How to measure Pressure with respect to Atmospheric Pressure using a Differential Pressure Transducer?

I understand that connecting the pressure source to 1 port of the differential pressure transducer and leaving the other port open to atmosphere will allow me to measure the pressure with respect to atmospheric pressure. However, since atmospheric pressure is changing all the time and therefore the pressure sensor diaphragm cannot be at zero, I do not get a STABLE ZERO Baseline. How do I achieve a stable zero baseline?