Bubble meters

Connect the tubing from the electronic bubble meter to the inset of the impinger/bubbler. 

Figure 2. For calibration, the cassette is attached to an electronic bubble meter as shown in the illustration.

Calibrate personal sampling pumps before and after each day of sampling, using either the electronic bubble meter method or the precision rotameter method (that has been calibrated against a bubble meter). 

The electronic bubble meter method consists of the following ... 

Press the button on the electronic bubble meter. Visually capture a single bubble and electronically time the bubble. The accompanying printer will automatically record the calibration reading in liters per minute. 

The precision rotameter is a secondary calibration device. If it is to be used in place of a primary device such as a bubble meter, care must be taken to ensure that any introduced error will be minimal and noted. The precision rotameter may be used for calibrating the personal sampling pump in lieu of a bubble meter provided it is (a) Calibrated with an electronic bubble meter or a bubble meter, (b) Disassembled, cleaned as necessary, and recalibrated. It should be used with care to avoid dirt and dust contamination which may affect the flow, (c) Not used at substantially different temperature and/or pressure from those conditions present when the rotameter was calibrated against the primary source, (d) Used such that pressure drop across it is minimal. If altitude or temperature at the sampling site are substantially different from the calibration site, it is necessary to calibrate the precision rotameter at the sampling site where the same conditions are present. 

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. 

The needle valves of Fig. 1 are operated choked, so that there is a constant flow independent of downstream resistances. However, the valve shown upstream of the bubble meter is very important. It is adjusted so that the pressures at the switching valve and upstream are hardly perturbed by a switch. The flows of both mixtures are adjusted to be identical by the needle valves and the bubble meter. When all is properly regulated, the ball in each rotameter makes a short, quick ( ls) excursion at the moment of the switch, and identically for the return to the first position of the valve. 

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. 

Nitric oxide at about 50 ppm compressed with very pure nitrogen in gas (flinders is provided for this purpose, and the true concentration is established by comparison with that of a compressed-gas tank that can be obtained from the National Bureau of Standards, as a standard reference material. The nitric oxide meter is calibrated repeatedly at several concentrations of nitric oxide, and the mass flow meters are recalibrated often with absolute bubble meters. 

All pneumatic parameters, including spht ratios, can be set from the keyboard to ehminate routine use of the bubble meter. 

Of the RAS parameters, the gas flow rate has the greatest effect on volatility and must therefore be carefully controlled (Deibler et al., 2001). The flow rate of the effluent should be periodically measured to ensure flow rate consistency. The flow rate can be measured with a simple bubble meter or an electronic flow meter. Be sure the meter used is appropriate for measuring the magnitude of the anticipated flow rate. 

Figure 1. Schematic diagram of the tubular reactor system. (1 gas cylinder 2 rotameter 3 reactor 4 furnace 5 temperature controller 6 gas chromatograph 7 bubble meter)...

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

Bubble meters are used calibrate laboratory meters that have low flow rates. The gas passes through a tee at the bottom connected to a balloon filled with soapy water. The balloon is squeezed to force the water into the neck and then a bubble forms as the gas passes through the liquid. The bubble ascends in the body of the vessel and passes the graduated marks. The volumetric flow rate is determined by recording the time the bubble takes to pass two graduated marks. Remember that reference conditions to calculate the volume displacement correspond to the pressure and temperature of the bubble meter. 

The uncertainty in the volumetric flow rate is twice as high when measuring the time at 400 ml versus 800 ml. Note that the effect of humidity has been ignored in these calculations. If a bone dry gas is fed to the bubble meter, the soapy water will have a tendency to humidify the gas the volumetric flow rate will increase in proportion to the humidity and could be as large as 2%. 

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. 

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