RACNet is a monitoring sensor network to provide highfidelity visibility, in terms of spatial and temporal resolution, of data center environmental conditions for energy efficiency improvement. RACNet overcomes the high node density and harsh RF environment challenges in data centers to achieve over 99% reliable data yield and short data collection latency. It does so through a novel network architecture that decouples data collection from the construction of the routing tree. This design, coupled with the use of different frequencies along neighboring data collection trees, enables RACNet to support large-scale, dense networks while maintaining perfect data reliability. Results from simulations, testbed, and real world deployments indicate that RACNet outperforms previous data collection systems, especially as node density increases.