Load Cells in Geotechnical Applications
DEFINITIONS
Load cell capacity: The Capacity is the maximum axial load a load cell is designed to measure within its specification and over which the load cell is calibrated. Load cells have a safe overload percentage to provide a factor of safety during installation and loading.
Safe load limit or over range: The maximum load that can be applied without producing a permanent shift in the performance characteristics beyond those specified; expressed as a percentage of the measuring range which is 150%.
Ultimate overload: The maximum load that can be applied without physical destruction of the load cell; specified as a percentage of the measuring range which is 300%.
Understanding the expected load is crucial, together with the capacity of the load cell
PERFORMANCE
A load cell will perform within specifications until the safe load limit is exceeded but it is not recommended to select the load cell assuming it will be used within this range. Beyond the safe load limit, even for a very short period, the load cell may be permanently damaged. It may physically break at the ultimate load limit.
OVERLOAD
Overload is still the primary reason for load cell failure. Although the process of selecting the right load cell capacity looks straightforward at first sight, capacity selection requires a fundamental understanding of the maximum expected and allowing a certain margin above this value.
SELECTION
When selecting a load cell, you should choose one that allows operation in the middle 80% of the sensor’s capacity. To ensure accurate readings, never use a sensor to measure force below 5% of the sensor’s rated capacity. Leaving a buffer/margin at the top end of the scale helps protect the load cell.
ECCENTRIC LOADING
A certain degree of eccentric loading is inevitable within the construction industry due to the fact that the axis of the tie-back, rock bolt or strut is rarely at right angles to the surface onto which the load cell rests. However, eccentric loading should be minimised to avoid achieving an overload situation. For vibrating wire load cells, the individual sensors can be read to ascertain the degree of eccentricity. Excessive eccentric loading could lead to damage of individual sensors. Any damage can be checked by measuring the resistance of the individual cable conductors.
For strain gauge load cells, compensation is achieved through the Wheatstone bridge circuit.
ELECTRICAL NOISE
When installing load cell cables, care should be taken to keep them as far away as possible from sources of electrical noise such as power lines, generators, transformers and arc welders.
LIGHTNING PROTECTION
Load cells are not fitted with lightning protection and therefore if they or cables are exposed especially long term lightning protection should be installed.
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