Soap-bubble meters

We measured flow rate with a Varlan P/N 29-000086-00 Soap Bubble Meter. The pressure gauges were Heise bourdon-tube type with 0.1 psla divisions. The column was 16 feet of 1/4 inch copper tubing packed with Carbowax 20M loaded to 20% on Fluoropak 90. 

Soap-bubble meter A device for measuring gas flow rates in gas chromatography. 

Flowmeters.— The measurement of gas-flow rate is usually carried out with a soap-bubble meter although other methods involving rotameters and capillary manometers are available. The soap-bubble meter is an inexpensive, easy to operate, direct method which has good accuracy. 

The exit gas stream was cooled to almost 0°C using the ice bath, and most of the water vapor was condensed. The remaining gas was metered using a soap-bubble meter. 

The cell containing a homogeneous membrane of known thickness is pressurised with a chosen gas. The extent of gas permeation through the membrane is measured by means of a mass flow meter or by a soap bubble meter. More sophisticated set-ups employ a calibrated volume connected to the permeate side with the small pressure increase in the calibrated volume being measured with a pressure transducer. The gas permeability or nermeabilin- coefficient P can be detennined from the steady-state gas flow if the membrane thickness t is known, since... 

Fspiit can be measured by a soap bubble meter, while Fcoi can be found by measuring tm and knowing the column dimensions. 

With capillary columns, we have many different flows to measure with extremely large differences in the range of flows. Capillary columns may have flows lower than 1 mL/min, yet have septum purge flows in the 2-5 mL range and splitter vent flows in the 100-300 mL range. It is preferred to measure capillary column flows in terms of linear gas rates. It is acceptable to measure the other flows with electronically controlled flow measuring devices or soap-bubble meters. Since the split ratio for an analysis is determined by dividing the flow of the column into the vent flow volume from the splitter, we normally do both of these flows electronically. Any other flows that are to be measured are not critical and either manually or electronic measurement is acceptable. 

The flow rate through a capillary column whose inner diameter is less than 0.53 mm is difficult to measure accurately and reproducibly by a conventional soap-bubble meter. Instead, the flow of carrier gas through a capillary column is usually expressed as a linear velocity rather than as a volumetric flow rate. Linear velocity may be calculated by injecting a volatile nonretained solute and noting its retention time, tM (seconds). For a capillary column of length L in centimeters. 

Calibrate the detector tube pump for proper volume measurement at least quarterly. Simply connect the pump directly to the bubble meter with a detector mbe in-line. Use a detector mbe and pump from the same manufacturer. Wet the inside of the 100 cc bubble meter with soap solution. For volume calibration, experiment to get the soap bubble even with the zero ml mark of the buret. For piston-type pumps, pull the pump handle all the way out (full pump stroke) and note where the soap bubble stops for bellows-type pumps, compress the bellows fully for automatic pumps, program the pump to take a full pump stroke. 

FIGURE 12.10 A homemade soap bubble flow meter constructed from an old Mohr pipet, a piece of glass tubing, and a pipet bulb. 

It is obvious that the flow rate must be precisely controlled. The pressure from the compressed gas cylinder of carrier gas, while sufficient to force the gas through a packed column, does not provide the needed flow control. Thus a flow controller valve is built into the system. The flow rate of the carrier gas, as well as other gases used by some detectors, must be able to be carefully measured so that one can know what these flow rates are and be able to optimize them. Flow meters are commercially available. However, a simple soap bubble flow meter is often used and can be constructed easily from an old measuring pipet, a piece of glass tubing, and a pipet bulb. See Figure 12.10. With this apparatus, a stopwatch is used to measure the time it takes a soap bubble squeezed from the bulb to move between two graduation lines, such as the 0- and 10-mL lines. The commercial version uses an electronic sensor to measure the flow rate based on the bubble movement. See Workplace Scene 12.3. 

In this study, we used the modified Wicke-Kallenbach cell which is tubular membrane cell type. Permeation measurements were performed in the 293K-373K, Oatm-Satm range for H2, N2, CO2 and CH4. Feed gas and retentate gas were controlled by MFC(Mass Flow Controller, Tylan Co.) and BPR(Back Pressure Regulator). Permeate gas flux was measured by soap bubble flow meter, MFM (Mass Flow Meter, Teledyne Co.) and wet gas meter. Especially, MFM was used to measure kinetics of membrane permeation. Separated and retentate gas composition was analyzed by on-line GC(HP 5890 II, TCD type). Helium was used as carrier gas and sweeping gas. Temperature was detected by RTD(Hanyoung. Co.) at inlet, inner cell and skin of cell. Pressures were detected by pressure transducers(Deco Co.) at inlet and permeate part. 

Figure 31-2 A soap-bubble flow meter. (Courtesy of Varian, Inc., Walnut Creek, CA.)...

How does a soap-bubble flow meter work ... 

Bubble flow meters are the simplest laboratory flow meters, suitable particularly for the calibration of the other instruments at very low flow rates. They are based on the determination of the velocity of the motion of soap bubbles driven by the gas measured, in a calibrated tube. They can be easily prepared from commonly used pipettes. An advantage is that... 

---