A bug has been discovered in the design of the gravimetric system. Users should take immediate action to ensure that no data is invalidated.
The pump in the gravimetric system is intended to only be powered on when a filter is installed. The filter prevents any foreign debris from becoming undesirably lodged in the system. The problem with this design and method of use is that when a filter is not installed the hose is normally left open on the table.
When the hose is left open on the table foreign debris can be blown into the hose, or a bug can crawl in. Once the filter is installed and the pump is turned on, the debris will be sucked downstream. Large objects will become lodged in the critical orifice.
With the critical orifice obstructed there will be less air moving through the gravimetric system, and its measurement will become invalidated.
There are two simple fixes to the problem of obstructions in the gravimetric system.
1) Check that the vacuum gauge reads around 16” Hg when the pump is running. It may not be exactly 16” Hg because of the accuracy of the gauge and the altitude at which you are operating, but there will be a normal value.
When the system is obstructed the gauge will read higher because the pump is able to pull more vacuum at less flow.
2) You can also visually inspect the critical orifice through the clear hose to determine if there are any obstructions.
Preventative action is to install a ball valve immediately downstream of the filter cassette connector.
The ball valve should be left closed when the system is not in use. Before starting a test the ball valve is opened. Even with the ball valve in use the spaces between the valve and the filter should be inspected before starting a test. After the test has been started it should be verified that the vacuum gauge reads the correct value, and that the critical orifice is clear.
I hope this error has not caused you any problems. Please contact me if you have any questions, or need any assistance.
Aprovecho Research Center
PO Box 1175
Cottage Grove, OR 97424, USA