This test is particularly useful for gearbox and reciprocating equipment samples. Usually, 1 mL of sample is mixed one to one with a diluent. This solution is then gravity flowed through a glass tube that is nestled over a strong magnet at a slight upward angle to the flow direction. Two light paths are located along the base of the glass tube, spaced about 1/4 inch apart. The magnet will pull out of suspension ferromagnetic particles and deposit them along the bottom of the glass tube in both light paths. Larger, more ferromagnetic particles (generally from 0.1 to over 300 microns) will be pulled out first and are ‘measured’ in the first light path. Smaller, less ferromagnetic particles (generally considered to be 0.1 to about 5 microns) will be deposited along the initial third of the glass tube, and are measured in the second light path. The measurement actually consists of how much attenuation of transmitted light is measured at the beginning of the sample flow against the transmitted light at the end of the sample flow due to the buildup of particles in the light paths. The first light path measure the Ferro Direct Read Large (FDRL) because most of the large ferromagnetic particles will be deposited in this light path, and the second light path will measure the Ferro Direct Read Small (FDRS) because few of the larger ferromagnetic particles will be deposited in this light path. The values obtained are converted into empirical numbers that range from 0.0 to 180.0. Some laboratories will provide higher numbers by using dilution methods with small sample volumes.
The relation between the FDRS and FDRL is sometimes indicative of the type of problem that exists. If the FDRL/FDRS is greater than 2.0, this may indicate a higher than normal number of large particles and as such, a more severe wear situation. The sum of the FDRS and FDRL is also an indication of how much wear is occurring. These numbers should not vary greatly over time for a given sample point unless there is an issue.