Closed-loop dosing on a modern holding furnace or dosing furnace depends on a continuous, non-contact metal-level signal feeding the pump or shot valve, and three sensing families now dominate that loop: radar, laser triangulation, and eddy-current proximity [S3][S7].
Each method trades standoff distance, accuracy, and tolerance to fume, dross, and refractory wall heat, and that tradeoff decides whether a furnace can be left unattended during an automatic dosing cycle [S1][S2]. The dosing furnace stores molten metal and supplies a defined shot weight to the casting machine, so its level loop is the single control point that defines fill accuracy and overfill protection [S2].
What the level sensor actually controls inside a dosing loop
Per the dosing-furnace control architecture described in patent WO2002100575A1 (priority date 2001-06-11, Alcoa Corp.), a level sensor located in the dosing chamber feeds a pump speed reference, so the loop is closed on bath height, not on a fixed shot weight [S1]. In an automated cold-chamber die-cast cell, that bath-height signal is what paces ingot additions into the upstream holding furnace and gates the next shot [S2].
Two operating modes are typical: a low-level setpoint that triggers a charge signal, and a high-level setpoint that trips an overfill alarm and disables further metal transfer [S4]. The sensor therefore needs sub-millimeter repeatability at the setpoint, not just absolute accuracy, and the resolution spec is the number an automation engineer should compare first.
Sensor families compared on the criteria that matter
Three non-contact principles are in commercial use for molten aluminum, zinc, tin and bismuth below 1400°F (760°C), and they split cleanly by mounting position and process environment [S3][S4][S7].
Closed-vessel radar (FMCW at 80 GHz or 122 GHz) measures through the furnace roof with a sealed window or waveguide, with the OndoSense Furnace Protect unit claiming sub-millimeter accuracy on enclosed melting, holding and casting furnaces from outside the vessel [S3][S5][S6]. Laser triangulation sensors are specified for open crucibles, furnace wells, or dosing chambers with line-of-sight to the bath, where the laser spot can be aimed at the melt surface without smoke or heavy fume in the path [S7]. Eddy-current proximity disks sit 0.5 in (13 mm) above the bath and resolve ±0.04 in (±1 mm) at that gap, with a 5 in disk and a NEMA 3R electronics enclosure kept under 150°F (65°C) inside [S4].
On the four decision criteria that drive a spec, the comparison is straightforward: radar wins on standoff (mounted through the roof, immune to surface dross), laser wins on accuracy in clean open wells, and eddy-current wins on simplicity and cost for low-temperature non-ferrous melts where the 0.5 in (13 mm) air gap can be held [S3][S4][S7]. Ultrasonic, while listed as a non-contact option for molten metal, is sensitive to surface waves, dross, and fume and is rarely specified in the same loop as a dosing-furnace pump reference [S9].
Resolution, setpoint, and the numbers an automation engineer should pin
The dosing-loop setpoint is typically a level window, not a single point: a low setpoint that starts the upstream transfer, and a high setpoint that arms the overfill lockout [S4]. For an eddy-current disk at 0.5 in (13 mm) standoff, resolution of ±1 mm means a usable level window of 3-5 mm before the high trip fires, which is the practical limit on shot-weight repeatability if the dosing pump is volumetric [S4].
Closed-vessel radar moves that limit: published sub-millimeter accuracy over the full bath means the dosing chamber can be controlled on a level band of 1-2 mm, which in turn lets a dosing furnace hold shot-weight variation tight enough to feed an automatic automatic molding line downstream without operator trimming [S3][S5]. Electronics-side ratings are part of the spec: NEMA 3R minimum for the enclosure, internal temperature below 65°C, relay output SPDT (1 Form C) for a low/high contact, and 120 VAC 50/60 Hz power [S4]. Cabling distance is not a marketing line, it is a measured constraint, with a 5 ft (1.5 m) maximum recommended from the eddy-current sensor to the electronics box to keep the low-loss cable within its loss budget [S4].
Where each method is the wrong choice
Eddy-current proximity sensors are not recommended for molten magnesium or any fuming metal whose vapor can ionize the 0.5 in (13 mm) air gap and create a false-conductivity path between the disk and the bath [S4]. That same constraint rules them out for high-vapor aluminum alloys at the upper end of the 1400°F (760°C) rating, where a 122 GHz radar through a sealed roof window is the safer path [S3][S4].
Laser triangulation fails when the optical path is blocked by fume, smoke from a charge cycle, or floating dross, which is exactly the environment of an open dosing ladle being skimmed between shots, and that is why suppliers such as Precimeter specify it for clean wells rather than active dosing chambers [S7]. Ultrasonic sensors, while non-contact and inexpensive, are sensitive to the surface waves, gas bubbles, and acoustic noise of a stirred bath, and the published guidance is to keep them out of closed-loop dosing [S9].
Integration into the holding furnace, transport ladle, and dosing pump
A real cell stacks three metal-handling units, and the level sensor sits at the metering one. The holding furnace holds already-molten metal in the required temperature range and acts as a buffer between the melter and the cell; the dosing furnace stores molten metal and supplies a defined shot to the casting machine; the transport ladle or launder connects them [S2]. The level sensor that closes the dosing loop therefore needs to be electrically and mechanically isolated from the holding furnace control, so that a setpoint trip on the dosing chamber does not propagate upstream as a false low-level charge request to the automatic level controller on the holding furnace [S1][S2].
In practice this is wired with the dosing-furnace level sensor providing a pump speed reference on a dedicated 4-20 mA or 0-10 V analog channel, with the high-level contact wired to a hard-wired interlock that disables the launder valve, rather than over the same network that handles the upstream metal material temperature control [S1][S8]. The dosing-furnace level signal should also be available as a dry SPDT contact for the PLC, so that the alarm path survives a network dropout [S4].
What changed in 2026 and what to watch for the next cycle
Two 2026 product updates from OndoSense moved the closed-vessel radar story forward: a level-monitoring solution for closed melting furnaces published 2026-07-08, and the aluminum-industry release of the same non-radioactive radar approach on 2026-06-28 [S5][S6]. Both releases explicitly target enclosed holding, melting, and casting furnaces and quote non-contact, high-precision measurement of molten metals from outside the vessel, which is the configuration that an unattended dosing cell needs [S3][S5][S6].
For the next procurement cycle, two signals are worth tracking: the published drop-in radar window assemblies for retrofit on existing refractory roofs, and the metal powder and dosing-foundry cells where laser triangulation is being qualified on aluminum dosing wells alongside radar on the upstream holding chamber [S5][S6][S7].
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