Emerson Exchange 365

Lowering alarm setpoint for toxic gas detectors

Hi,

for toxic gas detectors of many/all manufacturers, is there a limit for alarm setpoint whether in the low or high range where accuracy will degrade around the new setpoint. I want to lower alarm setpoint and make sure accuracy stays the same around the new setpoint.

Any ideas from experience please.

Thank you

john

• 19 Sep 2019 7:01 AM
• Cancel

1 Reply

• Edward Naranjo

  

  • 19 Sep 2019 10:12 PM

  Most toxic gas detector manufacturers set no limits on alarm set points. Alarm levels should not be set at values below the measurement error. For example, if a hydrogen sulfide gas sensor has an accuracy of ± 3 ppm, alarm levels at 3 ppm or less are of little benefit, since the measurements are undistinguishable from 0 ppm and likely result in false positives.
  Alarm set points should be set as close to ambient conditions as possible consummate to avoiding false alarms. Doing so maximizes the probability of detection success.
  
  When establishing alarm set points with new detection equipment or when detectors are installed in new operations or around new process equipment, it is useful to follow these steps:
  
  1) Set up the alarm concentration at a regulated level like OSHA’s permissible exposure limit and retain it for a minimum of 30 days
  2) Review trend data to identify peak and average gas concentrations
  3) Progressively decrease the alarm set point until the minimum setting is reached that the device’s accuracy can support or that results in nuisance alarms. If nuisance alarms become a problem, the alarm set point should be increased to the last setting at which they did not occur.
  
  One approach for incrementally adjusting alarm set points downwards is by retaining high-high alarm at the last setting that provided no false alarms and then resetting the high alarm level downward to the next incremental setting. During this exercise, the high alarm could be programmed so that do executive action is taken on its activation.

  • Cancel