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A large chamber (1.22 x 2.44 m) was constructed at West Texas A&M University for ammonia (NH3) measurements. A series of trials were conducted at a large commercial feedlot to assess the performance of the chamber. The trials were conducted over a two day period in April 2005. Samples were analyzed in a mobile lab parked as close as possible to the sample site. A Thermo Environmental Instruments 17C NH3 analyzer calibrated to 45 ppmv NH3 gas was used to determine NH3 concentrations. A visually located dry area in the feedyard pen was selected and measured for a 30-minute period. Clean sweep air was delivered to the chamber at a flow rate of 100 L/min via a large trailer mounted air compressor, equal to 0.3 exchange volumes per minute. NH3 concentrations measured were 20 ppm at this location. A fresh urine spot was located and the chamber was placed over the spot. NH3 concentrations at that location were 42 ppm. Both measurements were taken from the center of the chamber top. Nine different sampling ports in the chamber surface were sampled over a 60-minute period. It was determined that, while using the air delivery system in the chamber, NH3 concentrations depended greatly on the NH3 production of the surface below the sampling port. Removal of the air delivery system from inside the chamber and sealing all but the sampling port farthest from the air manifold resulted in NH3 concentrations of 21 ppm. Immediate testing inside the footprint of the large chamber with a NC State chamber, which has a much smaller footprint was done. Six location within the footprint showed a range of 2.41 to 15.2 ppm with the flowrate adjusted to .3 exchanges of clean air per minute.