Differential pressure transmitter debugging didn't return to zero?

Two differential pressure transmitter a display normal, another is for negative pressure; Two of the same line differential pressure transmitter, a normal display, another was displayed as negative pressure; Looking for our company to solve; And feedback the photos are as follows: from the point of view, the reason of the problem can't clearly; Advising clients to turn off three of two instrument valve group; Is displayed as follows: at this point, the basic can confirm that the left side of the table installed vertically; Lead to zero; So remove the watch case, after the zero below: at this time, the valve in the open, open, then close the valve in the high and low voltage, display is normal; Below: the installation of the differential pressure transmitter must be installed vertically, if the installation is not vertical, cause its not zero, the zero is zero, certainly shows no; Therefore, the correct approach is, regardless of whether or not the vertical installed, have suggested an artificial zero; In addition, the measurement of differential pressure transmitter is the top and the bottom of the storage tank pressure, due to the bottom of the tank in addition to the top of the tank pressure, and the pressure of the weight of the liquid; After high and low voltage offset, and the rest of the differential pressure is the weight of the liquid, so we need to strictly pay attention to the high and low voltage interface, application of high and low voltage switch, need to use the core program, non-professional workers do not automatic operation;