Electrical Measuring Instruments | Types Accuracy Precision Resolution Speed

One introductory classification groups types of measuring instruments by the main form of their sensing or output:

1. Electrical measuring instruments
2. Mechanical measuring instruments.
3. Electronic measuring instruments.

This article focuses on electrical measuring instruments. The three groups overlap: an instrument may sense an electrical quantity electronically and present it through a mechanical pointer. Electrical instruments measure quantities such as electrical power factor, power, voltage and current. An analogue electromechanical electrical instrument has moving parts whose inertia and damping limit its response speed. A digital instrument has different limits, including sampling rate, filtering, bandwidth and display update time.

Electrical instruments can also be classified by how their measurement values are established:

Absolute Measuring Instruments

In the historical absolute-instrument classification, the measured value is calculated from instrument dimensions, physical constants and observed deflection rather than read from a scale calibrated against another instrument. Examples include the Rayleigh current balance and tangent galvanometer. Modern metrology more often describes primary reference procedures and measurement standards.

Secondary Measuring Instruments

A secondary measuring instrument gives a direct indication after calibration. Its result is linked through a documented calibration chain to suitable measurement standards, with uncertainty stated at each relevant step. Most working voltmeters, ammeters and power meters fall into this practical category.

Another classification describes how the instrument obtains its indication. Two common methods are deflection and null measurement:

Deflection Type Instruments

Deflection type instruments produce an indication that changes with the measured quantity. In an analogue instrument, the pointer settles where deflecting torque is balanced by controlling torque. A permanent-magnet moving-coil ammeter is one example.

The diagram shows a moving coil and pointer in the field of a permanent magnet. For a radial and substantially uniform field, the electromagnetic deflecting torque is proportional to coil current: T_d = K.I, where T_d is deflecting torque and K is the instrument constant.

K depends on the coil turns and area and on the magnetic field strength. The control springs produce an opposing torque proportional to pointer angle. At equilibrium, the two torques are equal, so the deflection angle θ is proportional to current when the constants remain stable.

Null Type Instruments

A null instrument compares an unknown quantity with a known opposing quantity. The operator or control system adjusts the known value until the detector indicates zero difference. The unknown is then calculated from the calibrated balance condition. Two requirements are:

1. The opposing quantity or its relationship to a measurement standard must be known.
2. The detector must distinguish balance from a relevant unbalance with adequate sensitivity.

The balance mechanism also needs a controlled means of adjustment toward the null point.
The practical differences between deflection and null type of measuring instruments are:

1. A null method can achieve low measurement error because the result depends on a calibrated balance quantity and a sensitive zero detector. Its accuracy still depends on standards, circuit loading, detector threshold and the complete uncertainty budget.
2. Null methods can detect small differences near balance, while a deflection instrument provides a continuous direct indication over its range. Neither method is always more sensitive; performance depends on the design and operating conditions.
3. A direct-deflection instrument often follows changing inputs without an iterative balance step. Manual null methods are usually slower, although automatic electronic null systems can respond quickly.

Electrical measuring systems commonly provide one or more of three functions.

Indicating Function

An indicating instrument presents the current measured value. The indication may be an analogue pointer, a digital display or another readable output.

Recording Function

A recording instrument stores measured values with time or another independent variable. Older recorders used paper; modern systems commonly use electronic memory, data files or networked logging.

Controlling Function

A controlling instrument uses a measured value in a feedback or protection function to change a process. The measurement, decision logic and final control element may be separate parts of one system.
Performance characteristics of electrical measuring instruments and measurement systems are commonly divided into static and dynamic characteristics:

Static Characteristics

Static characteristics describe performance when the measured quantity is constant or changes slowly enough for transient response to be negligible. They include accuracy, precision, sensitivity, resolution, repeatability, reproducibility, linearity and drift. Three related groups are:

1. Accuracy:
   Measurement accuracy is the qualitative closeness of agreement between a measured value and a reference quantity value. Precision instead describes agreement among repeated results under stated conditions. Instrument performance is usually specified through maximum permissible error or uncertainty rather than a numeric value called accuracy. Error limits may be stated in forms such as:
   1. Error at a specified point
   2. Percentage of full-scale range
   3. Percentage of reading or reference value.
2. Sensitivity:
   Sensitivity is the change in indication divided by the corresponding change in the measured quantity. Resolution describes the smallest input change that produces a perceptible indication change, while display resolution is the smallest meaningful difference between displayed values. Noise, friction and range can affect resolution.
3. Reproducibility:
   Measurement precision covers the closeness of repeated results under stated conditions. Repeatability uses the same procedure, operator, instrument, location and short time interval. Reproducibility deliberately changes specified conditions such as operator, instrument, location or time. Drift is a separate gradual change in indication or metrological properties. Common descriptions include:
   1. Zero drift
   2. Span drift
   3. Zonal drift

Dynamic Characteristics

Dynamic characteristics describe how the indication follows a time-varying input. Important measures include delay, rise time, step response time, time constant, settling time, overshoot, bandwidth and dynamic error. Response speed is meaningful only with a stated input change and a specified tolerance around the final value.