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Feature
How a pressed-in and bolted-on strain-gauge weighing system functions
In the world of inventory monitoring in dry bulk storage bins, steel tanks, and other vessels, weight-measurement systems calculate the material weight in a vessel by converting a distance or level measurement to volume and then to weight. Weight-measuring systems, on the other hand, directly measure the material weight. This eliminates conversion errors that often occur in indirect weight measurement using level-measurement sensors. As a result, weight-measuring systems have an advantage — and are often required — in applications where high-accuracy material measurements are essential.
However, a drawback to directly weighing material in a vessel is the difficulty and expense of retrofitting an existing vessel with conventional weighing sensors (load cells). Strain-gauge sensors that can be pressed in or bolted onto the vessel's support structure simplify this task and reduce its cost. Unlike conventional stationary load cells, which are typically mounted under a vessel's legs or other support members while the vessel is lifted by a crane or other equipment, pressed-in and bolted-on strain-gauge sensors are simply attached to the vessel's legs or other support members. This eliminates a need to lift the vessel and may even allow it to remain in use during sensor installation.
A strain-gauge weighing system includes multiple pressed-in or bolted-on strain-gauge sensors, cables, typically one (and sometimes more than one) junction box, and a remote controller-display device.
Pressed-in sensors are attached by being inserted or pressed into a hole drilled into the vessel's leg, frame, or other support member. Bolted-on sensors are simply bolted onto the member.
photoOne junction box is typically located just before the controller-display; cables from each sensor lead into this box, and another cable connects the box to the controller-display. On a large vessel, a junction box (in this case called a terminal box) may also be located near each sensor simply to connect the sensor's short prewired cable (if so equipped) to an additional cable (up to 50 feet long) that runs to the remote controller-display.
In operation, as material is added to or removed from the vessel, the vessel's support members deform. The strain-gauge sensors mounted on the members measure the deformation as strain. Each sensor sends an electrical output signal (in millivolts DC) that's proportional to the increasing or decreasing strain to the controller-display via the terminal boxes (if so equipped) and the junction box.
The junction box typically averages the signals from all sensors before sending one averaged output signal to the controller-display. The controller-display converts this signal into a weight measurement in pounds, tons, or other appropriate units and displays it for the operator. The controller-display also provides operating power to the sensors via the junction box, performs calibration functions, and provides an interface that allows the operator to not only read the material weight in the vessel but set up and adjust the controller-display and the weighing system.
The number of pressed-in or bolted-on strain gauge sensors an application requires and where the sensors are attached will depend on the vessel size, the support structure's configuration, and the specified weighing accuracy. To recommend the correct number of sensors and their locations, the weighing system supplier will need accurate information about the vessel dimensions, support structure configuration, and required weighing accuracy.
For more information about strain-gauge weighing systems, you can read the article “Improving dry bulk inventory management with pressed-in and bolted-on strain-gauge weighing systems.” by Joe Lewis, BlueLevel Technologies Inc. You can also find articles listed under “Weighing and batching” and "Level measurement" in PBE's Article Index.
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