Instrumentation Questions Part One

What is a Measurement Signal?

A measurement signal is a representation of physical quantity that conveys information regarding the state of a system. This information can be anything from temperature, pressure, and voltage to more complex data such as speed or even biological signals. The signal acts as a bridge between the physical phenomena being measured and the representation of that phenomena, often through electronic systems.

The Importance of Measurement Signals

Measurement signals are vital in various applications, ranging from industrial automation and environmental monitoring to healthcare diagnostics. They are essential for: 

• Measurement signals which are foundational for gathering real-time data. 
• They help in regulating parameters within a system, ensuring safety and efficiency.
• Signals allow for ongoing surveillance of processes, helping to detect and address issues promptly.

By providing accurate data, measurement signals enable better outcomes in engineering and scientific research.

What is a Signal?

A signal is essentially a fluctuating quantity that conveys information. In a broader sense, signals can take various forms, including electrical signals, optical signals, or even sound waves.

Example of a Signal: Consider a temperature sensor in an oven. As the oven heats up, the sensor produces an electrical signal that varies with temperature changes. This signal can be represented graphically on a screen, demonstrating how the temperature increases over time.

While this example focuses on an electrical signal, it serves to illustrate the essence of what a signal is: a means of conveying information about a physical process or state.

Basic Types of Signals

Signals can be categorized based on several criteria. They can dividend into two following types:

1- Analogue Signal

Analog signals are continuous in nature and represent information in a varying format. For example, the audio signal from a microphone is an analog signal, where the amplitude and frequency can continuously change. Another Example An analog clock is a perfect metaphor, as the hands continuously move across the face rather than jumping between discrete numbers.

In another explanation A signal is said to be analogue if the amplitude of the physical value representing it can take an infinite number of values within a given time interval, these signals can be classified as following:

• Continuous signal: These signals exist over a continuous range. It is a signal which varies slowly over time. Example include voltage output from an analog sensor over time.: temperature, flow rate, level.
• Aperiodic Signals: These do not repeat and can occur at any time. An example might include random noise.
• Shape: The shape of this signal is important: cardiac pressure, chromatography impacts.
• Frequency: It is the frequency spectrum which carries the desired information: voice analysis, sonar, spectrography.

2- Digital Signal

Digital signal represent data in discrete steps or binary values (0s and 1s). They are not continuous and the values remain fixed during specific intervals. For Example A digital thermometer displays temperature in distinct values – for instance, 25°C, 25.1°C, or 25.2°C – but doesn’t show values in between those numbers.

In another explanation A signal is digital if the amplitude of the physical value representing it can only be a finite number of values. This finite number of values is generally a power of 2, these signals can be classified as following:

• Discrete Signals: Signals that exist only at specific intervals.
• Periodic Signals: These repeat over a specific time interval. For example, the output of a clock oscillator.
• Logic (also known as ON/OFF): It provides information on the status of a two state system. Example: an open or closed valve.
• Pulse train: Each pulse is the image of a state change. Example: an incremental encoder gives one finite and known pulse per revolution.
• Sampling: This is the digital image of an analogue signal. Example: temperature, flow rate, level.

Standard Signals

1- Pneumatic
Standard pneumatic signal
200 – 1000 hPa
3 – 15 psi
2- Electric
Standard electric signal
4 – 20 mA
1 – 5 V

Role of Sensor Transmitter

• The role of the sensor is to gather information, it will sample a physical value via a process pulse tapping and convert it into an electrical or pneumatic value.
• The role of the transmitter is to transmit information, it will retrieve an electrical or
  pneumatic value from the sensor output and convert it into a standardized signal usable by
  acquisition equipment such as recorders, controllers or the analogue input boards of
  instrumentation and control systems.
• The combined sensor and transmitter form a measuring instrument capable of measuring
  various physical values (temperature, pressure, etc.) which are essential for the operation
  and efficiency of an industrial site.

General Technology of a Sensor Transmitter

A sensor transmitter consists of three main elements:

• Test body: mechanical element which reacts by deformation or displacement.
• Transducer element: sensing element linked to the test body. It converts the reactions of the test body into an electric or pneumatic value.
• Converter: its role is to convert the mechanical displacement (or the deformation) into a standardized pneumatic, electrical or digital value which can be transmitted to a receiver (e.g. DCS or local pneumatic controller)

 

The physical value to be measured at the sensor input is called the "Measurand". It depends on any case which it can be:

• A pressure
• A flow rate
• A temperature
• A level
• An analysis

This measurand varies from a min. value to a max. value, it is preferable to talk about a "percentage variation": the min is 0% and the max is 100%.

In order to be understood by the receiving devices, this information must be in a specific form called a SIGNAL which, in this case, is the transmitter output signal. So the transmitter output signal can be:

• An air pressure.
• An electric current.
• A digital signal.

Qualities of Sensor Transmitter

Like all measurement instruments a sensor transmitter must have certain qualities:

1. Accuracy: Difference between the real value and the measured value.
2. Repeatability: Several measurements of the same value must give the same reading.
3. Sensitivity: The greater the variation in the reading for a small variation in the measured value, the greater the sensitivity.
4. Measurement scale: The sensor's minimum and maximum readings must correspond to the minimum and maximum values of the phenomenon to be measure.

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