A fume hood is a valuable piece of safety gear for any laboratory that works with chemicals. The can only do their job with proper use, which requires regular safety checks and calibration to make sure that everything is working properly. Those checks involve making sure that air is flowing properly and that there are no dangerous leaks in the system. There are a few different ways to carry them out, and it’s best for laboratories to do so regularly. In most cases, annual checks combined with additional calibrations after any changes are made to the hood or to the area around it are sufficient.
1- Tracer Gas Test
A tracer gas test is an easy way to make sure that gas can flow out of the fume hood at an appropriate rate. The test begins by placing a diffuser device inside the fume hood. The device releases a specific gas into the system at a standard rate, which is usually four liters per minute. Most of these tests use sulfur hexafluoride for the gas because it is heavier than the air inside the fume hood, but some testers do prefer to use other gases. A vapor analyzer is also placed outside the fume hood where the user’s face would normally be. If the hood works properly, it will vent the gas out of the system without exposing the vapor analyzer to a dangerous dose.
2- Face Velocity Test
This test exists to make sure that air passes out of the front of the fume hood at an appropriate rate. The individual who is testing the system should open the fume hood’s sash and divide the open area into an imaginary grid of rectangles. The tester then uses a velocity probe to measure the rate at which the air is flowing in the middle of each imaginary rectangle. The probe should be used in averaging mode and held in place for at least ten seconds to make sure that each measurement is as accurate as possible. If the air is not flowing at the appropriate rate, or if one part of the hood is losing air at a much greater or lower rate than the others, then the hood likely needs adjustment to ensure that it performs adequately.
3- Small Volume Smoke Testing
A visual test offers the easiest way to make sure that air is circulating inside the fume hood at an appropriate rate. A small volume smoke test starts by releasing a small amount of white smoke inside the fume hood, either through a tube/bulb system or through a swab of titanium tetrachloride. The smoke should circulate through the perimeter of the sash without any irregularities. If the flow reverses or shows any other unusual patters within a localized area of the fume hood, then the hood is malfunctioning and needs to be repaired before it can be used. A problem also exists if the smoke escapes from the fume hood and enters the laboratory at any time.
4- Large Volume Smoke Testing
Large volume smoke tests make sure that the fume hood can handle a lot of fumes at the same time. A large amount of smoke gets slowly pumped into the fume hood through a tube, and the tester watches the flow pattern. If any of the smoke escapes into the laboratory during the test without immediately getting sucked back into the hood, the fume hood fails. If the fume hood can handle the constant flow of smoke without losing any smoke to the environment, then it passes the test.
5- Professional Support
Most tests are fairly simple, but the devil is always in the details. Calibrating a fume hood does require some skill, especially after a problem has been detected. In most cases, the easiest way to calibrate the fume hood and be assured of a good result is to call for a professional testing company. They can perform all of the standard tests that indicate if a fume hood needs calibration or not, and then make any necessary changes. They can also provide the special equipment which is necessary for these tests, which is an asset for smaller laboratories. Fume hoods are vital safety tools, so it’s always worth getting it right rather than risking an error.