Page 309 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

temperature (sensor input) results in a large change in voltage (sensor output). Some sample
output units are volts/Celsius, millivolts/dB sound change, etc.
• Dynamic (or span) range: This metric defines the range of inputs from physical phenomena
whereby the sensor output falls within acceptable accuracy levels when the sensor input is within limits, i.e., within its dynamic range. Should the sensor input exceed these ranges—either over or under—it may have inaccurate measurements because it is outside of the dynamic range. The “range” of a sensor is the region between the limits within which a quantity is measured. The upper range value is the highest quantity measurable by the subject sensor while the lower range is its lowest. The “span” of a sensor is the algebraic difference between the sensor’s upper and lower range values. An example of this concept is an optical sensor adjusted to the wavelength of visible light (e.g., a human eye) that observes wavelengths outside of its dynamic range (e.g., infrared or ultraviolet (UV) light). In this example, the sensor is receiving input from both within and outside of its dynamic (or span) range and is interpreting the visible light while discarding (or inaccurately measuring) the light waves outside of its dynamic range. The difference between range and span is demonstrated by the following example—if a gauge measures the pressure in a closed tank, the range of this gauge could be 100
180 psi
(7
12 bar) while the span is 80 psi (5.5 bar).
• Accuracy: A comparison is made between the actual output of a sensor to the output with an ideal sensor with the difference (or expected error) defined as its accuracy. Typically, this is expressed as a fraction of the full-scale output (FSO)/reading. For instance, the manufacturer of a pressure sensor may guarantee the accuracy of the sensor to 3% of FSO, meaning that it will output a value within 3% of its true value. Sensors will never be “all things to all people” in that typically the wider a range for a sensor the less resolution (defined below) and accuracy that will be reflected. As an example, two pressure-sensitive depth gauges of similar quality will have a resolution/accuracy inversely proportional to their range (i.e., a 0
330 ft (0
100 m) range sensor will have twice the resolution/accuracy of a 0
660 ft (0
200 m) range sensor with all other parameters being equal). The accuracy is typically stated in percentage (which, of course, would not change as a percentage of output between like sensors). The resolution and the accuracy of a sensor are generally related, but they are not directly linked, i.e., a high-quality instrument may have a low resolution but be highly accurate (e.g., a deep-rated digital quartz depth gauge as used on a WCROV), while a low-quality instrument may have a high resolution with low accuracy (e. g., a shallow-rated pressure-sensitive depth gauge as used on an OCROV).
• Hysteresis: The “hysteresis” of a sensor is its dependence upon the output (for any given change in input) value’s history of prior excursion of the input and the direction of the current excursion (Figure 12.2). This means simply that the output values obtained while increasing the input will be different from the output values obtained while decreasing the input. Hysteresis is based on the inherent physical characteristics of the materials used to construct the instrument. Hysteresis can be mechanical or magnetic. As a sensor is cycled up and down its dynamic range, the track of the sensor’s output may not exactly correlate over time or scale with actual conditions. In other words, hysteresis is the measure of the lead/lag time of the sensor’s input to the sensor’s output and/or the differing accuracy of the sensor as the dynamic range is cycled from top to bottom versus bottom to top. An example of the lead/lag hysteresis would be a pressure sensor on an ROV still in the TMS that is being lowered vertically to the job site at a rate of 30 m/min (meters per minute) versus 90 m/min. With a 0.05% hysteresis, if the winch is
12.1 Theory 301