ASHRAE 110 AU
The last of the ASHRAE 110 tests is performed in a working lab. It is usually done with the normal equipment in place. Assuming we have a history of an acceptable AM and AI test, we are now testing the impact of the users work practices on performance.
We often find that hoods that fail to perform at an acceptable level on the AU test are a result of changes that have occurred in the laboratory ventilation system. These systems are very complex and need regular maintenance to perform as designed. Other times it is simply work practices that can be corrected.
Regardless of the reason, this test will verify if the performance of the hood is acceptable.
ASHRAE 110 AI
This test is performed with empty hoods in the lab before it is occupied. This is a form of commissioning.
If you have the AM test for the hood, it can then be compared to the AI test results. If there are major differences in the performance, that is an indication that there is a problem with the building HVAC or the Laboratory Ventilation System that needs to be addressed.
ASHRAE 110 says:
Room ventilating systems, both supply and exhaust, including the fume-hood exhaust, shall be in full normal operation. Airflow systems in the laboratory shall be properly commissioned prior to beginning the test. This includes calibration of airflow controls; calibration of automatic temperature controls; balance of supply air; completion of a duct traverse on the exhaust duct.
If initial testing indicated that the hood is not performing as expected based on the AM test, testing should stop until the building issues have been resolved.
Once the hoods have an acceptable AI test we now know that the lab is safe for the user. All labs should have AI testing before occupancy to verify the overall system is working as designed and that there is an acceptable safety level.
ASHRAE 110 AM
This test is done in a test lab with near perfect conditions. The purpose of this test is to allow the user or purchaser to compare fume hoods from different suppliers. By comparing the test from each supplier you can see how well the fume hood is designed and how well it will perform compared to the other. You can also compare the energy costs necessary to operate the hood.
However, this test in no way indicates how the hood will perform in your lab. It simply tells you how will it perform compared to others in perfect conditions. Since your lab is much more complex than a test lab you will have many other factors impacting performance. Additionally, each lab will have a different laboratory ventilation system which also has a major impact on performance.
So the usefulness of this test is to determine the design quality of the hood, but not the performance in an actual lab.