Pressure gauge diaphragm issue
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A problem was identified on site: when installing a pressure gauge on a gasketed valve, the diaphragm of the plastic case of the pressure gauge came into contact, which affected the pressure reading. When does a large diaphragm appear, and when does a small diaphragm appear?
Seal gasket, press it onto the diaphragm! Why are you being rude like this?
Figure 1 shows a diaphragm pressure gauge; to isolate the medium and prevent corrosion, a larger diaphragm is used to increase the load-bearing area, and it is generally connected using flanges. Figure 2 shows a diaphragm pressure gauge, used for measuring low pressures; it can be connected via threads or flanges. Figure 3 shows the lined valve, which is of relatively poor quality
For pressure gauges with mechanical pressure transmission, the lower the pressure to be measured, the larger diaphragm is required； For pressure gauges with liquid-filled pressure transmission, the larger the measurement chamber, the larger diaphragm is required. A mechanically-operated pressure gauge relies on the pressure of the medium to push a diaphragm, causing it to move and thereby driving the pointer. Since the force generated by the diaphragm = medium pressure × diaphragm area, when the same displacement (thrust) is required, a larger diaphragm is needed when the medium pressure is low, while a smaller diaphragm is sufficient when the pressure is high. Although such pressure gauges can use the principle of force balance to reduce the displacement of the diaphragm, they are often divided into different ranges, with diaphragms of varying sizes used for each range. In pressure gauges that use liquid as a pressure medium, the liquid contained varies when the elastic element changes. This change is accommodated through diaphragm displacement. For the same volume change, the large diaphragm only needs to bend slightly, while the small diaphragm needs to bend significantly in order to adapt. Since the stress generated by the bending of the diaphragm affects the forces acting on the elastic element, it is possible to use a smaller diaphragm in some cases, but manufacturers usually opt for diaphragms of the same size.
In any case, the diaphragm must not be compressed
The size of the diaphragm’s corrugations depends primarily on the pressure to be measured; the dimensions of the standard gasket (the inner diameter of the flanges) also need to be taken into account. The first image clearly shows an unreasonable design – the sealing surfaces on both sides of the diaphragm are too narrow, which means that the gasket will inevitably compress the corrugations on the diaphragm