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About Performance Specifications

Instrument performance is specified by range, resolution, accuracy, and precision.

Range is defined by the highest and lowest readings the instrument is expected to produce. The lowest reading, if zero, is often omitted: 0-20 psi or 20 psi. Other examples: 3000 microstrain, 10 degrees, 50 mm, 700 kPa.

Sensitivity is the smallest change that can be sensed by a sensor. Sensitivity typically decreases as range increases. Examples: 0.01 mm, 0.1 inch.

Resolution is the smallest change that can be displayed on a readout device. Resolution typically decreases as range increases. Examples: 0.01 mm, 0.1 Hz, 0.025% Full Scale.

Repeatability or Precision is the degree to which a reading deviates from the mean of a series of readings taken under identical conditions. This parameter is always expressed as a ± value, such as ± 0.5mm or ±0.25% FS.

Accuracy is the degree to which a reading deviates from an absolute value. Accuracy is always expressed as a ± value, such as ± 0.5mm or ±0.1 %FS (full scale).

Discussion

Costs generally increase with sensitivity, resolution, repeatability, and accuracy, so it is important to specify just the performance levels that are needed. Higher performance levels do not necessarily provide more useful information.

Sensitivity and resolution are similar because both refer to the smallest change that can be that can be sensed or displayed. Resolution is more common in specifications. A truly complete specification for resolution will include not only the resolution value but also the readout that was used: 1 arc second using EL/MEMS data recorder.

Repeatability and accuracy are similar parameters because both refer to deviation or error. In most geotechnical applications, it is sufficient to have instruments with good repeatability, since we are more concerned with changes than with absolute values. We compare current and intial readings to learn if pore-pressure has increased or if movement is occuring. The absolute value of the pore-pressure or the tilt of the inclinometer casing is of lesser interest.

Sensitivity and resolution are quite different from repeatability and accuracy. The first two are concerned with "smallest value" while the second two are concerned with "greatest error." Sensitivity and resolution are usually many times smaller than repeatability and accuracy.

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