them immediately in terms of for example pore size and distribution.
It can be seen thatwe have considerably improved and rendered more reliable the method and apparatus proposed intheASTM by removing very largely the reliance on manual dexterity and skill, by rearranging the circuitry shown inthatASTM in orderto improve the performance and reliability ofthewhole and by standardising ourtestliquid used to wet the material under test.