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Basic Measurement Principles and Sensor Features
Measurement Principles
This section describes the physical operating principles of beta-emitting basis weight sensors. Those already
familiar with the measurement principles of Honeywell basis weight sensors may want to go on to next section
"Features of the Precision Basis Weight Sensor."
Beta Particles and Basis Weight Measurement
Beta particles (or betas) are electrons emitted from atomic nuclei during nuclear decay. After leaving the
nucleus they may be thought of as an electron beam suchas in a cathode ray tube (CRT), found in a televisions
and computer monitors.
Beta particles from nuclear decay are not of a single energy but are emitted in a continuum of energies up to a
maximum value. This maximum energy value depends on the type of source capsule or isotope. Higher energy
betas are more penetrating and therefore can be used on heavier products. The most commonly used capsules, in
order of increasing maximum energy (the number signifies the particular isotope used), are as follows:
Promethium-147 (Pm-147)
Krypton-85 (Kr-85)
Strontium-90 (Sr-90)
The emitted beta particles will interact with the sheet in two different ways. It may be scattered from the sheet,
or it may lose some or all energy in the sheet. The betas that pass through the sheet and into the receiver enter
an ionization chamber. This is the detector. The ion chamber outputs a small current (approximately one nano-
ampere) which is proportional to the energy deposited in the ion chamber. The current from the ion chamber
goes through a short wire to an amplifier whose output is an analog voltage on the order of 0 – 10 volts. This
signal is sent to an electronic circuit and is read by a computer which averages the signal for some prescribed
time interval. Then using proprietary algorithms, the software converts the average signal to a calculated basis
weight of the product. The more material in the beam of betas, the more scattering and absorption, therefore the
smaller the signal. Beta particles are absorbed nearly uniformly by all substances, because as normal variations
in the chemical composition have very little effect on the absorption or basis weight reading. That is, the
absorption is dependent on the basis weight and not on color, texture, state of matter, etc.
This is a principal advantage of using beta sources in basis weight sensors. This means, however, that the air in
between the source capsule and the ionization chamber (as well as any debris in the beam) will absorb beta
particles just as the product being measured will.
Correctors
To accurately measure the product, several correction algorithms (correctors) are added. An ideal basis weight
sensor signal would change only when the sheet's basis weight changed. Unfortunately, despite the designer's
efforts, there remain external influences which affect the signal. For example, any increase in the mass between
the source and receiver causes a larger basis weight reading. Several factors can cause this: dirt build up on
windows, increase in air mass due to temperature change, or change in the distance between heads. These
effects should remain small relative to the raw or uncorrected basis weight reading. To compensate, these
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