Learning Unit II: Process Instrumentation III - Telemetering and Signal Converters

Flashcards covering telemetering mediums, pneumatic and electrical transmission systems, optical fibre principles, and industrial signal converters based on Learning Unit II Process Instrumentation III.

Telemetering

The practice of conveying measurement and process control information over a distance from a source to a central control centre with the aid of a distributed control system.

Basic Telemetering System Elements

The four fundamental functional components of a telemetering system: an input converter, a transmitter, a transmission line, and a receiver.

Pneumatic Transmission Line Limit

The practical distance boundary of approximately $300\,\text{m}$ for pneumatic lines, beyond which electrical transmission is required due to propagation delays.

Live Zero ($20\,\text{kPa}$)

The baseline pressure of the standard $20-100\,\text{kPa}$ pneumatic range that enables immediate detection of pipeline air leaks, simplifies calibration, and ensures adequate driving pressure for dynamic response.

Pneumatic Transmission RC Analogy

An equivalent circuit model where the volume of pneumatic system elements represents capacitance and the inside diameter of the pipeline represents resistance to calculate system time constants.

Live Zero ($4\,\text{mA}$)

The base current level of a standard $4-20\,\text{mA}$ current loop that enables immediate detection of an open circuit or line break, distinguishing a fault from a true zero measurement.

Two-Wire Current Transmission System

A configuration where a single pair of conductors delivers power supply voltage ($24-48\,\text{V}$) to the transmitter and carries the returned $4-20\,\text{mA}$ signal across a $250\,\Omega$ load resistor to produce a $1-5\,\text{V}$ drop.

Frequency Transmission

An electrical transmission technique in which process data is modulated into frequency variations to avoid signal degradation over long distances, commonly used for capacitive, reactive, and transformer measurements.

Pulse Duration Telemetering

A telemetry method where the magnitude of the measured voltage or current variable is represented and conveyed by the width or duration of transmitted electrical pulses.

Pulse Count Telemetering

A telemetering technique where the value of the measured variable is determined solely by the number of pulses counted per second, regardless of the individual wave shape.

Pulse Code Telemetering

A digital transmission method where an analog signal value is converted into a binary number consisting of a consecutive sequence of pulses representing zeros and ones.

Parity Checking

An error-detection mechanism in digital communication where an extra bit is added to each code group to ensure the total count of 1s is always even (even parity) or always odd (odd parity).

Frequency Division Multiplexing (FDM)

A multiplexing method that divides the available transmission bandwidth into separate frequency channels to send multiple signals simultaneously, separated at the receiving end by band-pass filters.

Time Division Multiplexing (TDM)

A multiplexing technique that divides the transmission medium into discrete time slots allocated to specific signals, separated at the receiving end by a de-multiplexer.

Index of Refraction ($n$)

The optical parameter defined as $n = \frac{c}{v}$, expressing the ratio of the speed of light in a vacuum ($c = 3 \times 10^8\,\text{m/s}$) to the speed of light within an optical medium ($v$).

Fibre Optic Cable Structure

A thin cylindrical optical transmission line consisting of an inner core with refractive index $n_1$ and an outer cladding with refractive index $n_2$, where $n_1 > n_2$ to facilitate total internal reflection.

Numerical Aperture (NA)

A quantitative measurement of the light-gathering capability of an optical fibre, defined by $\text{NA} = \sin(\theta) = \sqrt{n_1^2 - n_2^2}$, where $n_1$ is the core index and $n_2$ is the cladding index.

Minimum Angle of Internal Reflection ($\theta_{\min}$)

The threshold angle required to sustain total internal reflection at the core-cladding boundary, given by $\theta_{\min} = \sin^{-1}\left(\frac{n_2}{n_1}\right)$.

Injection Laser Diode (ILD)

An optical source characterized by higher cost, higher temperature sensitivity, shorter lifetime, smaller size, faster rise time, and higher coupling efficiency, suited for long-distance optical transmission.

Light Emitting Diode (LED)

A cost-effective optical source with lower temperature sensitivity, longer lifetime, larger physical size, and a response rise time of $20-30\,\text{ns}$, typically restricted to application distances $\le 1\,\text{km}$.

Hybrid Optical Sensor

A category of optical detector used to interface existing pneumatic or electronic plant instrumentation to optical transmission networks.

Fibre Optic Sensor

A sensing device that utilizes optical technology directly to sense process variables and transmit the data optically to a remote optoelectronic interface.

Current to Air Converter

An electro-pneumatic converter that transforms a $4-20\,\text{mA}$ DC current into a proportional $20-100\,\text{kPa}$ pneumatic signal using a coil in a permanent magnet, a flapper-nozzle system, and feedback bellows.

Pneumatic to Current Converter

A signal converter that transforms a pneumatic pressure input into a proportional direct current output (such as $4-20\,\text{mA}$) utilizing a variable capacitor moved by a pressure capsule, an AC carrier amplifier, and a synchronous rectifier.

Current to Voltage Converter

A signal converter that changes an input current into a proportional voltage signal (e.g., $1-5\,\text{V}$) utilizing an isolated chopper circuit, transformer, rectifier, and output amplifier.

Voltage to Current Converter

A converter that transforms an input voltage into a proportional current output using an operational amplifier integrator, chopper circuit, isolating transformer, rectifier, and feedback network.

Millivolt to Current Converter

A device that converts the low millivolt electromotive force (e.m.f.) produced by a thermocouple into a standard $4-20\,\text{mA}$ current signal using a potentiometer input bridge with cold junction compensation and a solid-state chopper.

Resistance to Current Converter

A signal converter that converts resistance changes measured by a resistance temperature detector (RTD) into a standard $4-20\,\text{mA}$ output using a Wheatstone bridge input circuit.

Frequency to Current Converter

A signal converter that translates variable frequency pulse inputs from turbine flow meters or tachometers into a $4-20\,\text{mA}$ DC signal using an isolating transformer, input amplifier, and Schmitt trigger.