The issue of pressure test differential pressure

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The project I’m working on recently encountered pressure difference issues during pressure testing, which has left me quite puzzled. I would like to ask for your help in resolving this problem: Testing system: instrument air, general-purpose air; Testing pressure: 1.903 MPa; Pressure stabilization time: 1 HOUR. Since the pressure gauges have not arrived yet, two pressure gauges with a range of 0–6 MPa were used instead! During the pressure testing, they are placed at the highest points, with the lowest point at the bottom. The problem now is that the pressure reading at the high point is consistently lower than that at the low point, but during my last pressure test, the high point ended up being lower than the low point. The pressure difference is 0.14 MP. DNV says it’s because of the pressure gauge? I don’t know how to pull it either right now. I hope the experts here can give me some advice!