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6.
Oil Sample Preparation
6.1 – Preparing an Oil Sample for Testing
Gravimetric settling of wear particles in lubricating oil starts immediately after a sample is left standing.
To obtain a representative sample from a larger sample, the particles must be evenly dispersed. To make a
homogenous mixture, the following procedure is recommended:
1.
The oil should be in a clear container to allow for observation of the oil and large contaminants. Make
sure the container is two-thirds full to allow for agitation to completely mix the particles into the oil,
thus giving the sample a homogenous mixture.
2.
Heat the oil to approximately 150°F (65°C). This is according to ASTM standard procedure to keep
the particles suspended as long as possible.
3.
Remove from the heat source and vigorously shake the bottle.
4.
With the pipettor and clean pipette tip, remove 1 ml of oil and dispense into a clean sample vial.
5.
Add 1 or 2 ml of heptane to the 1 ml of oil in the sample vial. The viscosity of the oil determines the
amount of heptane added to the oil in the sample vial. For high-viscosity fluids add 2 ml of heptane to
reduce the viscosity. This will allow the viscous oil to flow along the precipitator tube at a similar rate
as lower viscosity fluids. For low-viscosity fluids 1 ml of heptane is enough to provide fluid flow
through the precipitator tube. It does not matter if 1 or 2 ml of heptane is used, as long as the required
1 ml of oil is used for each test. However, higher viscosity samples moving at too slow of a rate will
affect particle deposition by increasing the amount material deposited on the DL compared to the DS.
6.
The quicker a sample is tested, the better the results. Allowing the sample to settle in the test vial may
cause bunching of particles in the precipitator tube since most of the material will be collected at the
bottom of the vial and deposited in the precipitator tube around the DL sensor. To avoid this, use the
prepared sample as soon as possible or re-mix the sample before testing so that wear particles are
properly dispersed.
6.2 – Dilutions
Readings on the DR-7 greater than 90 in the 1:1 or 1:2 dilution modes are not linear due to the particles
piling on top of one another. When the DR-7 readings reach 90, the test is invalid and should be repeated
using a dilution of the base sample.
1.
To prepare dilutions, start with the base mixture. In a clean sample vial, place 1ml of the base
mixture. To this add 9 ml of diluent oil. This will make a 10:1 dilution.
2.
In another clean sample vial, place 1ml of the 10:1 dilution. To this, add the heptane as done to make
the base solution (1 or 2 ml). This is the 10:1 sample to be run on the DR-7.
3.
On the RUN DR tab MENU display, change the dilution ratio to 10:1 and the readings will be
corrected for this dilution. The upper limit will automatically be changed to 900 instead of 90. If the
DL and DS readings go over 900 a 100:1 dilution must be made.
4.
To make a 100:1 dilution, start with 1 ml of the 10:1 dilution and add 9 ml of the diluent oil. Take 1
ml of this dilution and the standard amount of heptane in a clean sample vial to run the 100:1 dilution.
Change the dilution ration to 100:1 in the menu so the final DL and DS readings will be corrected for
the dilution and the upper limit of the test will be changed to 9000.
5.
When the dilution factor is changed, the DR-7 will automatically multiply the DL and DS readings to
give you the correct dilution readings and the limits of the test will be increased. This means that in a
normal test the display will warn the user if the reading goes over 90. In a 10:1 dilution, that number
increases to 900 if the DR-7 is set at that dilution ratio. The 100:1 would have a high limit of 9000.
NOTE:
If the dilution factors are not changed in the menu, the DL and DS readings must be multiplied
by the dilution factor to get the correct readings. In a 10:1 dilution the DL and DS readings would have to
be multiplied by 10.
