Rate of occurrence of failures based on a nonhomogeneous Poisson process: an ozone analyzer case study

Environ Monit Assess. 2013 Feb;185(2):1463-71. doi: 10.1007/s10661-012-2645-6. Epub 2012 May 18.


Atmospheric pollutant monitoring constitutes a primordial activity in public policies concerning air quality. In São Paulo State, Brazil, the São Paulo State Environment Company (CETESB) maintains an automatic network which continuously monitors CO, SO(2), NO(x), O(3), and particulate matter concentrations in the air. The monitoring process accuracy is a fundamental condition for the actions to be taken by CETESB. As one of the support systems, a preventive maintenance program for the different analyzers used is part of the data quality strategy. Knowledge of the behavior of analyzer failure times could help optimize the program. To achieve this goal, the failure times of an ozone analyzer-considered a repairable system-were modeled by means of the nonhomogeneous Poisson process. The rate of occurrence of failures (ROCOF) was estimated for the intervals 0-70,800 h and 0-88,320 h, in which six and seven failures were observed, respectively. The results showed that the ROCOF estimate is influenced by the choice of the observation period, t(0) = 70,800 h and t(7) = 88,320 h in the cases analyzed. Identification of preventive maintenance actions, mainly when parts replacement occurs in the last interval of observation, is highlighted, justifying the alteration in the behavior of the inter-arrival times. The performance of a follow-up on each analyzer is recommended in order to record the impact of the performed preventive maintenance program on the enhancement of its useful life.

MeSH terms

  • Air Pollutants / analysis*
  • Air Pollution / statistics & numerical data*
  • Brazil
  • Environmental Monitoring / methods*
  • Ozone / analysis*
  • Poisson Distribution*
  • Statistics as Topic


  • Air Pollutants
  • Ozone