MPX2100DP only measures depression

I'm trying to read out the analog differential pressure sensor MPX2100DP. In my first approach I used an oscilloscope to visualize the change of voltage. My basic setup was only the sensor connected to +9V and GND. The other pins were connected to the oscilloscope. The tube to produce pressure by blowing in is connected to the pressure side of the sensor. I think I connected everything correctly like mentioned in the data sheet (http://cache.freescale.com/files/sensors/doc/data_sheet/MPX2100.pdf). My setup is shown in this image:

The first strange thing is that the sensor shows max voltage difference (55 mV) when both inlets of the sensor are free (no pressure). I didn't expect it this way. When I increase pressure in the tube the sensor does nothing. When I'm doing a depression in contrast I can see a change of voltage as shown in the second image. Does anyone of you guys have an idea what could be wrong?

idle state here Idle state here

with depression in tupe

This is a differential sensor. This means it will compare the vacuum of one port to the positive pressure of the other port. If you are applying negative pressure to the port that you expect to respond to positive pressure, then it must mean you have the ports reversed. Try plugging the hose into the other port and then blow into it. This should result in an increase in voltage in the out+ pin, as you expect.