Introduction to Load Cells

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. 

Load Cell History

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 Performance Comparison
Type	Weight Range	Accuracy (FS)	Apps	Strength	Weakness
Mechanical Load Cells
Hydraulic Load Cells	Up to 10,000,000 lb	0.25%	Tanks, bins and hoppers.  Hazardous areas.	Takes high impacts,  insensitive to temperature.	Expensive, complex.
Pneumatic Load Cells	Wide	High	Food industry, hazardous areas	Intrinsically safe.  Contains no fluids.	Slow response.  Requires clean, dry air
Strain Gage Load Cells
Bending Beam Load Cells	10-5k lbs.	0.03%	Tanks, platform scales,	Low cost, simple construction	Strain gages are exposed,  require protection
Shear Beam Load Cells	10-5k lbs.	0.03%	Tanks, platform scales,  off- center loads	High side load rejection, better  sealing and protection
Canister Load Cells	to 500k lbs.	0.05%	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  Load Cells	0-50k lbs 0-200 lbs. typ.	1%	Small scales	Small, inexpensive	Loads must be centered, no  load movement permitted
Other Load Cells
Helical	0-40k lbs.	0.2%	Platform, forklift, wheel load,  automotive seat weight	Handles off-axis loads,  overloads, shocks
Fiber optic		0.1%	Electrical transmission cables, stud or bolt mounts	Immune to RFI/EMI and  high temps, intrinsically safe
Piezo- resistive		0.03%		Extremely sensitive, high  signal output level	High cost, nonlinear output

Load Cell Operating Principles:

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.