Electrical measuring of instruments Theory

Types of measurements

There are following types of measurements

           

           1. Primary measurements

           2. Secondary measurements

           3. Tertiary measurements

1.Primary measurements :-  

the measurement obtained directly by observation and compensation is called primary measurement. It needs no conversion, e.g.  measuring length , time, mass of a body etc.

2. Secondary measurements:-  

these measurement involve one conversion, e.g. measurement of pressure. In this pressure will be converted into a suitable form by a transducer and fed to the instrument.

   3.Tertiary measurements:- 

   the measurements  involve two conversion of the quantity under measurements, e.g. temperature measurement by thermo-couple , in this measurements the temp is first converted into voltage, which is again converted into the legible form through a meter.

Some important terminology

Important terms related to measurements are defined below:-

     

      1.  Instrument :-  

       

       Instrument is a device to measure value or magnitude of a quantity or a variable.

     

      2. True value :-  

       

      the true value of a quantity under measurement may be define as the average of an infinite no. of measured values when the deviation tends to zero”.

It is not possible to measure exact true value of the quantity

      

      3. Error :-  

      

       The difference b/w the “measured value” and “true value” of a quantity is called the error.

     4. Accuracy :- 

     

      Accuracy means “conformity to truth ‘ . the accuracy is the “closeness with which an instrument reading approaches the true value of the quantity being measured.

Accuracy = (measured value - true value ) / true value