Back to the Basics: Limit to Adjustment of Calibration

In normal uses, most analyzers will require adjustment to their sensitivity to make up for a gradual reduction in sensitivity over time. 

The normal amount of adjustment of the signal should be well understood by the maintenance personnel so that problems other than loss of sensitivity are not corrected by the improper use of the "span" adjustment. 

For Example:

• some analyzers have little or no loss of sensitivity over time and therefore the need to increase the "span" adjustment might indicate a pneumatic leak that should only be corrected by finding and sealing the leaking joint. 
• other analyzers have the need to greatly increase the span are indicative of the need to re-tune the analyzer replace the sensing element. 

A log of routine maintenance should be kept so that abnormal amounts of adjustment are recognized and a full test of the analyzer is performed.

Since they have direct bearing upon safety, the controls for adjusting the alarm levels and the calibration (zero and span) should be accessible only to authorized personnel who have been trained in the correct procedures for adjustment, and who can make decisions regarding the proper operation of the process.