Thread Content
Abstract: Zero-point migration can be divided into three types: no migration, negative migration, and positive migration. The positive and negative pressure chambers are connected to the pressure measurement points at the lower and upper parts of the container respectively, with the positive pressure chamber being at the same level as the zero liquid level； The pressure transfer tube connecting the negative-pressure chamber to the pressure measurement point at the upper part of the container is filled with the same gas as that above the liquid level in the container. Zero-point shift can occur in three ways: no shift, negative shift, and positive shift; these will be explained below: 1. No shift. As shown in the figure above, the positive and negative pressure chambers are connected to the pressure measurement points at the lower and upper parts of the container, respectively, with the positive pressure chamber being at the same level as the zero liquid level ； The pressure transfer tube connecting the negative pressure chamber to the pressure measurement point at the upper part of the container is filled with gas at the same pressure as that above the liquid level in the container. Since the density of gas is much lower than that of liquid, the static pressure difference between the pressure measurement point and the negative pressure chamber is very small and can be ignored. Let the pressures acting on the positive and negative chambers of the differential pressure transmitter be P+ and P-, respectively. Then: P+ = P0 + Hg, and P- = P0. Therefore, △P = P+ – P- = Hg. It can be seen that when H = 0, △P = 0; in this case, the differential pressure transmitter is not subjected to any additional static pressure ； When H=Hmax, △P=△Pmax. This indicates that the differential pressure transmitter does not need to be relocated. II. Positive transfer. When installing a differential pressure transmitter in practice, it is often not possible to ensure that the transmitter and the zero-level are at the same level. As shown in the figure above, assuming that the pipe connecting the negative pressure chamber to the pressure measurement point at the top of the container is filled with gas, and ignoring the static pressure generated by this gas, the pressures acting on the positive and negative pressure chambers of the differential pressure transmitter are respectively P+ = Hg + hg + P0 and P- = P0. Therefore, △P = P+ – P- = Hg + hg = Hg + C. It can be seen that when H = 0, △P = C; thus, the differential pressure transmitter experiences an additional positive pressure difference. To make its output I > 4mA. To make I = 4 mA when H = 0, it is necessary to find a way to eliminate the effect of C. Since C>0, positive migration is required. III. Negative transfer. As shown in the figure above, when the gas in the space above the liquid in the container is condensable, such as water vapor, or when the medium being measured is corrosive, isolation fillers filled with an isolation fluid are often installed between the positive and negative chambers of the differential pressure transmitter and the pressure sampling points, in order to maintain a constant height of the liquid column acting on the negative-pressure chamber. Let the density of the isolation fluid be ρ2. Then P+=h1 g+H g+P0 and P-=h2 g+P0; therefore, △P = h1 g+H g–h2 g = H g–B, where B = h1 g–h2 g. It can be seen that when H=0, △P = –B