Single point calibration is the simplest type of calibration, recommended for applications that require precise measurements under only one load or torque. If the force sensor is known to be linear, repeatable and has the correct slope within the required measurement range, a single point calibration can be applied to adjust any zero offset error (zero balance). Single point force sensor calibration also helps to verify "output drift" in order to correct any deterioration in sensor performance over time.
Two point calibration is more complex and precise than single point calibration. In two point calibration, the weighing load cell offset is adjusted at two different output values, resulting in a relatively accurate straight line across the entire force measurement scale. It is usually recommended to perform a two point calibration because the weight and torque sensors are known to have reasonable linearity within the measurement range (or rated output). This basically rescales the output by correcting the slope (weighing load cell sensitivity) and offset (zero balance) errors. With the new zero offset and slope (weighing load cell sensitivity), a linear equation characterizing the sensor output can be determined.
Some critical applications require high accuracy within very specific measurement ranges for force sensors. In these cases, a five-point calibration of the weighing load cell and curve fitting are needed to characterize the calibration curve and achieve measurement output within a specified output range. Usually calibrated at 0%, 20%, 40%, 60%, 80% and 100% of the desired measurement range.
During the five-point calibration of the force sensor, output readings are increased and decreased to determine the repeatability and hysteresis of the force measurement system (sensor + signal conditioner). Since most force sensors are paired with reading displays or signal conditioners to form turnkey force measurement systems, the instrument should always be connected to the sensor and calibrated with standard equipment from the torque sensor calibration laboratory as a system. This process is repeated twice or more depending on the application requirements. Use the differences in output to calculate non-repeatability (or repeatability) and linearity (accuracy).
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