Advertisement
Features of the Precision Basis Weight Sensor
This section contains an explanation of the major features of the Model 4202 Precision Basis Weight Sensor,
starting at the source capsule and continuing through the chain of major features. A new source body is the most
prominent feature of the Precision Basis Weight Sensor. The source body holds a Promethium 147 radioisotope
"capsule" configured as a line source, with the long axis aligned in the machine direction. This arrangement
allows for increase in signal strength without loosing CD streak resolution. The source body is designed
specifically for the characteristics of Promethium, and is not appropriate for higher energy or gamma emitting
radioisotopes such as Krypton-85 or Strontium-90. The receiver is based upon the Close Geometry Receiver
used in Source 9.
A normally closed stainless steel shutter provides radiation protection. That is, the shutter is forced closed by a
spring unless the linear pneumatic actuator over powers the spring and opens the shutter. The loss of either
electric power or pneumatic (air) pressure will allow the spring to close the shutter. An orifice in the air line
slows the action of the shutter to insure smooth repeatable positioning and long life. All mechanical parts
involved in shielding the radioactive capsule or connecting those shielding parts together are made from
stainless steel for resistance to melting in case of fire. If the temperature exceeds a preset value a fire safety pin
will activate, causing the shutter to close until the mechanism has been disassembled.
The shutter, while several times thicker than needed to stop the radiation from Pm, is much thinner than
required for an isotope such as Krypton-85, allowing the source capsule to be located very close to the source
head window. This minimizes the air gap and optimizes the geometry for delivering large numbers of beta
particles to the receiver, ensuring an accurate, highly repeatable measurement.
Source 12 has the normal Honeywell-Measurex flag, here called Flag1. Periodically, the sensor goes offsheet
and measures the signal with just this Flag1in the beam. This measurement is called reference or standardize.
By comparing the current Flag1 reading to the reading at calibration, the sensor measures the dirt build up. A
dirt correction is based on this standardize flag reading. (This method will attribute to dirt a change anything
which has changed since last standardize, not just dirt build up on the windows.)
Source 12 has a second flag assembly. Both flags are activated by linear pneumatic actuators identical to the
shutter's. The flags lie in separate planes both directly opposite the thick shutter. Because they are in separate
planes, both flags can be inserted into the beam path simultaneously. In addition to the normal Honeywell
Measurex three point standardization, this allows two point verification. These two points being Flag2 and
Flag1 + Flag2. Source 12 software supports this new standardization in the following way: three point
standardization provides the usual correction factors, which are then applied to evaluate the weights of Flag2
and Flag1 + Flag2. The combination of Flag1 and Flag2 is called Flag12 (flag one two). The weights of the
flags should be constants since corrections have been applied for temperature, Z, dirt, new background, and air
readings. The readout of the weights thus constitutes a true quality indicator which can be tracked over time.
Differences in the weight readings from the weights at calibration time are referred to as Flag2 error and Flag12
error.
Calculating the weights of Flag2 and Flag12 is fundamentally different from the common but sometimes
misleading practice of calculating the weight of the single flag (as in Source 6 and Source 9). Calculating the
weight of the single flag is not as independent as measuring at another weight. This is because the dirt
correction is based on the single flag ratio just as is the weight of the single flag. At this ratio, the nature of the
dirt correction tends to compensate exactly, whether or not the correction is appropriate. Using a second flag, at
a different weight and ratio, there is both statistical and systematic independence. Thus the Flag2 and Flag12
errors are much better quality indicators than the old style flag weight, and also far better than common attempts
to use the F/A (Flag to Air) ratio as a quality indicator. The latter is true because the F/A ratio is, by design, the
basis of a corrector, and therefore expected to change, much as the air gap temperatures will. To allow for
Advertisement
