Thickness Monitoring

Thickness monitoring methods

When you need to measure something’s thickness, a tool such as a ruler can be used to a resolution of about 1 mm – however, trying to measure by eye with any more accuracy than this is unreliable.

For measuring smaller dimensions, the micrometre (similar to a vernier calliper) was invented. These use a precise screw thread to measure small distances with a resolution of about 0.01 mm, and digital or mechanical versions are relatively inexpensive.

Micrometres are still widely-used in low-volume manufacturing where processes are performed by hand. However, for mass production, taking repeated manual measurements using a micrometre is impractical as it would place a significant bottleneck on the production line and reduce its speed.

Micrometre used for measuring by hand.

For performing accurate measurements on a production line, a robotic Coordinate Measurement Machine (CMM) can be used. These use a probe to measure the 3D geometry of a manufactured part and can be automated to perform the inspection much faster and more accurately than a human operator. Robotic CMM inspection is used in high-volume production of parts with complex geometry, such as components in a car engine.

However, thin materials such as paper only have one important measurement to monitor – their thickness. Using a micrometre for this is too slow and could easily damage the material, while a CMM system is unnecessarily complex and could still slow the rate of production. To solve this problem, specialised radioactivity-based thickness monitoring instruments have been developed to provide a fast, non-contact method of measuring the thickness of thin sheet materials.

Thickness monitoring using radioactive isotopes

Alpha (α), Beta (β) and Gamma (γ) are the main types of radiation, with each having different penetration power through materials. All three types of radiation are radiation particles, with alpha being the heaviest and most ionising and gamma being the lightest and least ionising.

• Alpha (α) radiation, a helium nucleus, is typically blocked by a sheet of paper and will only travel a few centimetres through the air.

• Beta radiation, a fast-moving electron, can penetrate through a sheet of paper but will usually be blocked by a thin metal plate.

• Gamma radiation, a high-energy photon, is the smallest particle and has the most penetrative power, only being blocked by a thick metal plate of a material like lead or iron.

The relationship between material thickness and the amount of radiation attenuation can be used to provide a non-contact measurement. By knowing the initial radiation beam intensity and measuring the intensity of the beam after it has passed through the material, the amount of attenuation and corresponding material thickness can be calculated.

As the radiation beam intensity can be measured continuously, this technique can provide a way to inspect thin materials in real-time as they are manufactured on a non-stop production line.

Radioisotopes used in thickness monitoring

Beta radiation is most commonly used for thickness monitoring applications, as it has the most suitable penetration power; alpha radiation would be blocked by even a thin sheet of paper, while gamma radiation will pass through most thin materials - although gamma radiation is sometimes used for thicker sheet metal materials.

A radioactive source with a long half-life is ideal for this application, as this means the rate of activity will be constant for a long time. This is useful as it means the radioactive source will not have to be changed often, and the activity level will be almost constant each day.

Thickness monitoring instrument

The instrument needed to perform radioactive thickness monitoring is sometimes called a radioactive gauge.

When implemented in the manufacturing process for a thin sheet material like paper or aluminium foil, the radioactive gauge is generally used to both monitor and control the product thickness. These products are usually produced using rollers which form the material into a sheet through compression and drawing.

Diagram of a radioactive thickness monitoring instrument (radioactive gauge) incorporated into a roller production system. - StudySmarter Originals

1. After the sheet material has passed through a set of rollers, it passes under a radioactive source with a known intensity.

2. A detector on the other side of the material measures the intensity of radiation that passes through the sheet and feeds this data to a computer processor.

3. The processor calculates the material thickness based on the known initial radiation intensity, the measured intensity of radiation that passed through the sheet, and the attenuation properties of the sheet material.

4. This calculated thickness is compared to the target thickness. If the material is too thick or thin, the computer adjusts the force on the rollers to correct the error.

Thickness monitoring examples

The most obvious application of radioactive gauges is in controlling the thickness of sheet materials during their manufacture. Depending on the material being produced, these use either beta or gamma emitters depending on the penetration power needed. However, the technology has also found several other uses in similar non-contact inspection applications. Here are some examples.

Thickness monitoring applications

• Paper production (Beta)

• Aluminium foil production (Beta)

• Metal sheet and plate production (Gamma)

Thickness Monitoring: Other industrial applications

• Fluid level monitoring – detecting when a container has been filled to a certain level (Gamma)

• Quantities/compositions of raw materials travelling on a conveyor belt (Gamma)

• Analysing density distribution of materials within a closed container (Gamma)

Commonly-used radioisotopes for gauge applications

• Krypton-85. Beta emitter with a half-life of 10.8 years.

• Caesium-137. Beta emitter with a half-life of 30.17 years.

• Americium-241. Primarily an Alpha emitter, with gamma radiation byproduct and half-life of 432.2 years. Used as a gamma emitter in radioactive gauge applications, as the alpha particle is blocked by a few cm of air.

• Cobalt-60. Beta emitter with a half-life of 5.3 years, decaying into Nickel-60 which emits gamma rays.