CHECKING
THE DENSITOMETER
Calibration
and maintenance of the densitometer is necessary because an
inaccurate density reading can be misinterpreted as a process
variation. To monitor your densitometer and enable readings
from two or more densitometers to be reliably compared, regularly
check your densitometer. Reading these stable samples
measures the stability of the instrument as a whole, and of
the filters and photometric components specifically. Instruments
may differ in characteristics of the photoreceptor, spectral
cutoff of the filters, amplifier linearity and sensitivity,
and degree to which standard diffuse density readings are
approximated.
To use the check plaques:
1. Set up and calibrate the
densitometer as recommended by the manufacturer.
2. Read the check areas on
the plaque and record their density values daily.
3. Compare each set of newly
recorded values to the previous ones to detect any instability
or trends away from normal.
4. If erratic daily readings
are obtained, take corrective measures before the densitometer
is used further for process control. During the first three
to five weeks of use, the densitometer's characteristic reading
level for each check plaque area becomes apparent.
5. The averages of the readings
obtained during the first three to five weeks of use are adopted
as aim values, control values are calculated, and tolerance
limits are assigned. Thereafter, the performance of the densitometer
is judged with reference to these values.
Details of this procedure are
contained in the instruction sheet that accompanies each check
plaque. If a pattern of erratic readings or a trend away from
normal is determined by inspection of the readings and application
of tolerance limits, some action is required. This action
may include:
- Readjustment of the densitometer
- Replacing faulty components
such as filters or electronic parts (tubes, photoreceptor,
etc).
- Determination of new aim
values. New aim values must also be determined if filters
are replaced or if it becomes necessary to correct the densitometer
for a long-term drift that is not related to routine maintenance,
such as a slow deterioration of electronic parts.
Variation in control plots
can also be caused by reading procedures. Always read control
strips with the emulsion side up. Read both the reference
strip and the processed control strip in the same manner.
While these strips have been designed to provide minimum variation
within a patch, the center of the patch always should be read
to minimize potential development variations at the edges
of each step in the control strip.
CALIBRATING
FLOWMETERS WITH A GRADUATED CYLINDER
For machines with automatic
or semiautomatic replenishment, check the settings on the
replenisher and wash water flowmeters daily and readjust them
as needed. Calibrate each flowmeter before use to determine
the volume of replenisher solution or wash water delivered
for a particular setting. Calibrate the flowmeters using the
actual replenisher solution in use. Calibrating them with
water may not be valid because of differences in viscosity
and specific gravity. Make a calibration curve for each flowmeter.
To calibrate your flowmeters follow these steps:
1. Disconnect the replenisher
or wash water line at a convenient location between the flowmeter
and the line to the processor.
2. With the replenisher or
wash water running, measure the flow at a selected flowmeter
setting (e.g., at 10 percent of the scale), using the graduated
cylinder and a stopwatch. Read and record the setting, as
well as the volume of replenisher solution or wash water that
is collected in 60 seconds.
3. Repeat Steps 1 and 2 at
five or six different settings, covering the entire flowmeter
scale.
4. On graph paper, create calibration
curves for each flowmeter. Identify the graph paper with the
processing machine identification and the solution checked.
Label the horizontal axis "Flowmeter Scale" and
the vertical axis "Flowmeter Rate (mL per minute)."
5. Plot the volumes obtained
in Steps 2 and 3. Draw a smooth curve through the points.
To locate a flowmeter setting for a particular rate, draw
a horizontal line from the rate wanted on the vertical axis
to the curve drawn. From this point on the curve, draw a dotted
perpendicular line down until it intersects the horizontal
axis. The point at which it intersects gives the flowmeter
scale reading for that particular replenishment rate and flowmeter.
6. Finally, recheck the delivery
rate of the flowmeter using the setting obtained.
In addition to the initial
calibration, verify the volume being delivered by each flowmeter
periodically. To do this, disconnect the line at a convenient
location and measure the volume collected in 60 seconds; compare
this volume to the volume expected at that setting. If the
collected volume does not agree within 5 percent, clean the
flowmeter and recheck the rate. If the correct volume still
is not delivered at that setting, recalibrate the flowmeter
as described in the steps above.
CALIBRATING
SMALL MIXING TANKS USING A GRADUATED CYLINDER
Small mixing tanks (5 gallons
or less) can be calibrated with a graduated cylinder. To do
this, fill the cylinder to its highest mark with water, and
pour the water into the container being calibrated. Repeat
until the container is filled with the volume of solution
you are calibrating it for. Mark the level and identify it
with the volume it corresponds to. A pail made of metal or
rigid plastic can be calibrated in this way, and the pail
then used to calibrate larger tanks-up to 30 gallons or so.
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