This article describes the various methods of measurement. Instrumentation & control engineers are responsible for designing and maintaining control systems that are used to measure and control the processes. Instrumentation & Control use various methods of measurement which involve sensing, processing, and transmitting any physical quantity like temperature/pressure/flow and level.
Methods of Measurement
Some of the most common methods of measurement used in instrumentation and control engineering include:
- Direct Method
- Indirect Method
- Comparative Method
- Substitution Method
- Null Method
- Fundamental Method
- Transposition method
- Coincidence method
- Deflection method
- Complementary method
Types of measurements
1. Direct Method
The direct method of measurement makes use of a measuring instrument that provides a direct reading of the quantity that is being measured. Direct Method is considered the most accurate and the most reliable as it eliminates the need for additional calculations/estimations.
The Direct Method is widely used in the process and control industries. Examples of the instruments used in the direct method include digital voltmeters for measuring the voltage, digital thermometers for measuring the temperature, and pressure gauges for measuring the pressure.
Direct Method is also used to measure various process variables such as temperature, pressure, flow, level, and voltage directly with instruments such as thermocouples / RTD, pressure transmitters, flow meters, and level transmitters.
2. Indirect Method
The Indirect Method of measurement uses an indirect relationship between the quantity being measured and another quantity that is more easily measured. The Indirect Method of measurement often requires additional calculations or estimations to obtain the final value. The Indirect Method is usually used when direct measurement is not possible or practical.
The Indirect Method is widely used in the process and control industry for measurements such as the level in any tank/vessel, and fluid flow flowing through a pipe, where the quantity being measured is inferred from another measurable quantity.
Examples of indirect methods are measuring the resistance of any unknown resistor by determining the voltage/current across it. Calculating the flow rate of a fluid through a pipe by measuring the differential pressure drop across a restriction like an orifice.
3. Comparative Method
The Comparative Method of measurement compares the quantity which is being measured to any known standard. The Comparative Method is commonly used for calibrating instruments, where the measurement is compared to a known reference standard to determine the accuracy of the instrument.
The Comparative Method is most commonly used in the process and control industry for calibrating instruments and checking their accuracy. For example, calibrating a pressure gauge or calibrating a pressure transmitter using a known pressure source like Scandura Pressure Pump, or a Dead Weight Tester.
4. Substitution Method
The Substitution Method of the measurement substitutes the quantity which is being measured with a related quantity of any known value. The Substitution Method can be used when direct measurement is not possible or practical.
The Substitution Method is used in the process and control industry for measuring quantities that cannot be directly measured. For example, determining the capacitance of an unknown capacitor by measuring the time constant of an RC circuit using any known resistor.
Another example of the substitution method is the use of frequency response analysis for measuring the transfer function of a process.
5. Null Method
The Null Method of measurement measures the difference between two known quantities to determine the value of one. The Null Method is used when it is difficult to measure a quantity directly, but it is easy to determine the difference between two measured quantities.
The Null Method is also used in the process and control industry for measuring quantities that are difficult to measure directly. For example, measuring the resistance of any conductor is known by determining the difference in potential between two points on the conductor and then measuring the current flowing through the conductor.
Another example of the Null Method is the use of a null balance to measure the weight of an object.
6. Fundamental Method
The Fundamental Method of measurement determines the value of the quantity being measured by using a basic principle or law of physics. The Fundamental Method is most often used to measure fundamental physical quantities such as length, mass, or time. For example, measuring the length of a given rod we use the principle of interference of light.
For measuring the mass of an object we apply Newton’s laws of motion. The Fundamental Method is used in the process and control industry to measure the length of a shaft using a proximity sensor.
The choice of measurement method completely depends on the nature of the process variable which is being measured, the accuracy requirements by the end user, and the practicality of the measurement method. All of the above methods have their own advantages and limitations, and they are often used in combination to achieve the most accurate and reliable measurement results.
7. Transposition Method
In this method of measurement, a direct comparison is done. The measured value of the quantity is first balanced by an initial known value A of the same quantity, and after that, the value of the measured quantity is put in place of this known value and is balanced again by another known value B.
If in both cases, the position of the element indicates, then the value of the quantity to be measured is AB.
8. Coincidence Method
This is a differential method of measurement. In this method, a very small difference between the value of the measured quantities and the reference is calculated by the observation of the coincidence of certain lines or signals. Example- measurement by Vernier caliper, micrometer.
9. Deflection Method
In the deflection method, a deflection of a pointer on a calibrated scale shows the value of the quantity to be measured,
10. Complementary Method
In this method, the value of the measured quantity is combined with a known value of the same quantity. The combination is then so adjusted that adding these two values is equal to the predetermined comparison value. For example, the determination of the volume of a solid by liquid displacement.
