Today (Wednesday July 18th) at around 4:15 pm I was taking a small tour group from VECC into the Target Hall. In the process of discussing the mechanical design of the Target Modules and South Hot Cell turntable I inadvertently lifted the Aluminum Hatch Cover of the Hot Cell about 2 inches in an effort to explain something. This tripped the ventilation alarm before I realized what I was doing. I evacuated the visitors from the Target Hall, checked booties and gloves at the check station before having the visitors step over the boot box. I called operations and let them know what I had done. The Target Hall was locked out and is awaiting a survey for the morning of July 19th before work can proceed.