from the clean filter. This change in reading value is
used with the calibration chart in figure 3-6.
20. Find this value on the left of the chart, then
move horizontally until the reference line is
intersected. Read vertically at either the top or
bottom of the chart to determine the amount of
contamination in either milligrams per gallon or in
milligrams per liter.
NOTE
Each contaminated fuel detector has its own
calibration chart that is marked with the same
serial number as the unit.
You should recognize that this instrument is
only a secondary standard and does not replace the
requirements for periodic laboratory analysis; it
supplements the laboratory analysis. Extensive field
tests have demonstrated that the calibration chart
with this unit is valid for most fuel samples.
However, there are occasional samples that do not fit
the normal pattern. It may become necessary to
establish a new or modified calibration chart in a few
unusual cases where the contaminants in a particular
system do not follow normal patterns. Duplicate
samples sent to the laboratory for gravimetric
analysis can give a cross-check on the instrument and
quickly pinpoint these unusual situations.
Light Intensity Adjustments
If insufficient adjustment is available to obtain a
reading of 0.60 milliamps on the meter as outlined in
step 12, proceed as follows: Set the rheostat at
midscale and note the meter reading with the light on
and the photocell in the measuring position, and no
filter in the receptacle, Unplug the instrument and
open the back. Loosen the light bulb holder slightly.
See figure 3-7. If the meter reads below 0.60
milliamps, slide the bulb holder up. If the meter reads
over 0.60 milliamps, slide the bulb holder down. The
filament of the light bulb should be horizontal after
the change is made. Temporarily close the case, plug
in the instrument, turn on the light, and check the
meter reading. It is not necessary to obtain an exact
0.60 reading by adjustment of the light bulb because
final adjustments will be made by use of the rheostat.
When a suitable position for the light bulb has been
found that will permit adequate adjustment by the
rheostat,
Figure 3-6.—Calibration chart.
3-18
