In the past many methods have been used to measure the flow rate of a gas. One of these methods, which has been made portable, consists of a flow tube, for which the volume is known very accurately between two fixed points which are provided with sensors, a frictionless piston made up of a soap film, and a base unit to which the flow tube is attached, including means for recording the time of travel of the frictionless piston between the two points. Generally, a reservoir of soap solution is provided near the base of the flow tube. A film of the soap is applied to a "bubble maker", a generally circular ring which is suspended above the reservoir. Gas filling the flow tube causes the soap film to move off of the ring and to proceed through the flow tube, passing the sensors one at a time. The time of travel of the film, or "bubble", between the sensors defining the fixed volume or critical volume area is proportional to the rate of flow of the gas through the flow tube. Initial "wetting" of the sidewalls of the flow tube has been taught as a method for reducing the friction acting against the film as it is carried up the tube by the gas flow. To that end, the prior art has taught that it is preferable to allow several film bubbles to traverse the flow tube and burst prior to sensing the passage of the film and measuring the flow rate.
Numerous devices which have been developed in the past, one of which is illustrated in U.S. Pat. No. 4,860,590 issued to Buck, use optical sensors to detect when the soap film crosses the fixed points. The systems may further include a microprocessor based timing circuit which receives the detection signals from the sensors and records the time at which each sensor was passed, thereby yielding the time that the bubble spent traversing the critical volume area of the tube. The microprocessor in the base unit can additionally calculate the rate of flow of the gas by dividing the fixed volume of the tube by the measured time required for the soap film to pass through that volume. Thus, the flow rate of the gas is computed based on the fixed volume of the flow tube and the time required for a bubble to traverse that volume. Devices of this type typically claim accuracies of +/-0.5%.
Sources of error are inherent in the systems which have been used in the past. One source of error, particularly associated with mass-produced meters, is the inability to readily establish the volume of the flow tube. There is great difficulty in maintaining strict tolerances, and the cost of manufacturing flow tubes with precise dimensions having specifically placed sensors can be prohibitive. Absent strict manufacturing standards however, one cannot be guaranteed that a given flow tube has the exact intended volume between the sensors once it is assembled with the base unit. In most of the prior art metering systems, the inexact volume of the tube is compensated for by moving the sensors along the height of the tube until the volume readings indicate the desired preset volume value. In the above-referenced Buck patent, there is taught a means by which the volume of an individual flow tube can be approximated and stored by the microprocessor and switches associated with the base unit once the meter has been assembled. The Buck system provides a gas having a known flow rate through the assembled meter and calculates the flow rate for that gas as it passes through the flow tube' s fixed volume, using the stored value of the "ideal" volume of the tube in the calculation (i.e., the volume which the flow tube was manufactured to contain). The calculated flow rate is then compared to the known flow rate and any deviation is assumed to be due to a difference between the ideal and the actual volume magnitude of the flow tube. The difference in the magnitudes is then compensated for by adjusting the stored value of the volume to reflect the actual volume of the flow tube. The stored value is adjusted by DIP switches (representing a matrix of volume values) which are coupled to the microprocessor. Thereafter, the microprocessor will calculate the flow rate using the actual measured volume of the flow tube which is the combined value of the ideal volume stored in the microprocessor of the base unit and adjustments as stored the DIP switches coupled thereto. This latter Buck system does not require that the sensors be moved during the calibration, which is preferable given the sensitivity of the sensor components and the alignment challenges attending the placement of same. However, in the Buck system, any time that a different flow tube is provided to the base unit, re-calibration and consequent re-adjustment of the DIP switches is required. What is desirable is a means to precisely determine the volume of the flow tube and digitally record this value in nonvolatile memory associated with the tube itself rather, that the base unit, thus allowing for exacting flow rate calculations without maintaining exact machining dimensions and without the necessity of repositioning system components, thereby reducing manufacturing and assembly concerns and costs while still achieving a high accuracy with the flow device.
Another source of error in gas flow meters is the generation of bubbles, soap films introduced into the flow tube in the absense of gas flow. This can result when a film of the soap has been applied to the bubble maker and the ring comprising the bubble maker has substantially the same diameter as the flow tube. Mere contact with the sidewalls of the flow tube may result in transfer of the soap film to the flow tube, which can effect sensing of "flow" in the absense of any gas flowing in the tube. Furthermore, the presence of the film in the tube will necessarily affect the next intended measurement.
A further design inefficiency of gas flow meters involves the length of the flow tube, beyond the critical volume area between the sensors. Prior flow meter designs have had difficulty in completely bursting the soap film after it has traversed the critical volume area of the flow tube. Typically, the soap film is allowed to proceed to an upper surface, beyond the critical volume area, at which the film essentially collapses or bursts with the soap solution then running back along a given path to the reservoir below. A difficulty is encountered when the films, or the so-called "bubbles", don't collapse or burst completely, resulting in a multitude of small malformed bubbles at the upper end of the tube. Known designs encourage the formation of small bubbles since the bursting surface at the upper end of the tube generally is joined to the upper sidewalls of the tube itself. The build up of smaller bubbles can result in excessive flow of bubbles and/or liquid along the sidewalls of the flow tube and consequent interference with the sensors. The incomplete bursting problem therefore necessitates leaving considerable time between measurements so that the small bubbles have sufficient time to eventually dissipate before their presence can interfere with the sensors and the subsequent measurements. Increasing the time between successive flow measurements, to allow the small bubbles to burst themselves, can be time consuming and therefore unacceptable to many users of the flowmeter. An alternative solution, which has been adopted by many flow meter manufacturers, is to increase the length of the flow tube from the critical volume area, at the later encountered sensor, to the upper end of the tube at which the film is to be burst. Due to the portability of the device, it is evident that the ability to fully burst the bubble at a controlled point would desirably reduce the size of the flow tube and the time between successive measurements.
Another problem is particularly associated with the prior art flow meters which utilize optical sensors. Many of the prior art portable flow meters which incorporate optical sensors have difficulty operating in varying ambient light conditions. A large amount of background light, such as the condition which exists when operating out of doors or near an outside window, causes the optical sensors to erroneously fail to detect a bubble passing through the tube. Although prior art references, such as the Buck patent, teach that the use of infrared sensors can eliminate the problem, infrared sensors are nonetheless sensitive to the infrared components of ambient light which can adversely affect the operation of the sensors. It is therefore desirable to have a method of filtering the infrared portion of the ambient light, to an acceptable level for the infrared emitter detector pairs which are incorporated into the flow meter device.