Measurement Principles

Here’s a breakdown of some common measurement principles used in electrical instrumentation:

1. Ohm's Law (Voltage, Current, Resistance)

• Principle: Ohm’s Law states that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance. This relationship is given by the formula:

  Formula: V = I x R

  Where:

  • V = Voltage (Volts)

  • I = Current (Amperes)

  • R = Resistance (Ohms)

• Application: This principle is fundamental in measuring voltage, current, and resistance using instruments like voltmeters, ammeters, and ohmmeters. It helps in designing systems and diagnosing faults in electrical circuits.

https://youtu.be/HsLLq6Rm5tU?si=Wd551KMElujYPB7N 

 

2. Electromagnetic Induction (Current and Voltage Measurement)

• Principle: Faraday’s Law of Electromagnetic Induction states that a voltage is induced in a conductor when it experiences a change in magnetic field. This is given by the formula:

  Formula: EMF = - N x dΦ / dt

  Where:

  • EMF = Electromotive Force (Volts)

  • N = Number of turns in the coil

  • Φ = Magnetic flux

  • t = Time

• Application: Electromagnetic induction is used in devices like transformers, current transformers (CTs), and voltage transformers (VTs), which are essential in measuring high currents and voltages in electrical power systems. It’s also used in inductive sensors for detecting the presence of magnetic fields.

https://youtu.be/3HyORmBip-w?si=amUBAt8yFcTNciZC 

 

3. Thermocouples and Seebeck Effect (Temperature Measurement)

• Principle: The Seebeck effect refers to the generation of an electromotive force (EMF) or voltage when two dissimilar metals are joined at two junctions, and a temperature difference is maintained between them. The amount of voltage generated is directly proportional to the temperature difference.

  Formula: V ∝ ΔT

  Where:

  • V = Voltage (mV)

  • ΔT = Temperature difference (°C)

• Application: This principle is used in thermocouples, which are widely employed in industrial processes to measure temperature accurately, especially in high-temperature environments like furnaces, engines, and boilers.

https://youtu.be/PccE4WcfnAw?si=KdU74pnakIaPENBx 

 

4. Wheatstone Bridge (Resistance Measurement)

• Principle: The Wheatstone bridge circuit is used to measure unknown electrical resistance by balancing two legs of a bridge circuit. The condition of balance is achieved when the ratio of two known resistances is equal to the ratio of one known resistance and one unknown resistance, allowing for the calculation of the unknown value.

       Formula: ​R1 / R2 = R3 / Rx

Where:

• R1, R2, R3​ = Known resistances

• Rx​ = Unknown resistance

• Application: This principle is essential in strain gauges and precision measurement of resistance, which is critical for calculating parameters like strain, displacement, and pressure.

https://youtu.be/ZqAM_wQ35ow?si=qEHG0fLoLhwSpG6I 

 

5. Piezoelectric Effect (Pressure and Force Measurement)

• Principle: The piezoelectric effect is the ability of certain materials (such as quartz) to generate an electric charge in response to applied mechanical stress. The generated voltage is proportional to the applied force or pressure.

  Formula: V ∝ F

  Where:

  V = Voltage

  FFF = Applied force or pressure

• Application: This principle is widely used in pressure sensors, accelerometers, and vibration sensors in industries that require precise force or pressure measurement, such as automotive and aerospace engineering.

https://youtu.be/_XABS0dR15o?si=ob5vgCdZHT5lWkdf 

 

6. Hall Effect (Magnetic Field and Current Measurement)

• Principle: The Hall effect occurs when a magnetic field is applied perpendicular to the flow of current in a conductor, causing a measurable voltage (Hall voltage) to develop across the conductor. The magnitude of this voltage is directly proportional to the strength of the magnetic field and the current passing through the conductor.

  • Formula: VH = I x B / n x q x t​

  Where:

  • VH​ = Hall voltage

  • I = Current

  • B = Magnetic field strength

  • n = Charge carrier density

  • q = Charge of the carriers

  • t = Thickness of the conductor

• Application: Hall effect sensors are used in devices to measure magnetic fields, current (non-invasively), and proximity sensing. It is a reliable method in automation systems and current sensing circuits.

https://youtu.be/wpAA3qeOYiI?si=ikonyUIuIMkjoq6P 

 

7. Capacitive Sensing (Displacement and Level Measurement)

• Principle: Capacitive sensors work based on the change in capacitance between two conductive plates. The capacitance changes with variations in the distance between the plates, the dielectric material, or the surface area. This principle can be applied to measure distance, displacement, or the level of fluids in a tank.

Formula: C = ε0​ x εr x A / d​

Where:

C = Capacitance

ε0​ = Permittivity of free space

εr = Relative permittivity of the material

A = Area of the plates

d = Distance between the plates

 

• Application: Capacitive sensing is commonly used in level sensors (liquid level in tanks) and proximity sensors (for detecting objects or displacement in industrial automation systems).

https://youtu.be/o4_6yu-GIDU?si=ogpeESlnkRjzUIk3 

 

8. Inductive Sensing (Proximity and Position Sensing)

• Principle: Inductive sensors work based on changes in inductance, which occurs when a metal object comes close to a coil of wire through which an alternating current is flowing. The changing magnetic field induces a change in the coil's inductance, which is used to detect the object's position or proximity.

• Application: Inductive sensors are used in industrial automation for detecting metallic objects and in position sensing systems such as encoders or motion control devices.

https://youtu.be/TEOCHrnotrk?si=67c8x3vTndg1Q_1N 

 

Importance of these principles

• Accuracy: These measurement principles ensure accurate readings of electrical and physical quantities, crucial for system performance and safety.

• Control and Automation: They provide the foundation for control systems in industries like power generation, manufacturing, and aerospace.

• Safety and Reliability: Many of these principles help monitor systems in real time, ensuring that critical parameters stay within safe limits