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Monitoring atmospheric pressure in a trench is crucial for ensuring worker safety, especially in scenarios where there is a potential for hazardous atmospheric conditions, such as the presence of toxic gases or a lack of oxygen. The correct approach to checking atmospheric pressure is to conduct the assessment before workers enter the trench. This ensures that the environment is safe for work, as conditions can change rapidly and may not remain stable over time.

Assessing the atmospheric pressure prior to entry allows for proper safety measures to be implemented, such as ventilation or the use of protective equipment if hazards are detected. By checking only once before entry, it helps to ensure that no dangerous changes have occurred that could put workers at risk as they prepare to start their tasks in the trench.

Other frequencies of checks, such as once every hour, every 15 minutes, or once a day, could potentially expose workers to hazardous conditions during the time lapses between checks. Therefore, verifying atmospheric conditions immediately before workers enter the trench is the most appropriate and safest protocol.