Issue: Fixed CEMS (Continuous Emissions Monitors) Only Measure at One Location CEMS are fixed units that typically only measure from one sample location which is usually on a main centralized stack. This main stack is often fed by several separate emissions sources throughout the plant such as boilers, heaters, engines, ovens, and furnaces. If the CEM is measuring emissions that are higher than the regulated or permitted limits, it can be difficult to determine the source of the high emissions which could be from any or several of the separate sources that feed into the main stack.