Basics and Classification of Measuring instruments


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Measuring instruments are devices that measure physical quantities that are obtained real time and compared with standard quantities with predefined units that yields a numerical value relating the subject under study and the referenced quantity. Most measurement instruments are subject to errors.

Stages of a general measurement system

PSE : This stage is in direct contact with thequantity under measurement and consists of various sensing elements.

DCE : this stage converts data from one from to another. Eg: Voltage to frequency, V to I conversions. etc

DME : This tage converts the level of the signal, that is amplification or attenuation of signal

DTE : Consisting of transmission medium like optic fibers, cables etc

DPE : This stage is the display, recorders or storage elements.

Important Definitions

Instrument : A device that gives the value of a quantity.
Accuracy : The lowest possible difference in value that the instrument achieves (measured value) in comparison to the true value.
Precision : Gives the reproducibility of the measurement system. While an instrument has high precision does not mean that it is accurate.
Resolution : The smallest change in the input (measured value) that the instrument reacts to.
Sensitivity: Ratio of the change in output to the change in the input.
Reproducibility : Measure of repeatability of reading over a period of time. this is different from precision, where precision is defined by the repeatability of the same value for the foxed value of variable during successive measurements, Reproducibility is more related to the aging effects of the system.
Repeatability : Repeatation of a reading of an instrument from a given set of readings.
Error : Deviation of the measured value from the true value.

Types of error

Gross Error : Human errors due to misreading of instrument, incorrect adjustment, improper application and computational error.
Systematic Error : Caused due to defective or worn parts of the instrument and also effects of environment on the instrument.
Random Error: Errors whose cause cannot be determined due to random variation in parameters or system.

Standards

For meaningful results in calibration, the measured variables have to be accurate which is quantified by standards

Standards of EMF

Primary standard - Weston cell saturated

  • Potential of saturated Weston cell E = 1.01864 volts
  • Contains CdSO4 crystal, Hg2SO4 ( A amalgam of Cd+Hg)
  • Variation in emf with temperature - 40𝞵V/0C
  • Variation in potential with time -1 𝞵V/Year
  • Maximum current 100 𝞵A
  • Internal resistance 600-800 Ω
    Primary standard Weston cell saturated

Secondary standard - Unsaturated Weston Cell

  • Potential of unsaturated Weston cell E = 1.0180 to 1.0194 volts
  • Does not have CdSO4 crystal
  • Porous plug is used to hold electrode in place
  • Variation in potential -30 𝞵V to -50 𝞵V/Year

Laboratory standard - Zener diode circuit

Standards for Resistance
Manganin is used as the standard resistance

  • Contents of Manganin:

    Ni - 4%
    Cu - 84 %
    Mn - 12%

  • Characteristics of manganin

    High resistivity
    Low temperature coefficient
    Low thermal expansion with Copper

  • Errors in Resistance standards

    Skin effect
    Stray inductance and capacitance
    Could be contact resistances

Standards for mutual inductance

Primary standard - Bifilar Winding

Used also to reduce inductive effect of resistance

Primary standard - Campbell type

Consists of marble cylinders with screw threads that has copper coil without insulator curled around under tension

Secondary standard -Two coils of copper wound on bobbin of marble separated by flange.

Standards for self-inductance
Primary standard Campbell type
Secondary standard Copper wire covered in silk wound over marble former
Standards for Time
Primary standard Atomic clock
Secondary standard Rubidium crystal
Standards for Frequency
Primary standard Caesium beam, Hydrogen Maser
Secondary standard Rubidium crystal, Quartz crystal

Classification of Instruments

Basic classification

Mechanical : Having stable static conditions, they do not read dynamic (transient) conditions of the measuring variable.
Electrical: More rapid indication. Involves conversion of electrical quantity to mechanical movement on the scale
Electronic: Having the fastest response to follow both static and dynamic changes in the system

Classification based on output

Primary : These absolute measuring systems gives the magnitude and value of the variable in terms of the physical quantities of the instrument which need not be compared to any standard values. They are used in national laboratories for standard measurement
Eg: Tangent galvanometer, Raleigh current balance

It is time consuming, every time a reading is made, it is computed for obtaining the quantity under measurement.

Secondary : These instruments are used on calibration with absolute or primary instruments or with other secondary instruments.
All instruments used for measurement after calibration are secondary instruments and they give a value directly which is meaningful after calibration.

Classification of Secondary instruments

1. Based on the electric parameter it depends on
Depending on the underlying principle of operation, the secondary instruments are classified as below. The definitions are self explanatory.

2. Based on the operation nature

Indicating : Indicate the quantity to be measured by means of a pointer which moves on a scale. Examples are ammeter, voltmeter, wattmeter etc

Recording : Indicating instruments that continuously records the values of the measured variable on a chart or dial. The recording is generally made by a pen on a graph paper which is rotated on a dice or drum at a uniform speed. The amount of the quantity at any instant may be read from the traced chart.

Integrating: Records the total quantity of the measured variable over a period of time. Eg: kilowatthour (kWh) meter


Recording instruments - Strip chart recorder