Pneumatic Load Cells
Pneumatic load cells also operate on the force-balance principle. These devices use multiple dampener chambers to provide higher accuracy than 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
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. 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.
Miniature — Subminiature and miniature compression load cells are designed to perform in high capacity loads with minimum available space. Heavy-duty 50.8 mm (2”) diameter compression load cells have a low profile, the small size accommodates test benches, industrial weighing applications and prototype structures. They should include a twist-lock connector and a cable connection. There are miniature load cells in metric configurations, ranging from 0 to 100 and 0 to 50,000 Newton range. Sought after features: stainless steel rugged construction, built-in load button, high-accuracy and a 5-point NIST traceable calibration included.
Strain—Strain load cell sensors are suitable for accurate dynamic and static measurement. Designed with a grid of fine grade wire or foil that is bonded to a carrier matrix backing, proportional variance of electrical resistance is in linear variance with grid strain. The strain is found by measuring change in resistance when force is applied to the carrier matrix, which is bonded to the surface. The carrier matrix and adhesive bond work together to transmit strain or change in resistance to the grid. Adhesive and carrier matrix also dissipate heat and insulate against electrical noise, which can act as interference and alter readings. The Wheatstone Bridge Circuit Theory is widely used in static strain measurement for its outstanding sensitivity.
Beam—Low-capacity bending-beam load cells made of aluminum alloy have a capacity range from 1 to 500 kg. Used for OEM force measurement and weighing applications, industrial benefits include cost efficiency. Single point load cells are also aluminum alloy with 1 to 500 kg capacity ranges. Heavy-duty shear beam load cells are manufactured in corrosion resistant, nickel-plated, carbon-steel alloy. Shear beam and double beam load cells are also used in multiple cell applications like tank weighing and industrial process control. Cantilever or bending beam load cells are used for static weight, dynamic weighing, as well as force measuring operations.
Platform—These hermetically sealed load cells are best for applications requiring water tightness and high accuracy, such as industrial food processing, weighing, and automatic weighing stations. Resistive load cells built with bonded foil strain gages can be accurate to ±0.02% of full scale, they offer off-center load compensation which is useful for building scales that are accurate, even when objects to be measured are placed anywhere on a loading platform. S-Type—S-beam load cells receive compression output readings. They are designed to provide best performance in compact and versatile units, suspended loads, tank weighing, and hoppers.
Canister Style—This type of load cell is used for single and multi-weighing applications, also hermetically sealed and water resistant. The heavy-duty design canister load cell sensors are environmentally sealed to accommodate harsh environments, and are best for axial compression applications. Hydrostatically Compensated—Used for submerged operations like marine weighing, underwater platforms, in pit flooding environments, and on dry docks. All stainless steel construction is very reliable in harsh underwater conditions.
Tension/Compression—Tension/compression load cells are versatile with low profile and welded stainless steel design. They are highly accurate in monitoring compression and tension forces. Industrial load cells with threaded load connections are constructed to measure tension or compression forces in harsh industrial environments. Bi-directional units range from 25 to 10,000 pounds in 2” diameter (FSO Linearity of ±0.15%).
Evaluating Load Cell Selection
- Measurement Duration: Short-term duration with a Tare, or longer measurement durations where Tare is not applicable
- Output Requirements: Digital USB, wireless, digital RS232/RS485, analog mV/V, 0 to 5V, 4 to 20 mA
- Measurement Speeds: 1 Hz, up to 100 Hz, faster than 100 Hz
- Direction of Loading: Tension, compression, combination of both
- Percent Accuracy Requirement: As a percentage of reading, or percentage of full scale output
- Operating Temperature Conditions: Room temperature, outdoor temperature, well-controlled environment, harsh environment with wide range temperature and humidity changes
- Options for Mounting: Fastening sensor to both sides, unfastened mount
- Certifications Requirement: Trade applications: NTEP or OIML certifications, testing or measurement ASTM E-74 or R&D certification
- Operations Cost: Totals depend on small quantities, or bulk volumes