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1.
Introduction
The Direct Reading Ferrograph is a trending instrument, providing readings which measure the amount of
metallic particles, both large (DL) and small (DS) in a known oil sample. The numbers given do NOT
relate to any other numbers such as parts per million (ppm). The DL and DS numbers of the DR-7 will
correspond to those reading generated by a DR-5 and will be slightly higher than a DR-3. In turn these
numbers can be used to calculate the wear particle count (WPC).
The DR-7 is much more versatile than its predecessors. Running on a Windows platform allows users to
quickly upgrade DR software to the latest version, interface through the internet, and set up e-mail
capabilities. The USB connections can easily be used to set up multiple DR-7 units together and connect
to the user lab software. The provided Ethernet port allows users to network into their current systems and
to each externally.
The first Direct Reading Ferrograph was developed in the early 1970’s as the whole science of
Ferrography was being developed under a contract with the US Navy. Early DR Instruments were large
and had to be adjusted before every test. The large density particles and small density particles had to be
read separately by turning a switch. Over the years, the Direct Reader (DR) progressed from the original
manual instrument to a Duplex. The Duplex was a manual DR with a Ferrogram Maker in the same
instrument. The DR-2 was the next version which contained the first attempt at a self-adjusting
instrument. Throughout the years the DR-3, DR-5 and DR-6 series of Instruments were created, finally
with this current DR-7 design.
During the development of the Ferrography instruments, it was discovered the most sensitive locations for
detecting a changing wear situation were found, this point was determined to be at the entry point of the
magnets, at a position approximately 5 millimeters downstream. Particles of 5 microns or greater
collected at the entry point. Particles with the size of 1-2 microns collect at the 5 mm position.
Accordingly, the Direct Reader Ferrograph was designed to quantify particles into these two size ranges.
This is where the Density Large (DL) and Density Small (DS) numbers come from. These densities are
found by shining a monochromatic light through the bottom of the glass part of the precipitator tube. The
amount of light cut off by the particle builds up in the glass tube, and therefore blocks the sensors, and
this blockage is read by sensors. The DR then converts this attenuated light into the DL and DS readings.
The digital output has a range of 0 to 190 units. A reading of 100 units corresponds to approximately one-
half of the area covered and is the recommended upper limit. For readings greater than 100, the
instrument response is non-linear due to the particles piling on top of one another so that less light is
attenuated. Trico recommends not using a reading over 90 and the instrument indicates a warning
message to indicate that the sample should be diluted.
Because this is a trending instrument, results on the same sample will vary due to accuracy of sample
preparation, sample temperature, settling of the particles, and other factors. Comparative unit testing
proves that a given sample can produce results repetitively within a range on a single DR-7 and across
multiple units.
