There are two aspects to filter monitoring, checking the airflow to ensure the pre-filter is not clogged with dust and checking the exhaust air for chemical contaminants to ensure the main filter has not reached the breakthrough point. The results of the filter monitoring should be recorded on a log as per the attached sample logsheet. Airflow can be measured with an anemometer and a number of designs are available including propeller, hot wire and vane anemometer. In addition, most units are fitted with a low airflow alarm, which indicates a low airflow situation as well as fan failure. The use of high quality centrifugal fans ensures that airflow is maintained even as the pre-filter airflow resistance increases.
Adsorption takes place in a filter bed in what is known as the active filter zone.
As the filter is used this active zone progressively moves up the filter bed until it approaches the top surface of the filter. At this point there is an initial breakthrough by the contaminant gas, and thereafter the percentage of contaminant gas that escapes filtration increases. Monitoring can be performed with gas detection tubes such as draeger or gastec. Most of the units have test ports allowing the use of this equipment in a zone immediately above the main filter. The filter is challenged with a known chemical, and the concentration of this chemical in the exhaust air is measured with a suitable detection tube.