What are my mistakes by using sensors? - sensors

Question

I slowly make my first tries with using sensors. And i want to show the actual air pressure (My smartphone defently have a airpressuresensor.)
I look at some code and copy some code, and build the app. Now it doesnt work, and I dont know why. Does sensors need sth. in the manifest or so?
There are no errorcodes, just a applicationcrash...
the evil line seems to be, because if I build a try and catch around the app dont crash, but nothing happends. : sMgr.registerListener((SensorEventListener) this, sensor, SensorManager.SENSOR_DELAY_NORMAL);
Here is the code:
public class MainActivity extends ActionBarActivity {
public Handler handler = new Handler();
public double a;
SensorManager sMgr;
Sensor sensor;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
sMgr = (SensorManager)getSystemService(SENSOR_SERVICE);
List<Sensor> sensors = sMgr.getSensorList(Sensor.TYPE_PRESSURE);
if(sensors.size() > 0) {
sensor = sensors.get(0);
sMgr.registerListener((SensorEventListener) this, sensor, SensorManager.SENSOR_DELAY_NORMAL);
}
}
public void onAccuracyChanged(Sensor sensor, int accuracy) {
}
public void onSensorChanged(SensorEvent event) {
float presure = event.values[0];
Log.d("PressureSens", ""+presure);
TextView textView = (TextView)findViewById(R.id.text);
textView.setText(""+ presure);
}