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A ductless (also known as recirculating) fume cupboard is an enclosure usually manufactured from stainless steel and acrylic or glass which uses an internal fan to move air through one or more carbon filter(s) to absorb chemical fumes and protect the wider environment as well as the user from harmful gases.

The number of carbon filters required in your fume cupboard depends on the size of the unit and the variations of chemicals in use. For complex work with multiple chemicals used in high quantities, a ducted fume cupboard would be advised which extracts air to the atmosphere at a safe location (usually the roof of the facility).



How do I know when to replace my carbon filters?

At Absolute Carbon Filters, we are regularly asked 'how do I know when to replace my carbon filters'. So, we asked fume cupboard specialist engineer, Chris Downey, of Crowthorne Group for some advice.




There are three ways to test if a carbon filter needs to be changed and these are:

1.  A Carbon Breakthrough Test is a definitive way of telling if a carbon filter needs to be changed. Your engineer uses 100mm of Isopropanol, poured into a beaker and placed on a heating panel within the work area of the fume cupboard. As the chemical is ‘burnt off’ within the cabinet, the GASTEC Gas Detector Tube System is used above the filter to test exhausted air for fumes. Exhausted air is pulled though a GASTEC tube using their specially designed pump, if escaping fumes are detected the inside of the tube changes colour indicating the filter is no longer adequately absorbing fumes and so is due to be changed.

Find out more about how the GASTEC system works here: https://www.gastec.co.jp/en/product/detector_tube/summary/

2.  Indicator or Sniffer Test offers a strong indication as to whether the carbon filter needs to be changed. Alcohol is sprayed into the work area of the fume cupboard and a sniffer machine is used to detect any alcohol being expelled via the carbon filter into the environment.

You may be wondering why alcohol is used, Chris explained that alcohol is absorbed by any type of carbon filter (except Mercury filters), so is the chemical of choice when testing carbon filters in ductless fume cupboards.

3.  When our customers want to test if a carbon filter needs changing for themselves, they can use a simple airflow test by holding an anemometer against the front aperture of the cabinet. The anemometer reading should be close to the reading of the last service or exactly as shown on the display on the cabinet, if a reduced airflow reading is recorded this would indicate that the filter is becoming saturated and so air is passing more slowly into the fume cupboard and through the filter. 

In this instance, an engineer should be called out to validate the airflow test with the Carbon Breakthrough Test or Sniffer Test and replace the pre-filter and/or carbon filter as necessary.



When should I call an engineer?

Another question commonly received at Absolute Carbon Filters is ‘When should I call an engineer?’ There are four main reasons all with the same level of importance. 

1.  Our advice is that your fume cupboard should not go 12 months without a service visit.

2. There is the smell of chemicals in the laboratory! A simple indicator that there is something wrong and an engineer should be called.

3. If the fume cupboard is sounding an alarm or otherwise indicating a warning, this would indicate there is an issue with the internal workings or airflow; an engineer will be able to diagnose the issue and advise a solution if a fix cannot be made during the initial visit.

4. If there is an obvious physical fault such as broken sash, hole in the filter etc. then an engineer should be called.



How long will my filter last?

By far the most common question our customers ask is ‘How long will my filter last’? The honest answer is, we don’t know. The longevity of your filters will depend on the frequency of use and the test process carried out. What we do advise is that your carbon filters are changed at least every two years as an absolute minimum. 



Find out more...

If you would like to find more commonly asked questions, visit the FAQ section of our website.

To find out more about which carbon filter to use for your application, please get in touch with us today at info@absolutecarbonfilters.co.uk