What a weighing system does
An industrial weighing system measures mass, usually by measuring the force that gravity exerts on a load. It might be a floor scale for pallets, a platform under a mixing vessel, a set of sensors under a storage tank or silo, or a weighing section on a conveyor. In each case the arrangement has the same basic parts: a structure that carries the load, one or more load cells that sense the force, and an electronic instrument that turns the signal into a number.
Weighing is attractive where other measurements are awkward. Volume-based methods assume a known density, while weighing measures how much material is actually present. That is why it is common in batching, inventory control, filling and shipping.
How a load cell works
A load cell is a metal element designed to deform very slightly under load. The most common type uses strain gauges: thin foil patterns bonded to the metal. When the metal flexes, the foil stretches or compresses, and its electrical resistance changes by a tiny amount. Several gauges are wired together in a bridge circuit, which converts that small resistance change into a small voltage that is proportional to the force applied.
The signal is then amplified and converted to digital form by an instrument, often called an indicator or transmitter. It can be shown on a display, or sent on to a control system. Control systems often receive it through programmable logic controllers that use the weight for batching, alarms and interlocks.
Load cells come in several shapes, each suited to a different mounting situation:
- Compression cells sit under a load and are squeezed by it. Common under tanks and platforms.
- Tension cells hang a load from above, for example a suspended hopper.
- Shear beam cells are bar-shaped and often used in platform scales and tank weighing.
- Bending beam and single-point cells are used for smaller scales.
Other sensing technologies exist, such as hydraulic and some types using vibrating elements, but strain gauge cells are by far the most widely seen.
Weighing a tank or silo
Tanks and silos are often weighed on a set of load cells, commonly three or four, supporting the vessel. The instrument adds the outputs from all cells to find the total.
Here is a purely hypothetical example. Suppose three cells under a tank read 1,200 kg, 1,250 kg and 1,150 kg. The total is 1,200 + 1,250 + 1,150 = 3,600 kg. If the empty tank, its fittings and its supports weigh 1,000 kg (the tare), the contents weigh 3,600 − 1,000 = 2,600 kg. The instrument stores the tare and subtracts it automatically. The numbers are invented for illustration only.
This is a different principle from level sensors, which infer quantity from height; those are the subject of tank level measurement. Weighing can be more useful when density varies or when the surface is hard to see, such as a rathole in a silo or foam in a tank. Neither method is always better, and some plants use both as a cross-check.
Where errors come from
Even a well-built system can read wrong. Common causes include:
- Mechanical interference. Anything that carries part of the load or pushes on the vessel adds error. Rigid piping connected to a weighed tank is a classic problem, since it can transfer force. Flexible connections and carefully arranged pipe supports reduce this, along with flexible hoses and similar arrangements.
- Uneven or shifting loads. Wind, vibration from agitators, and material impact during filling can disturb readings.
- Poor mounting. Cells must be level and aligned. Tilted or twisted mounts can introduce side loads, which cells are not designed to measure.
- Temperature. Changes can affect the cells and the structure. Good cells include compensation, but large swings can still matter.
- Electrical issues. The signal is small, so poor cabling, moisture, electrical noise and stray currents can affect it. Good grounding systems and surge protection help, especially outdoors where lightning is a concern. Welding on or near a weighed vessel can also pass current through a cell, and the procedure for this should be planned with the manufacturer’s instructions in mind.
- Overload and shock. Cells can be permanently damaged by loads well beyond their rating, including sudden impacts. Mechanical stops are often fitted to limit the movement.
- Drift and wear. Over time, readings can shift.
Calibration and checks
Calibration compares the system with known reference weights, or with another method that has been traceably verified, and adjusts the instrument so that readings agree. The exact procedure and frequency depend on the application and on local rules. In situations where weight is used for trade, such as selling by weight, legal metrology requirements typically apply and are managed by approved bodies. That makes regular calibration more than a good habit; it can be a legal requirement.
Between formal calibrations, operators can watch for signs of trouble: a zero reading that no longer returns to zero when the vessel is empty, readings that wander without any material movement, or a sum of cells that changes when load is moved from one side to another.
Practical notes on use
Weighing tends to be best used with a well-understood process. In batching, the system adds one ingredient until a target weight is reached, then closes a valve or stops a feeder; a feeder’s behavior near the end of a batch, such as the extra material still in flight, affects accuracy and is usually handled by cutting off slightly early. Feeders themselves are discussed under screw feeders. When the whole feeder and hopper are weighed, the control system can adjust speed to hold a steady rate, known as loss-in-weight feeding.
Instruments that sense process variables, such as weight, pressure and temperature, are collectively a large part of process control sensors.
Safety
Load cell supports carry heavy loads. Work near weighed vessels, including loosening mounts or fitting new cells, can change how a structure is supported and should be planned by qualified personnel. Isolate energy according to your plant’s lockout tagout procedure, support the vessel properly, and follow the manufacturer’s manual and local regulations. This article is general information, not engineering advice.