The ACG+ product page on the Analox Sensor Technology Website has all relevant product literature available for download. Take a look at the ACG+ oil mist detection document (within the manuals section). This describes oil mist detection and how the ACG+ can be used as an early indicator of compressor issues. 

Because many of the mechanisms that generate oil vapour/mist also generate other contaminants such as VOC’s, CO and water vapour, the ACG+ is suitable for monitoring compressors using both synthetic oil and mineral oil. 

The ACG+ monitors for these contaminants which, when detected, can give an indication that problems may be present with the compressor (resulting in the generation of oil mist). These indications can be used as a prompt to perform an oil mist test using the sample support feature.