As we know,the sensor is to convert a physical quantity to be measured into another physical quantity that can be read and processed. In modern control, this physical quantity is an electrical signal; the transmitter converts the primary electrical signal of the sensor into a standard electrical signal,for example, a current signal 4--20mA, 0--20mA, the voltage signal 0--10V, 1--5V. The primary pressure sensor is a strain induced by the millivolt signal changes, if the sensor has been enlarged with plastic circuit, the output of standard current or voltage signals, such sensors can also be called pressure transmitters; pressure transmitter called , Relative to the early pressure sensor is the output of millivolt signal, the majority of modern pressure sensors have been directly output standard signals, so now the pressure sensor and pressure transmitter may be combined.
In industry e.g. applications with low pressure ranges that need a measurement of vacuum, require absolute pressure sensors for high accuracy. For example, in meat packing, a vacuum of a defined quality must be generated, so that a maximum shelf life can be safely ensured. The residual amount of oxygen in the packaging (i.e. the residual pressure relative to vacuum) is directly proportional to the shelf-life of the packaged food. Using a relative pressure sensor in this case, would lead to measurement uncertainty due to atmospheric changes of up to +/- 30 mbar.
As the maximum residual pressure in the food packaging should be 65 mbar, this would lead to substantial fluctuations in the expiry date that could no longer be guaranteed.