Miles D. LaHue, Ronald J. Teck, Geza Dombi, and Herman D. Axelrod
Notional Center for Atmospheric Research1 Boulder, Colorado 80302
I An Improved Design for SOUP-Bubble Flowmeters
I
Flow-rate measurements are often necessarv. .. in gas chromatography, gas dilution systems, atmospheric air sam~ling,and limiting orifice calibration, to mention a few.-perhaps the most common device employed for this purpose is the soap-bubble flowmeter. It is inexpensive, easy to operate, and has good accuracy. Several soap-bubble flowmeters have been d e ~ i g n e d . ~ Typically, the flowmeter is constructed from a simple graduated huret from 10 to 50 ml in volume with a small rubber squeeze bulb at one end for the soap solution reservoir (Fig. 1). Flows ranging from a few ml/min to a couple of hundreds of ml/min can he measured quickly and accurately. One needs simply to record the length of time necessary for a single bubble to travel through a specified( volume as determined by thegraduation marks on the flowmeter. The flow rate or gas velocity is then computed by dividing the volume by the elapsed time required for the bubble to travel that volume. T o realize the accuracy that can he achieved with the soap bubble flowmeter, several prerequisites must be met. The walls of the flow tuhe must be completely wetted with the soap solution prior to a measurement. If this is not done it is often impossible to form a bubble and, in any case, the bubble will soon distort and eventually hreak before the measurement can be completed. The formation of a layer of foam in the soap reservoir must be avoided, since this can both interfere with the formation of the bubble and hinder its progress up the flow tube. Also, foam on the walls within the measuring volume can alter the volume, thus giving erroneous results. Finally, i t is desirable that only one well-formed buhhle be in the bubbles flow tube during a single measurement. Multiwalled hubblers are difficult to read accurately and, by their very geometry, produce erroneous results; we believe this phenomenon is due to the bubble itself actually displacing a small part of the volume it is measuring as it passes u p the flow tube. Although accurate flow-rate measurements of more than 200 ml/min are difficult to achieve with the usual 50-ml buret, they can readily be obtained using a 250-ml buret. However, these larger flow-rate meters exhibit several idiosyncrasies which make them difficult to use. Foam is readily formed in them; it is difficult to form a single buhhle in the flow tube; and an unusually large rubber bulh is necessary to contain a supply of soap solution sufficient to wet the inside of the flowmeter walls. A few minor modifications to the standard flowmeter desim will avoid the frustration and loss of time that so often accompanies the use of the larger (e.g., 250 ml) flowmeter. The reservoir A (Fig. 2) holds a quantity of the soap solution which is used to wet the walls of the flowmeter. Wetting the walls is accomplished by holding the flowmeter in a horizontal position and rotating it along its horizontal axis. The rubber bulb C holds an additional 'The National Center for Atmospheric Research is sponsored by the National Science Foundation. 2Nelson, Gary 0.."Controlled Test Atmospheres," Ann Arbor Science Publishers, Inc., Ann Arbor, Michigan, 1971, p. 28.
quantity of soap solution which is used for the bubble formation. The sidearm B is %-in. I. D. tubing, which allows excess soaD in the flowmeter to return to the rubber hulh. Without provision for this return, the soap solution in the rubber bulh could he d e ~ l e t e dduring use and eventually allow air to he sucked back into t i e rubber bulb; the inevitable result would be the undesirable formation of foam. It would be very difficult to fill the rubber hulh with the soap solution without the sidearm. The small bore of this sidearm allows the soap solution level to be raised in the gas intake arm D, so that a bubble can be formed and at the same time excessive flow u p the sidearm B back into the reservoir A can he avoided. The dashed "level line" indicates the relative location of this arm required to achieve the correct soap-solution level. The gas intake arm D is slightly enlarged a t E and the slope is fairly steep; this allows any soap solution to drain back immediately into the rubber bulb before it could cause additional bubbles to form. We believe that these modifications reduce or eliminate most of the inherent problems encountered in using the large diameter soap-bubble flowmeters. These modifications should allow one to enjoy the convenience of using these larger bore flow meters without worrying about some of the usual problems associated with them.
Flgure 1. Typical soapbubble flowmeter.
Figure 2. Modified soap-bubble flowmeter.
Volume 50, Number 12. December 1973 / 867
