Rapid Determination of Very Small Gas Flows A Soap-Bubble Method LESLIE SILVERhIAN
AND
ROBERT 51. THO1\1SON, Harvard School of Public Health, Boston, ;\lass.
I
S PRODUCIXG very small gas concentrations by the dilution method, and for other purposes, it is necessary to meter small volumes of gas a t low rates (1 to 10 ml. per . minute). The usual gas-displacement measuring devices require long periods of time or the use of very small gasometers when measuring these small flow rates. The following method eliminates the above difficulties, is simple to use, and requires no elaborate equipment. It may be used to calibrate needle valves, capillary flowmeters, or other indicating devices. The principle of the method is the formation and measurement of soap bubbles measured a t a capillary tip. The apparatus necessary consists of a small capillary tip, a 3-way stopcock> stop a magnifying glass, and an nated The latter Was simply a piece of thin Crosssection ruled paper mounted on a cardboard frame and illuminated from behind by a 15-watt lamp. The arrangement of apparatus is shown in Figure 1.
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Calibrated
'-'"' Volume Syringe Measurements No'
Mean Diameter .Urn. 4 12.1 6 15.5 4 17.8 5 20.2 4 22.2 del,iation from mean, lo,2%, rDa 011 ~ o l u m eof sphere = -.6 Of
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1.00 1.96 2.94 3.90 5.00 Average a
Based
Calculated5 Volume
Ratio of Calculated t o Actual Volume
M1. 0.91 2.25 2.92 4.34 5.76
0.91 1.15 0.99 1.11 1.15
the volume of the small spheres (less than 2 mm.) at the is negligible in comparison to their final volume. If a large &meter is used initially, both measurements should be made. For calibration purposes a series of five or more readings is made for each flow and averaged. The time for each reading is small-for example, at 7 ml. per minute each reading took about 32 seconds. Thus a series of readings can be obtained in a short time. Care should be taken to have the measuring station free from cross drafts and air currents which cause the bubble to oscillate. A cardboard screen can be placed around the measuring station.
A comparison of actual volumes with calculated volumes is shown in Table I. A calibrated 5-ml. syringe was used for the source. The average deviation from a sphere for the five bubble sizes measured is about 10 per cent. This difference is due to errors in diameter measurement, departure from sphericity, and errors in setting syringe volumes (hand adjustment was employed). I n use, volumes can be determined by measuring the diameter or by calibrating the measuring st'ation by means of a syringe. Resistance "measurements showed that the back pressure created by the bubble itself was less than 2 mm. of water. I n order to reduce the back pressure produced by capillary action on the tip walls below 12 mm. of water (before the bubble is formed), the tip should be 1.5 mm. or more in diameter . Measurements can be carried out using larger openings, but the departure from sphericity is greater. Flows as high as 2 to 4 liters per minute were measured using a small funnel (30-mm. opening). Bubbles as large as 75 mm* in diameter can be formed. For these measurements, a calibration of the bubbles using their horizontal diameter was made by means o f a 1 0 0 - ~ l . syringe. Repeated measurementson given volumes using the syringe Were found to check Within per cent, when the horizontal diameter was measured. However, because of the large departure from sphericity direct calibration for large bubbles is recommended-that is, the measuring station should be calibrated first by using a SYringeAll the measurements in this study were made using air or nitrogen a t room conditions (760 mm. of mercury and 25" C.) ; however, the method should be applicable t,o other gases.
CAPILLARY TP
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TABLE I. COMPARISOE OF ACTUAL WITH CALCULATED VOLUMES
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FIGURE 1. APPARATUS In order to measure flow or calibrate flowmeters the desired
flo~vposition or manometer reading is set with air by-passing the
capillary tip. A small beaker (5-ml. is adequate) is then filled with soap solution and brought in contact with the tip. As soon as the capillary tip has drawn up the soap solution, the small beaker is removed, The soap solution may be freshly prepared from commercial castile soap or a permanent mixture used ( I ) . Both soap solutions employed gave comparable results in measuring the flov of gases, or mixtures of density close to that of air which are relatively insoluble in soap solutions. With soap solution in the capillary tip the stopcock is turned towards the tip for flow measurement. The instant the bubble begins to form the stop watch is started and then stopped at any desired size measurement. Bubbles as large as 25 mm. in diameter are easily formed. The bubbles were found to be very stable (1.5to 2 minutes) and elastics. A bubble could be formed by a syringe filled with air and collapsed by evacuating it. This could be repeated three times and all diameters checked for the same syringe air volume. Timing the bubble just as it starts to form was simpler and gave results comparable to the exact method which consists of starting the watch at a definite diameter, stopping it at another, and then calculating the volume from differences in sphere volumes. Since the volume of a sphere varies as the cube of the diameter,
Literature Cited (1) Hodgman, C. D., and Holmes, H. N., "Handbook of Chemistry and Physics", 25th ed., p. 2389, Cleveland, Ohio, Chemical
Rubber Publishing Co., 1941-42. work was done on a project supported by a grant from the Industrial Hygiene Foundation of America, Inc.. Pittsburgh, Penna.
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