Level measurement in hot stone bins: an application guide
Hot stone bins are a critical component of any asphalt production plant, serving as the primary storage and separation point for heated aggregate prior to mixing.
Consistent asphalt quality depends on every stage of the production process performing reliably, and that includes the level control on your hot stone bins. When level instruments fail or give inaccurate readings, the wrong proportions of aggregate reach the mixer, and the consequences show up in every batch.
Purpose of hot stone bins
Aggregates, including gravel, sand, and crushed rock, must meet two essential conditions before they can be incorporated into an asphalt mix:
- Heated to remove residual moisture and reach the required process temperature.
- Sorted by size for precise mixing formulas.
The hot stone bin is where this heated, and sorted material is temporarily stored before moving on to the next stage in the process.
How a hot stone bin works
- Heating the aggregates – Raw stone from the quarry is dried and heated in a rotating drum, commonly referred to as a dryer.
- Screening by size – The hot material is elevated and passed over vibrating screens that separate it into distinct size fractions (e.g. fine sand, medium aggregate, coarse stone). This stage is also known as the screen house.
- Storage in bins – Each size fraction drops into its own compartment within the hot stone bin, where materials are kept both separated and at temperature.
- Controlled release – During asphalt production, precise quantities of each aggregate size are released from the bins into a weigh hopper. This ensures that every batch of asphalt meets the exact specification required for its intended application.
Think of hot stone bins as a bank of heated storage compartments, each holding a different aggregate size fraction and always ready to be mixed in exactly the right proportions for road building and repair.
Benefits of a reliable hot stone bin installation
- Accurate mix control – Stores different aggregate sizes separately to enable precise, repeatable batching.
- Consistent quality – Every asphalt batch is produced to the same specification.
- Temperature retention – Aggregates are maintained at the correct temperature for effective bitumen coating.
- Efficient operation – The bin acts as a buffer between continuous drying and batch mixing, smoothing out production flow.
- Process automation – Works with level instruments for reliable, fully automated control.
- Flexibility – Enables rapid changeover between different asphalt mix designs.
- Reduced waste and downtime – Minimises off-specification material and unplanned production interruptions.
Challenges of working with hot stone bins
- Segregation risk – Uneven material distribution can adversely affect mix quality.
- Instrument reliability and installation – Level instruments that are incorrectly installed or configured will produce inaccurate readings, undermining process control.
- Wear and abrasion – Bins, liners, and gates are exposed to constant high-temperature wear.
- Blockages and bridging – Hot aggregate can compact or arch, disrupting material flow.
- Heat loss – Inadequate insulation reduces thermal efficiency and can compromise mix quality.
- Maintenance access – Internal components can be difficult and hazardous to service safely.
- Calibration errors – Incorrect weighing or level readings will result in off-specification asphalt.
Why capacitance probes for level control?
VEGACAL 67 Capacitive high temperature probe for level measurement of bulk solids – available with threaded and flange options
Capacitance probes are a widely used and proven technology for level measurement and level detection in hot stone bins. Although radar has been successfully applied in many aggregate storage applications, hot stone bins often present challenges as bin shapes, restricted mounting positions etc, making capacitance measurement an attractive alternative.
The VEGACAL 67 capacitance level transmitter excels in these demanding conditions, delivering reliable level measurement with repeatable performance at process temperatures up to 400°C. It is available with both threaded and flanged process connections, offering flexible installation to suit any plant configuration.
For maximum convenience and safety, the instrument can be fitted with a head-mounted Bluetooth PLICSCOM display and adjustment module. This enables wireless access for configuration, diagnostics, and monitoring — allowing operators to manage the system efficiently without direct contact with the process.
With the VEGACAL 67 and PLICSCOM combination, you get a robust, high-precision solution designed for performance, safety, and operational efficiency.
The instrument can be mounted at either the top or bottom of each hot stone bin. Each sensor is fitted with an isolation tube of a specified length to maintain the required clearance from the external bin wall. A flexible stainless steel cable connects the instrument to an electrode that follows the contour of the bin from top to bottom, forming an integral part of the capacitance probe assembly.
The electrode can be fabricated in various forms, such as angle iron or steel rod, depending on the application. It is secured to the bin wall using isolation brackets, typically manufactured from ceramic to provide the necessary durability and high-temperature resistance.
A range of isolation bracket types is available, and these can be supplied specifically tailored to the application for optimum performance and longevity.
At VEGA we understand that no two hot stone bin applications are identical. Contact us today to organise a site visit and let one of our dedicated team of engineers assess your installation and discuss the right solution for your plant.
Alternatively, you can get in touch with your local sales engineer here or fill in our contact form and someone will be in touch to discuss further.
