Fill level inspection: camera, X-ray or high-frequency?

What are the three ways to check fill level on the line?

Every fill level inspection system answers one question for each container: is the liquid between the minimum and maximum level? The methods differ in how they find the liquid.

• Camera (vision). A camera images the container against a backlight and finds the liquid line as an edge. This is what addaScan builds.
• X-ray (and gamma). Radiation passes through the container at the expected fill height; liquid absorbs more of it than the gas above, so the detector can tell whether liquid is there.
• High-frequency. The container passes through an electric field, and the liquid in the neck changes that field.

A single fill level sensor, such as a photocell at the neck, is the simplest form of bottle fill level detection; one supplier of fill inspection systems notes that in the simplest case a photocell suffices (supplier documentation).

How does a camera check fill level, and what can it not see?

The camera sits on one side of the conveyor and a backlight on the other. Light passes through the empty neck and is bent or absorbed where there is liquid, so the liquid line shows as a clear edge. The station compares the edge with limits taught from good containers and rejects under-fills and over-fills. Fill level, cap and seal inspection describes the station in detail.

The same image shows the cap, so one station can check fill, caps and the tamper-evident band.

The limits are physical:

• Opaque containers, full sleeves and cans hide the liquid. No lighting fixes that.
• Tinted containers depend on how much light passes; a different light colour sometimes helps, but dark tints can hide the line.
• A label over the fill zone blocks the view; the check needs a clear band at the fill height.
• Height is not volume. The same level means the same volume only if the containers are the same inside.

How does X-ray fill level inspection work?

An X-ray fill level inspection unit sends a beam across the container at the level the filler should reach. The contents absorb more radiation than the gas above them, so the signal at the detector shows whether liquid is present at that height. One supplier describes the signal crossing the product at the expected level; a container that does not reach the minimum is treated as under-filled, one that reaches the maximum as over-filled (supplier documentation). another describes the same measurement of radiation absorption with X-rays or gamma rays (supplier documentation).

Its strength is that it does not need to see the liquid. Vendors offer X-ray for any container material, including metal (supplier documentation), and for opaque containers (supplier documentation). One supplier reports using it on completely opaque and sleeved bottles where an optical check was not an option (supplier documentation).

The cost is regulation. Suppliers note that radiation-based fill level measurement needs a special licence and trained staff to follow radiation protection rules (supplier documentation). In South Africa, the Hazardous Substances Act classifies electronic generators of ionising radiation, which include X-ray equipment, as Group III hazardous substances, and section 3 controls their sale, use, operation and installation; radioactive sources are Group IV. SAHPRA Radiation Control administers both (supplier documentation). Speak to SAHPRA and the supplier before you order.

What is high-frequency (microwave) fill level inspection?

Many people search for microwave fill level inspection; the suppliers cited here call the method high-frequency. In one supplier’s description, the container passes through a high-frequency electric field, the volume of liquid in the neck changes the field, and the change is measured and compared with reference values (supplier documentation). The same supplier adds that the technique made it possible to measure foamed-up liquids, and links its wide use to the licensing that radiation methods need.

One supplier offers high-frequency sensing for plastic and glass, with an option that measures liquid and foam together and calculates the level once the foam has gone, and offers X-ray for metal (supplier documentation). Another presents its high-frequency system as an alternative to photon (gamma or X-ray) systems, without the reporting to federal or state authorities that it says a photon option needs in its market (supplier documentation).

The frequency, the fit with your bottle and product, and the approvals needed in South Africa are questions for the supplier; we do not supply it.

Which method suits which container?

Start with the container, not the method. The table summarises what the methods can do as their makers describe them; anything marked “possible” still has to be confirmed on your containers and product.

For cans, a camera station still has work to do. It reads the date and batch code on the can end and finds visibly dented cans; see can inspection, which states plainly that the inside of the can is not checked. For returnable glass, empty bottle inspection checks the bottle before it reaches the filler, and crate inspection checks the crates.

What can each method tell you, and what not?

A level check, by any method, says the liquid reached a height. It does not say how many grams or millilitres are in the pack.

A checkweigher answers a different question. It does not care whether the container is clear, sleeved or metal, but it cannot see a crooked cap, a missing tamper band or a wrong code.

How do foam and sloshing affect fill level inspection?

Carbonated drinks, beer and some juices foam after filling, and every liquid moves when the container is pushed, transferred or stopped. One supplier notes that foam can mask the true level and that inspection should happen where the product is in a consistent condition, away from belt transfers that unsettle it (supplier documentation).

• Camera. The camera sees the top of the foam as well as the liquid below it. The station is placed where the surface has settled, and foam is included in the good samples so the limits allow for it. Whether that is enough is shown at the sample test.
• High-frequency. Vendors describe measuring foamed liquids, or liquid and foam separately (supplier documentation) (supplier documentation).
• X-ray. A beam at one height reads what is there at that moment, so placement after the surface has settled matters here too.

Sloshing is a mechanical problem before it is an inspection problem. Even spacing, smooth transfers and a stable guide rail help every method.

Where does fill level inspection sit on the line?

Usually straight after the filler and capper. One supplier lists fill level and closure checks among the inspections in the outfeed of the filler block (supplier documentation). There are three reasons to put it there:

• Fast feedback. A run of under-fills points to the filler while the cause is still there.
• One reject for fill and cap. A camera station that also checks caps uses the same reject and the same tracking.
• The view is still clear. For a camera, before the labeller or sleever covers the fill zone.

Full containers are heavy and move fast, so the reject must remove them without toppling the line, and a confirmation sensor should prove they left. The PLC and reject integration guide covers tracking and confirmation.

Does fill level inspection prove the net contents?

No. South African legal metrology is about the quantity in the pack. The Legal Metrology Regulations set tolerances by reference to SANS 458, and the NRCS may not permit the sale of prepackages that are deficient of the quantity marked. Where automatic filling or weighing instruments determine the quantity, users outside the “e” mark scheme must take steps to detect incorrect quantities and keep records of the checks (regulations 32, 34, 137 and 138) (supplier documentation).

The regulations set the outcome, not the equipment. A fill level check on every container is one way to catch gross under-fills between sample weighings; a camera, an X-ray unit or a high-frequency unit can each do that on the containers it suits. Proving the quantity still takes weighing or measuring, with the records the regulations ask for.

What does addaScan build, and what not?

We build camera stations on HIKROBOT hardware: fill level on clear, translucent and suitable tinted containers, caps, tamper bands, visible seals and codes, with the PLC link and reject. We do not supply X-ray or high-frequency fill level inspection. If your containers are cans, opaque or sleeved, we will tell you at the sample test that a camera cannot check their fill, and suggest you ask an X-ray or high-frequency supplier or rely on weighing. We can still quote the checks a camera can do on those lines.

See bottling and canning lines and food and beverage for where each check fits.