Error in Measuring Differential Pressure Using DPT


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I am trying to measure the differential pressure drop past a tube bundle. The water flow is at saturation temperature, and the boiling takes place in the tube bundle. The differential pressure readings are positive when the water is at room temperature. However, when the liquid temperature rises and boiling occur in the tube bundle, the pressure starts to show negative pressure. I can't really understand what's happening in the process.
Please make a sketch of the section of the process with physical placement of the connection points.

Are there impulse tubes or remote seal capillaries?

Dry or wet legs?

Actual temperatures over the course of the process?

magnitude of DP being measured? (and units?)
Steam pressure is greater than than the water supply pressure so the downstream low side DP port senses a higher pressure than the upstream water pressure at the DP's high side port; so DP goes negative. DP = High side minus low side.