A gas monitor may seriously under-estimate actual flammable gas levels.  This may occur if the target gas is different to the gas used for calibration. A correction factor can be applied to convert measured to actual levels. 

See Worksafe Alberta Bulletin  - LEL Correction

When sending your gas monitor for calibration, be sure to notify the service technician which flammable gas to use for calibration.  This will vary depending on the environment that the monitor is being used and will ensure the most accurate reading.  Some gas monitors allow users to change the flammable gas selection on-site.