Measurements Instrumentation

Reference data and engineering information about measurements instrumentation for process control applications.

measurementsinstrumentation

Overview

Engineering reference data for Measurements Instrumentation in process control.

Key Formulas

PID Controller

$u(t) = K_p e(t) + K_i \int e(t)dt + K_d \frac{de}{dt}$

Proportional-Integral-Derivative control.

Transfer Function

$G(s) = \frac{K}{\tau s + 1}$

First-order system.

Variables

Symbol	Description	Unit
$K_p$	Proportional gain	—
$K_i$	Integral gain	1/s
$K_d$	Derivative gain	s
$\tau$	Time constant	s

Measurements and instrumentation span several specialized engineering domains, each with distinct principles, tools, and standards.

Flow Measurement

Flow metering employs various principles to quantify fluid velocity or volume. Common technologies include:

Differential Pressure: Orifice plates, Venturi tubes, Flow nozzles.
Velocity-Based: Pitot tubes, Turbine meters, Vortex shedding meters.
Variable Area: Rotameters.
Positive Displacement: Oval gear meters.
Electromagnetic: Faraday's law applied to conductive fluids.
Ultrasonic: Doppler shift or time-of-travel methods.
Mass Flow: Coriolis effect or thermal dispersion.

Temperature Measurement

Temperature is measured using sensors that exhibit a change in a physical property with thermal energy.

Thermocouples: Generate a voltage due to the Seebeck effect at the junction of two dissimilar metals.
Resistance Temperature Detectors (RTDs): Measure the change in electrical resistance of a metal (e.g., platinum) with temperature.
Thermistors: Semiconductor-based sensors with a large, non-linear change in resistance.

Electric power (P) is measured using an ammeter (for current I) and a voltmeter (for voltage V). For DC circuits: $P = V \cdot I$ For AC circuits, real power accounts for the power factor (cosφ): $P = V_{rms} \cdot I_{rms} \cdot \cos\phi$

Process Instrumentation & Safety

P&ID (Piping and Instrumentation Diagrams): Schematics showing the functional relationship between piping, equipment, and control systems.
ISA Codes: Standardized alphanumeric codes used to identify and tag instrumentation on P&IDs (e.g., FT-101 for a flow transmitter).
Safety Colors: ANSI standards define colors for marking physical hazards (e.g., red for fire protection equipment, yellow for caution).

Signal Processing

The decibel (dB) scale, a logarithmic unit, is useful for comparing signal magnitudes, such as sound power or pressure. When adding signals from multiple sources, powers (not pressures) are summed.

References

Original Source

Source: engineeringtoolbox.com