Calibration Ammonia Sensor and Gas Concentration
The calibration gas will purge the air out of your chamber, so that it contains 50 ppm NH3.  This assumes that there are no reactions that change the mixing ratio of the ammonia (e.g. reaction with water in your chamber).  The length of the tube doesn't matter unless the gas reacts with your tubing, though length will decrease the pressure of the gas coming out of the tube.