This invention relates to diaphragm pumps operated by gas pressure, particularly but not necessarily exclusively for medical services.
U.S. Pat. No. 4,319,570 discloses a fluid-pressure operated diaphragm pump to be used as an aspirator or tracheal suction pump. It has a pumping chamber in part defined by a flexible diaphragm, the flexure of which creates a suction in a drainage line. In a further chamber separated from the pumping chamber by a secondary sealing diaphragm is a valve mechanism comprising inlet and outlet valve members which are opened and closed in opposition to each other as the diaphragm reciprocates, the opening of the outlet valve being triggered by the flexure of the diaphragm to a position of maximum pumping chamber volume and the opening of the inlet valve being triggered by a return spring that is operative when the diaphragm flexes to a position of minimum pumping chamber volume. The valve mechanisms required in this pump are relatively complex. They therefore carry an increased risk of malfunction in addition to being relatively expensive to produce. Cost is particularly important if the valve is to be disposable, as is often required for surgical apparatus.
Another pressure-fluid operated diaphragm pump is disclosed in U.S. Pat. No. 4,662,829 in which pumping and driving chambers are separated by a flexible diaphragm and the driving chamber is connected continuously to a pressure air source. The flexible diaphragm is stretched over an exhaust port of the driving chamber during the contraction of the pumping chamber, by virtue of the pressure differential between the pumping chamber and the exhaust line, until the continuing flexure of the diaphragm under the driving fluid pressure forces the port open. At that stage, the pressure in the driving chamber is released via the exhaust port, the diaphragm collapses onto that port, and the cycle begins again.
Although this pump has a relatively simple mechanism as compared with the previously described example, it has inherent disadvantages. In particular, in order for the diaphragm to maintain a seal with the driving chamber exhaust, a portion of its area must be held stationary during the pumping stroke despite the fact that the deflection of the diaphragm that is lost thereby reduces the pumping rate. It is possible to compensate for this to a limited extent by employing more flexible diaphragm materials but the risk of overstressing and rupture of the diaphragm is then increased, and such measures cannot make up for the limitation of the pumping rate for any given size of pump due to the fact that part of the diaphragm is not participating in the pumping stroke.