A new method for in-place testing of tandem HEPA filter installations.

A new in-place high-efficiency particulate air (HEPA) filter testing procedure has been developed, based on the use of an intracavity laser particle spectrometer for ultrasensitive detection of challenge particulate concentrations downstream of tandem HEPA filter installations. The feasibility of these measurements had initially been demonstrated on a small-scale laboratory system where overall protection factors of 10(9) were routinely measured. The primary scaling problem for systems up to 20 000 CFM was in producing sufficient challenge aerosol. This was accomplished by the design and construction of a high-volume thermal dioctyl phthalate generator. The results of acceptance testing of 13 tandem HEPA filter systems, performed with both the spectrometer technique and conventional light-scattering photometry, are displayed and problem areas discussed.