A load cell is a transducer which converts force into a measurable electrical output. Although there are many varieties
of load cells, strain gage based load cells are the most commonly used type.
Before strain gage-based load cells became the method of choice for industrial weighing applications, mechanical lever scales were
widely used. Mechanical scales can weigh everything from pills to railroad cars and can do so accurately and reliably if they are properly
calibrated and maintained. The method of operation can involve either the use of a weight balancing mechanism or the detection of the force
developed by mechanical levers. The earliest, pre-strain gage force sensors included hydraulic and pneumatic designs.
In 1843, English physicist Sir Charles Wheatstone devised a bridge circuit that could measure electrical resistances. The Wheatstone bridge
circuit is ideal for measuring the resistance changes that occur in strain gages. Although the first bonded
resistance wire strain gage was developed in the 1940s, it was not until modern electronics caught up that the new technology became technically
and economically feasible. Since that time, however, strain gages have proliferated both as mechanical scale components and in stand-alone load cells.
Today, except for certain laboratories where precision mechanical balances are still used, strain gage load cells dominate the weighing industry.
Pneumatic load cells are sometimes used where intrinsic safety and hygiene are desired, and hydraulic load cells are considered in remote locations, as
they do not require a power supply. Strain gage load cells offer accuracies from within 0.03% to 0.25% full scale and are suitable for almost all industrial applications.
Load Cell Operating Principles:
| Load Cell Performance Comparison
|Mechanical Load Cells
|Hydraulic Load Cells
||Up to 10,000,000 lb
||Tanks, bins and hoppers.
|Takes high impacts,
insensitive to temperature.
|Pneumatic Load Cells
||Food industry, hazardous areas
Contains no fluids.
Requires clean, dry air
|Strain Gage Load Cells
|Bending Beam Load Cells
||Tanks, platform scales,
||Low cost, simple construction
||Strain gages are exposed,
|Shear Beam Load Cells
||Tanks, platform scales,
off- center loads
|High side load rejection, better
sealing and protection
|Canister Load Cells
||to 500k lbs.
||Truck, tank, track, and hopper scales
||Handles load movements
||No horizontal load protection
|Ring and Pancake Load Cells
||5- 500k lbs.
||Tanks, bins, scales
||All stainless steel
||No load movement allowed
|Button and washer
0-200 lbs. typ.
||Loads must be centered, no
load movement permitted
|Other Load Cells
||Platform, forklift, wheel load,
automotive seat weight
|Handles off-axis loads,
cables, stud or bolt mounts
|Immune to RFI/EMI and
high temps, intrinsically safe
||Extremely sensitive, high
signal output level
|High cost, nonlinear output
Load cell designs can be distinguished according to the type of output signal generated (pneumatic, hydraulic, electric) or according to the way they detect weight (bending, shear, compression, tension, etc.)
Hydraulic load cells are force -balance devices, measuring weight as a change in pressure of the internal filling fluid. In a rolling diaphragm type hydraulic load cell, a load or force acting on a loading head is transferred to a piston that in turn compresses a filling fluid confined within an elastomeric diaphragm chamber. As force increases, the pressure of the hydraulic fluid rises. This pressure can be locally indicated or transmitted for remote indication or control. Output is linear and relatively unaffected by the amount of the filling fluid or by its temperature. If the load cells have been properly installed and calibrated, accuracy can be within 0.25% full scale or better, acceptable for most process weighing applications. Because this sensor has no electric components, it is ideal for use in hazardous areas.
Typical hydraulic load cell applications include tank, bin, and hopper weighing. For maximum accuracy, the weight of the tank should be obtained by locating one load cell at each point of support and summing their outputs.
Pneumatic load cells also operate on the force-balance principle. These devices use multiple dampener chambers to provide higher accuracy than can a hydraulic device. In some designs, the first dampener chamber is used as a tare weight chamber. Pneumatic load cells are often used to measure relatively small weights in industries where cleanliness and safety are of prime concern.
The advantages of this type of load cell include their being inherently explosion proof and insensitive to temperature variations. Additionally, they contain no fluids that might contaminate the process if the diaphragm ruptures. Disadvantages include relatively slow speed of response and the need for clean, dry, regulated air or nitrogen.
Strain-gage load cells convert the load acting on them into electrical signals. The gauges themselves are bonded onto a beam or structural member that deforms when weight is applied. In most cases, four strain gages are used to obtain maximum sensitivity and temperature compensation. Two of the gauges are usually in tension, and two in compression, and are wired with compensation adjustments as shown in Figure 7-2. When weight is applied, the strain changes the electrical resistance of the gauges in proportion to the load.
Other load cells are fading into obscurity, as strain gage load cells continue to increase their accuracy and lower their unit costs.