Cross-Physics Sensor Replacement in Industrial Control Circuits
Cross-physics sensor retrofits require matching phase delay, thermal drift coefficients, and input filter bounds before changing the transduction element.
Thermal Drift Coefficients
Metrological Stability Variable
Numerical values describe how much a measurement deviates from its calibrated value as the ambient temperature changes. Within a sensor specification, thermal drift coefficients define the expected error per degree of temperature shift. This drift is caused by the physical expansion of mechanical parts and the changing electrical properties of semiconductor components.
Measurement Offset
Changes in the zero point or the sensitivity of the device can lead to significant inaccuracies in the field. A pressure sensor might report a different value at noon than it did in the morning even if the actual pressure remained constant. These errors are usually expressed as a percentage of the full scale range.
Component Expansion
Thermal energy causes the physical dimensions of sensing elements to change. In an inductive sensor, the resistance of the copper wire in the coil increases with temperature. In a capacitive sensor, the gap between the electrodes may shift due to the expansion of the housing.
Specification Tolerance
Manufacturers test sensors in environmental chambers to determine these coefficients before they leave the factory. A device with a low coefficient is more expensive but provides more reliable data in outdoor or high temperature applications. Users must account for these errors when calculating the total uncertainty of a measurement system.