There are many instances when an indication of a flow rate of a gas needs to be determined. For example, in patients (e.g. mammals such as humans) that have respiratory issues such as chronic obstructive pulmonary disease (COPD), sleep apnea, etc., it is often required to provide assistance in filling their lungs with air, or inhalation. There exist devices that are interfaced to a patient's airway for providing such assistance by injecting a positive airway pressure towards and into the patient' airway, thereby assisting that patient with inhalation. In such devices, it is sometimes needed to determine when the patient is inhaling or exhaling, based upon the flow rate and direction of flow in such devices that are interfaced to the patient.
One type of device for providing such assistance is a Continuous Positive Airway Pressure (CPAP) device as described in, for example, U.S. Pat. No. 4,944,310. Continuous Positive Airway Pressure devices generally provide a gas pressure that is slightly greater than ambient air pressure into the patient's airway. Continuous Positive Airway Pressure devices work well for certain patients, but patients that have poor lung capability often find it harder to exhale due to the constant added pressure directed into the patient's air passages by the Continuous Positive Airway Pressure device. This is because the Continuous Positive Airway Pressure device continues to provide positive air pressure, even while the patient is exhaling.
Bi-level Positive Airway Pressure devices address this issue of exhalation as described above by detecting when the patient is exhaling and reducing the positive airway pressure until the patient completes exhalation and starts inhalation. In such, there are two different positive airway pressures delivered (hence bi-level), a higher positive airway pressure while the patient inhales and a lower positive airway pressure (e.g., atmospheric pressure) while the patient exhales.
To accomplish the bi-level positive airway pressure delivery, Bi-level Positive Airway Pressure devices of current have electrical transducers that senses when the patient is exhaling and an electrical circuit that receives an electrical signal from the transducers and responsive to that signal, modulates the positive airway pressure between two values. For example, U.S. Pat. Pub. 20140150793 describes such a Bi-level Positive Airway Pressure device that has a flow sensor connected to a controller. This device has a blower for providing the positive airway pressure. Upon detecting that a patient is exhaling, the controller sets the blower to operate at a lower speed (or off), thereby reducing the positive airway pressure until the patient stops exhaling, at which time the controller detects the end of the exhalation and restarts the blower.
The above-described Bi-level Positive Airway Pressure devices are known to function well, especially with patients that have very little lung capacity. Unfortunately, many such patients are not limited to bed rest and wish to be mobile. It is known to provide the pressure component for positive airway pressure by a portable device, typically portable Continuous Positive Airway Pressure (CPAP) devices. Such devices typically derive the pressure component for positive airway pressure from a small battery operated pump or through a compressed gas cylinder (e.g. air, oxygen, etc.). It is possible, especially if made small and light enough to be carried by the patient. The sensors, the connections to the sensors, and the added electronics make portability hard to accomplish, especially if a compressed gas tank is utilized. Further, the issues related to battery charge maintenance become an issue. Further, due to the electronic components, power supplies, etc., it is difficult to dispose of such a device, making single-use devices out of the question. Therefore, such devices are not envisioned as to be sold as disposable devices.
This being said, there are many medical as well as non-medical needs for detecting the flow of a gas such as air through a passage such as through a pipe or duct. Since there are many situations in which there is little impedance to such gas flow (e.g. in a heating/air conditioning duct system), gas pressure is not a good indication of flow or flow rates.
What is needed is a device that detects a gas flow.