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Red Alert Level = WPC +3s
8.2 – D
L
and D
S
Readings
Flow rates through the precipitator tube have some effect on particle deposition across the
precipitator tube assembly. All precipitator tubes are pressure dropped during assembly within
the limits needed to maintain a certain consistent flow rate. D
L
and D
S
readings are susceptible to
changes as the flow rate decreases. Decreasing the flow rate by a factor of 2 will see an average
overall increase of D
L
and D
S
reading of 15%. Therefore, more viscous oils need 2ml of heptane
to thin them to an acceptable viscosity.
Sample time is the easiest indicator of a sample that is too viscous. Typical sample times are
between 200 to 300 seconds. Sample times that are much greater will cause particles to
concentrate at the beginning of the precipitator tube, instead of precipitating along the entire
length of the tube area. Samples which take long periods of time to run indicate the viscosity is
too viscous and should be thinned to maintain comparable DR data.
When readings increase several units and then decrease, it is an indication that large non-
magnetic particles are passing by the sensor location. This can be caused by large dirt particles or
minute air bubbles. However, these do not affect the particles that have precipitated to the
bottom of the tube, since dirt particles will tend to remain in the middle of the fluid flow and air
bubbles will rise to the top portion of the tube.
As discussed in the previous section, when the D
L
and D
S
numbers trend close together, this
indicates larger particles have precipitated out and most of the particle concentration present in
the oil sample are small particles. This indicates the machine wear has reached an equilibrium
state where large particles are being generated at the rate in which they precipitate out or are
removed from the system through filtration or other means. The remaining smaller particles
gradually increase over time as they tend to stay suspended (See Figure 8.1 A&B).
Typically, in non-steady state machinery the value of the D
L
will be greater than the D
S.
Cumulative plots of the
data will show diverging lines, where the slop of the D
L
will be greater
than the
D
S.
This indicates increasing large particles.
On occasion, a sample will produce a D
S
value greater than the D
L.
For these cases, there is a
high concentration of small particles that are small enough that they stay suspended in solution
and tend to settle further down the precipitator tube after entering the magnetic field. Therefore,
with relatively low large particle concentration and a very high small particle concentration the
magnitude of the D
S
value can be higher.
All versions of DR instruments calculate values of the D
L
and D
S
based on percentage of light
blockage, amplifying, and calculating the signals into the D
L
and D
S
units. These numbers
generated for the D
L
and D
S
are unitless, based on the light emittance over each of the sensors.
The calculations conducted are based on known analytical ferrograph results in comparisons to
the DR making the values of D
L
and D
S
relative to size concentrations of wear particles. At very
low concentrations of both D
L
and D
S,
a zero value can be possible. A zero does not necessarily
mean there are no particles in the sample, but rather the concentration is so low that there is not
