Tag: Sensors
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Null vs. Deflection Type Instruments
Null Instrument The null technique is one of the operational modes for a measuring instrument. A null instrument employs the null method for measurement. In this technique, the instrument exerts an influence on the measured system so as to oppose the effect of the measurand. The influence and the measurand are balanced until they are…
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Analog vs. Digital Sensors
Analog Sensors Analog sensors provide a signal that is continuous in both its magnitude and its temporal (time) or spatial (space) content. Most physical variables such as, current, pressure, temperature, displacement, acceleration, speed, light intensity and strain tend to be continuous in nature and are readily measured by an analog sensor and represented by an…
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Variable Capacitance Sensors
Several mechanical input sensors, as well as some sensors which measure humidity or temperature, operate by the transduced quantity causing a change in capacitance, which in turn, is converted to an analog output voltage by an ac bridge circuit or other electronic system. The capacitive sensors have been designed to measure force by means of…
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The Principle of Operation of a Vortex Flowmeter
If a bluff (non-streamlined) body is placed in a flow, vortices detach themselves at regular intervals from the downstream side as illustrated in the diagram below: In flow measurement the vortex shedding frequency is usually a few hundred hertz (Hz). At Reynolds numbers over 103 the volumetric flowrate, Q, is directly proportional to the observed…
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The Principle of an Optical Pyrometer Temperature Sensor
A heated object emits electromagnetic radiation. At temperatures below approximately 400 °C this radiation can be felt as heat. As the temperature rises, the object starts to emit visible radiation passing from red through yellow to white. Intuitively we can use this radiation to qualitatively measure temperature as illustrated in the table below: Temperature Colour…
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Types of Ultrasonic Non-Destructive Testing (NDT) Techniques
The technology of ultrasonic testing has been used by production and industry since the development of the radio engineering process. The effect and device of the technology are that ultrasonic waves of acoustic type do not change the rectilinear trajectory of motion when passing through a homogeneous medium. The ultrasonic method is also used to…
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The Principle of Operation of Optical Encoders
An encoder is a device that provides a digital output as a result of an angular or linear displacement. The position encoders can be grouped into two categories: Incremental Encoders Let’s consider the basic form of an incremental encoder for the measurement of angular displacement of shaft shown below: The incremental encoder shown above consists…
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The Construction & Principle of Operation of Load Cells
A load cell converts a force (usually the gravitation force from an object being weighed) to a strain which can then be converted to an electrical signal by strain gauges. A load cell will typically have a local circuit as illustrated in figure 1(a) with four gauges; two in compression and two in tension and…
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The Conditioning of Thermocouple Sensor Voltages
Consider the circuit below: From the figure above, the sensing junction and reference junction are connected in series. When the junctions are at the same temperature the voltage output from the junctions is zero and the output from the amplifier is zero. When the junctions are at different temperatures, there is a differential voltage at…
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RTD 2, 3, 4 Wire Sensor Connections
The resistance temperature devices (RTD) can be connected directly to the controller peripheral amplifiers using 2-wire, 3-wire or 4-wire lead configuration. The wires in all these connections are in shielded cables. The RTD is usually driven from a constant current source (I) and the voltage drop (V) across the RTD is measured. RTD 2-wire connection…