pressure transmitter

Generally speaking, a "simple" pressure transmitter is measuring the differential pressure between the process pressure and atmospheric pressure (this is "gauge" pressure). In this respect, a simple pressure transmitter is a differential device.

Differential transmitters are often used for measuring relatively small differences between two process pressures, both of which may be far away from atmospheric pressure - e.g. the pressure drop between the inlet and outlet sides of a venturi, which can then give a measure of flow.

Provided the pressure range you want to measure is within the differential range of the instrument, you can measure "simple" or gauge pressure by just leaving one side of the transmitter open to atmospheric pressure.

you connect process pressure into high side of differential pr. transmitter and just leaving the low side of diff.pr.transmitter. that means you measure with respect to Atmosphere , this is called gauge pressure .which is in pressure transmitter. If diff .pr transmitter shows mmwc then convert it in bar. (10000mmwc=1bar)

Provided the differential range of the instrument, should be higher than process pressure range .

that means if your process pressure is 5 bar the diff. range of the instrument should be higher than 50000mmwc.

Just another way of saying what JohnDG and Tirtho have already said, when you get right down to it, all pressure transmitters are of the differential pressure type.

If you connect the high side to your tank or vessel, and just vent the low side, your dp transmitter becomes a gauge pressure transmitter.

If you evacuate all the air from the low side (and plug it, keeping the vacuum inside), and connect the high side to the tank or vessel, your dp transmitter becomes an absolute pressure transmitter.

How you use your dp transmitter depends on your application. For example:

Let's say that the atmosphere outside your room is slightly toxic (somebody's smoking and you're allergic to it ) so you need to keep the room at a slightly positive (higher) pressure than the outside. You connect your high side to the inside of the room and the low side to the outside of the room.

As long as your pressure reading is positive, you're safe. If it becomes negative, grab your gas mask and fix the ventilation system.

I have to mention that there are cases in which you need to grab your gas mask when there is a slightly positive gradient on the already positive difference between the exterior and interior

Yes. Simply vent the low pressure side to atmosphere.