# PRECISION INDEX

A measure of the magnitude of the random errors of a series of observations of some given quantity. If the precision index is large, most of the random errors of the observations are small. The precision index appears as a parameter in the normal (Gaussian) distribution law. While making a series of observations, the standard deviation can be calculated. The precision index is then calculated using a formula and a measure of the precision of the observing instrument is obtained.

## Related Terms

### PLUG METER

A variable-area flowmeter in which a tapered plug, located in an orifice and raised until the resulting opening is sufficient to handle the fluid flow, is used to measure the flow rate.

### PHOTONEPHELOMETER

A nephelometer that uses a photocell or phototube to measure the amount of light transmitted by a suspension of particles.

### PHOTOELECTRIC REFLECTOMETER

A reflectometer that uses a photocell or phototube to measure the diffuse reflection of surfaces, powders, pastes, and opaque liquids.

### PHOTOELECTRIC PHOTOMETER

A photometer that uses a photocell, phototransistor, or phototube to measure the intensity of light. Also known as electronic photometer.

### PHOTOELECTRIC FLUOROMETER

A device using a photoelectric cell to measure fluorescence in a chemical sample that has been excited (one or more electrons have been raised to higher energy level) by ultraviolet or visible light; used for analysis of chemical mixtures.

### PACHYMETER

An instrument used to measure the thickness of a material, for example, a sheet of paper.

### OUTPUTMETER

Analternating-currentvoltmeter connected to the output of a receiver or amplifier to measure output signal strength in volume units or decibels.

### OUNCE

1. A unit of mass in avoirdupois measure equal to 1/16 pound or to approximately 0.0283495 kilogram. Abbreviated oz. 2. A unit of mass in either troy or apothecaries' measure equal to 480 grains or exactly 0.0311034768 kilogram. Also known as apothecaries' ounce or troy ounce (abbreviations are oz ap and oz t in the United States, and oz apoth and oz tr in the United Kingdom).

### ORIFICE PLATE

A disk, with a hole, placed in a pipeline to measure flow.

### OPTIMAL CONTROL THEORY

An extension of the calculus of variations for dynamic systems with one independent variable, usually time, in which control (input) variables are determined to maximize (or minimize) some measure of the performance (output) of a system while satisfying specified constraints.

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