Wood splinters that jam coffee grinders. Stone fragments from the harvest that destroy milling equipment. Glass shards that trigger a six-figure recall from a single incident. Anyone processing bulk materials knows the risk: foreign objects enter the product stream via raw materials, harvesting, and upstream equipment. Manual sorting lines are slow, labor-intensive, and a contamination risk of their own.
Gravity feed metal detectors — also called free-fall metal detectors — are specialized inspection systems that detect metallic contaminants in vertically falling bulk materials and automatically reject them. Powders, granules, grains, flakes, and pellets pass through a round detection coil in free fall, with no mechanical contact and no conveyor belt. Gravity handles the transport; the detector handles the monitoring.
The range of applications is broad: nuts, coffee, spices, flour, plastic granulate, and pharmaceutical active ingredients. Wherever dry or free-flowing materials move through gravity chutes, gravity feed metal detectors form a critical protective barrier — shielding downstream machinery, preventing metal-contaminated end products, and supporting HACCP, IFS, and BRC compliance.
RaymanTech offers gravity feed systems for various pipe diameters, throughput rates, and industry requirements, alongside our broader industrial metal detector range — conveyor detectors, pharmaceutical metal detectors, and pipeline detectors. As a technology partner for product inspection, we support you from consultation through a trial run with your actual product at our German test center, all the way to ongoing service.
A gravity feed metal detector is mounted vertically in a gravity chute — typically a stainless steel fall pipe. Bulk material drops through a cylindrical detection aperture holding a transmitter coil and two symmetrically arranged receiver coils. As a metallic particle crosses the field, it disturbs the electromagnetic balance. Ferromagnetic metals such as steel noticeably change the signal's amplitude and phase; non-ferromagnetic metals such as aluminum or copper generate eddy currents. Austenitic stainless steel is, in practice, the hardest metal type to detect.
Unlike conveyor-belt detectors with a rectangular aperture, gravity feed systems use a round one: this creates a more homogeneous field in which the concentrated falling product stays mostly within the most sensitive zone. Typical aperture diameters range from 30 to 200 mm — the smaller the aperture, the higher the sensitivity. Modern signal processing detects a metallic particle within milliseconds; total response time to rejection is typically under 50 ms.
Product effect describes a product's tendency to generate a signal that resembles a metal particle. Moisture, salt content, mineral content, and temperature all influence the electromagnetic properties of the product stream. In free-fall applications, bulk density also fluctuates with the dosing rate — a pulsing material flow produces varying interference signals.
Product effect is a frequently underestimated challenge, particularly with spices and coffee. Modern gravity feed metal detectors counter it with several technologies:
Good product rejected in error is a significant cost factor for high-value bulk materials such as nuts, specialty coffee, or spices — with thousands of detection events per shift, losses add up fast. Common causes: insufficient product-effect compensation, overly sensitive settings, pulsing product flow, and outdated signal processing.
Effective countermeasures: a product trial with the real material before purchase (RaymanTech offers a German test center for this), multi-frequency or AI-assisted technology for high-product-effect goods, precise tuning of the rejection window to fall height and product speed, and regular re-learning after seasonal raw material variation.
Three basic mechanisms are available for free-fall applications:
Switching time is critical: it must be precisely tuned to fall height and product speed to minimize good-product loss without letting contaminated material slip through.
The food industry is the largest area of application: nut processing, coffee production, spice and herb processing, milling, sugar and confectionery, grain and seeds, milk powder and infant formula, and dried fruit. Here, the gravity feed metal detector often serves as a Critical Control Point (CCP) in the HACCP concept.
The pharmaceutical industry uses gravity feed systems primarily in tablet and capsule production, with strict requirements for data logging (FDA 21 CFR Part 11), audit trails, and validation (IQ/OQ/PQ).
The chemical and plastics industry: gravity feed metal detectors protect extruders, injection molding machines, and pelletizers from metallic contamination in the granulate stream. Plastic granulate has an almost negligible product effect, allowing very high detection sensitivities. ATEX-certified versions (Zone 22) are also used in potentially explosive environments.
The animal feed industry uses gravity feed detectors for pelleted feed and pre-mixes after mixing or pelleting.
Proven installation points: after the mixer or mill, before the filling station (the "last line of defense"), at goods receiving, and after product-specific steps such as roasting or drying. The highest product safety comes from multi-stage inspection strategies with detectors at several process points.
Technical installation requirements: sufficient fall height for a consistent fall speed, a defined distance between coil and reject point (typical system lengths 790–1,080 mm), vibration-decoupled mounting, a compressed air supply of 5–10 bar for the diverter valve, and careful grounding to prevent electromagnetic interference.
Sensitivity is expressed in millimeters. Reference values at 100 mm pipe diameter: ferrous from 0.5 mm, non-ferrous from 0.8 mm, stainless steel from 1.0–1.5 mm. The smaller the pipe diameter, the higher the sensitivity — at the cost of throughput. For high-product-effect goods, expect a sensitivity reduction of 30–50%. A product trial delivers reliable figures for your specific application.
In food production, the gravity feed metal detector is often defined as a CCP in the HACCP concept — with defined limits, regular verification (at least four function checks per shift), documented corrective actions, and complete record-keeping. IFS Food classifies foreign-body detection under Chapter 4.12 as a knock-out criterion; BRCGS requires risk-based testing intervals and documented trend analysis under Clause 4.10.3.
For food-industry use, common requirements include IP65 protection (IP69K for wet areas), stainless steel 1.4301 or 1.4404 for product-contact parts, dead-space-free CIP-capable construction, and tool-free disassembly. In explosive dust atmospheres (flour, sugar, starch), ATEX certification for Zone 22 is required.
If your contamination risk goes beyond pure metal — stone fragments, glass shards, or wood pieces, for example — a metal detector alone isn't enough. For these cases, RaymanTech also offers combo systems with x-ray inspection systems and optical sorting.
Gravity feed metal detectors generally offer the best cost-to-benefit ratio for pure metal inspection of bulk materials. The highest product safety comes from combining a magnetic separator as a pre-stage with the gravity feed metal detector as the main inspection step.
All free-flowing, dry to slightly moist bulk materials: powders, granules, grains, flakes, nuts, and pellets. What matters is that the material can flow freely through the fall pipe under gravity.
The diameter depends on required throughput and desired sensitivity. Rule of thumb: choose the smallest diameter that still covers your throughput. For exact sizing, a product trial with your actual bulk material is recommended.
The diverter valve is an open system for granulates and coarse powders. The sealed valve is dust-tight and better suited to fine, dusty, or high-value products.
The minimum is four verification tests per shift using standardized test pieces, plus additional tests after product changeovers, cleaning, or maintenance. IFS and BRC require a risk-based schedule and complete documentation.
Multi-frequency or AI-assisted detection can help separate product-effect signals more precisely from genuine metal signals and reduce false triggers. Rejection time should also be precisely tuned to fall height and product speed. A trial with your actual product before investing shows the real false-reject rate.
Yes. Vertical installation needs little floor space; the main requirements are sufficient installation height (typ. 790–1,080 mm system length), a compressed air supply, and a power connection (100–240 V AC).
Yes, modern systems detect ferromagnetic metals, non-ferromagnetic metals, and austenitic stainless steel. Stainless steel is, however, the hardest metal type to detect — its detection limit is typically 50–100% higher than for iron.
At minimum, IP65 protection, stainless steel construction, and hygienic design principles. For HACCP, IFS, and BRC, an integrated validation function with digital logging is advisable. ATEX certification for Zone 22 is additionally required in potentially explosive environments.
