Is this in-place filter test the same one as the filter manufacturer uses ?

NO, but it is based on the same principle. The photometer is the same, but the DOP is generated by air. This is done by using a special Laskin nozzle which creates an aerosol when it is immersed in DOP liquid and air passed through it. This aerosol, unlike that used to test the filters at time of manufacture, is not a monodisperse 0.3 micron cloud, but is polydispersed, ranging from 0.32 microns to 1 microns, averaging approximatly 0.45 microns. It is also possible to create a simillar polydisperse aerosol by heat atmosphere by purging with an inert gas such as nitrogen or argon. The concentration of DOP so obtained will be very high, and can be used for testing air streams 5,000 to 50,000 CFM