Hi,
I got my hands on cheap barometric pressure sensor MPX700D.
It's four pin connected GND, VCC+, OUTPUT+ and OUTPUT-.
How to use this kind of sensor?.
I can simply read analog voltage from OUTPUT+, but what about OUTPUT-, how to use that pin?
And if you have any hints, how to convert readings to understandable format as barometric pressure?
Thanks!
Where did you buy it ?
The BMP085 and BMP180 are very accurate baromic sensors. They cost 3 or 4 dollars (inclusive shipping).
Your pressure sensor is can be a relative pressure sensor or a baromic (also called absolute) pressure sensor, but I think it is not ment to measure the baromic value. It can measure vaccuum and pressure up to 700kPa. It has 60mV output with that 700kPa.
It contains a piezo bridge inside, so the OUTPUT+ and OUTPUT- are outputs of the brigde.
You need a OpAmp to amplify the signal.
Converting it to baromic pressure is useless, since it is not temperature compensated.
If you want to, you might use the analogReference(INTERNAL) and connect both OUTPUT+ and OUTPUT- to analog inputs, and calculate the difference. The baromic value might be 300% inaccurate (just a wild guess).
Thank you Caltoa.
Sensor was found from a local electronic store.
As always, it's easier to buy Arduino compatiple parts, but it's also interesting to hack these parts 
I'll follow your advices and study some more. Maybe if I compare the values from this sensor to a real analog
barometric meter, I understand how it compares...
This is using a piezo bridge pressure sensor without opamp.
It is very inaccurate.
http://playground.arduino.cc/Main/SPX3058D
More than accurate enough, for the purposes for which it was designed. It is not a barometer.