One last question.. What is the difference between the Absolute sensors and the Gauge sensors.. what I found that the Gauge sensors can't read pressure at 0 PSI, only Absolute can do this.. I found this :
First PSI is an incomplete, lazy, units of pressure measurement, it relies on assumption. Units should be specified as either psig. psid or psia. where:
psig = a pressure measurement made relative to standard atmospheric pressure
psid = a pressure measurement relative to the difference between two applied pressures
psia = a absolute pressure measurement relative to a perfect vacuum (0 psia) on the low side port.
So lets step back and talk about how pressure sensors work. They are all 'two port' pressure sensors where what is being measured is the pressure difference between two pressure sensing ports. On some the two ports are accessible to the user, and on most only the 'high side' pressure sensing port is brought out for user attachment.
Pressure sensors that are called differential pressure sensors bring those two ports outside the sensor for the user to hook up to. If you applied 100 psia to one port (the low side port) and 101 psia to the other (high side ) port, the sensor would report a 1 psid pressure measurement.
Pressure sensors that are called gauge pressure sensor have their low side port vented to atmosphere but not brought out for user attachment, so the low side port will always 'feel' whatever barometric pressure there is at the time, typically a nominal 14.7 psia. The high side port is where the user attaches his desired measurement pressure, and if left unattached to anything the sensor will report a 0 psig pressure measurement. And if 50 psia is applied to the sensing port then the sensor will report a 35.3 psig measurement.
Pressure sensors that are called absolute pressure sensors have their internal low side port evacuated and then sealed to as perfect a vacuum as they can obtain so 0 psia. The high side port is what the users attaches to and if left unconnected the absolute pressure sensor will report a 14.7 psia measurement which is nominal atmospheric pressure.
So check your pressure sensor product description carefully and if it just says it measures PSI then you should check it's datasheet to make sure what kind of pressure sensor it is. All pressure sensors with two exposed pressure taps are differential pressure sensors, and sensors with one pressure port can be either an absolute pressure sensor or a gauge pressure sensor. Know what you are working with and what you require. Any sensor needing to measure barometric pressure needs to be a absolute pressure sensor. And of course most single port pressure sensors are of the gauge pressure type.