In known pressure cells or gauges of this kind the rear cell wall is made of the same or similar solid insulating material as the metering diaphragm, e.g. of silicon or glass in order to obtain largely constant measuring values upon temperature variations, without any superimposed intrinsic spurious signals. The connection between the metering diaphragm and the rear cell wall is established by a fixing material consisting of a gold/silicon eutectic or low melting glass solder. Epoxy resins are also used as fixing material. The fixing procedure takes a long time and is extremely expensive. The surfaces to be bonded together must be smoothed very accurately. Consequently practically no gap is formed between the parts to be joined which normally would attract solder by capillary action when being heated. Therefore, during the soldering process or during the curing of the epoxy resin, the parts to be joined must be moved towards each other rapidly by ultrasonic energy so as to obtain a surface area connection between the front end face of the cylinder barrel and the rear cell wall. If the pressure cell is to be used for reference measurements and the rear cell wall is provided with a passage for communication with a reference pressure fluid, it may further prove necessary to evacuate the interior of the pressure cell during the soldering or curing of the epoxy resin so that the fixing material can be drawn in better from the outside towards the inside. This procedure is complicated still further because, after establishing the connection, the parts must be cooled free of stress so as to avoid any basic deformation of the metering diaphragm since this would impress a spurious signal from the very beginning. However, once all the operating steps have been accomplished perfectly, a high quality pressure cell is obtained which is largely free of hysteresis and can be used also for relatively high pressures or pressure differences (e.g. from 0.1 to 1000 bar), yielding measuring values of great accuracy and largely invariable by temperature influences.
In many cases of application, such as in electronic sphygmo-manometers (non-invasive blood pressure instruments) or for measurements on carburetors of internal combustion engines, and the like, the pressures to be measured lie just a little above or below normal atmospheric pressure, differing from the same by as little as something like .+-.0.5 bar, and the measuring accuracy required is not very high.
USPTO Backgrounds
USPTO Backgrounds