Key learnings:
- Sensor Definition: A sensor is defined as a device that detects and responds to physical input from the environment, converting it into a readable output.
- Range and Span: The range is the sensor’s measurable limits, while the span is the difference between the maximum and minimum values it can measure.
- Accuracy vs. Precision: Accuracy is the closeness to the true value, while precision is how close repeated measurements are to each other.
- Sensitivity: Sensitivity is the change in sensor output relative to the change in input.
- Linearity and Hysteresis: Linearity is the consistency of sensor measurements with an ideal curve, and hysteresis is the difference in output when input is varied in two ways.
Sensors are characterized by the values of several parameters. The important characteristics of sensors and transducers are listed below:
- Input characteristics
- Transfer characteristics
- Output characteristics
Input Characteristics of Sensors
- Range: The minimum and maximum value of the physical variable that the sensor can sense or measure. For example, a Resistance Temperature Detector (RTD) for temperature measurement has a range of -200 to 800oC.
- Span: The difference between the maximum and minimum values of the input. In the above example, the span of the RTD is 800 – (-200) = 1000oC.
- Accuracy: The error in a measurement is specified in terms of accuracy. Accuracy is the difference between the measured value and the true value, and it is quoted as a percentage of full scale or a percentage of reading.
Xt is calculated by taking the mean of an infinite number of measurements.
- Precision: The closeness among a set of measured values, which differs from accuracy. Let Xt be the true value of the variable X and let a random experiment measure X1, X2, …. Xi as the value of X. The measurements X1, X2,… Xi are precise when they lie very near one another, even when they do not lie near the true value Xt. When we call X1, X2,… Xi accurate instead, they lie close to the true value Xt and therefore also close to one another. Accurate measurements are therefore always precise.
- Sensitivity: The ratio of the change in output to the change in input. If Y is the output in response to input X, the sensitivity (S) is the change in Y divided by the change in X.
- Linearity: The maximum deviation between the sensor’s measured values and the ideal response curve.
- Hysteresis: The difference in output when the input is varied in two ways: increasing and decreasing.
- Resolution: The smallest change in input that a sensor can detect.
- Reproducibility: The ability of a sensor to produce the same output when the same input is applied under changed measurement conditions, such as a different operator, instrument or location.
- Repeatability: The ability of a sensor to produce the same output every time the same input is applied with all physical and measurement conditions kept identical, including the operator, instrument, ambient conditions and so on.
- Response Time: The time it takes for the sensor’s output to reach a specific percentage (like 95%) of its final value after a sudden change in input.
