pressure unit           (valuation)-Factor
===============         ===============
mbar = hPa              100 Pa
bar = 1000 mbar         100 kPa bzw. 0,1 MPa
PSI                     70 hPa
Inch WS / Inch H2O      2,5 hPa
mWS / m H2O             100 hPa
mmWS / mm H2O           0,1 hPa = 10 Pa
mmHg bzw. Torr          1,33 hPa = 133 Pa

The correct pressure unit is, by the way, since long time Pascal (Pa).

Pascal is derived SI unit from pressure. Named after Blaise Pascal. Defined as 

1 Pa = 1 kg/(m·s2) = 1 N/m2

One Pascal is the pressure, which a force of one Newton is exerting on an area of one square meter.

Absolute pressure 

    In space is no pressure existant, but vacuum. Coming into the earth's atmosphere,
    pressure is increasing continuously and will be on sea level 1013,25 hPa at normal condition.
    The barometric or ambient pressure is a value of absolut pressure (p,abs.) ... related to vacuum.
    Absolute pressure sensor has only one pressure port (tube or hole)

Relative pressure

    The relative pressure is a relative relation between two pressures, whereat one pressure is always
    the actual atmospheric pressure (barometric or ambient pressure).
    Pressure (p,rel.) ... related to ambient pressure
    A relative pressure sensor has normally one pressure port (if you use a differential pressure sensor,
    one port has to be kept open to ambient).

Vacuum

    Vacuum can be named as negative pressure, too. Reference point is here also the actual atmospheric pressure.
    In a container you have vacuum, if this pressure inside is lower than the ambient pressure.
    The maximal possible vacuum is according to amount alike the ambient pressure.
    If you have a barometric pressure of 900 hPa, max. vacuum can be -900 hPa
    A vacuum sensor is in principle like a relative pressure sensor, not measuring pressure,
    but negative pressure (vacuum).

Differnetial pressure

    Is the pressure difference between two pressures.
    Popular application is measurement of flow speed with aid of a blind.
    A differential pressure sensor has 2 pressure ports.
    The pressure difference can have positive or negative value, pending on reference point, p1 or p2.
