Comparison of Deflection and Null Type Instruments

Comparing the advantages and disadvantages of the two types instruments.

  1. The accuracy of null type of instruments is higher than that of deflection type. This is because the opposing effect is calibrated with the help of standards which have high degree of accuracy. On the other hand, accuracy of deflection type of instruments is dependent upon their calibration which depends upon the instrument constants which are normally not known to a high degree of accuracy.
  2. In the null type of instruments, the measured quantity is balanced out. This means the detector has to cover a small range around the balance (null) point and therefore can be made highly sensitive Also the detector need not be calibrated since it has only to detect the presence and direction of unbalance and not the magnitude of unbalance. On the other hand, a deflection type of instrument must be larger in size, more rugged, and thus less sensitive if it is to measure large magnitude of unknown quantity.
  3. Null type of instruments require many manipulations before null conditions are obtained and hence are apparently not suitable for dynamic measurements wherein the measured quantity changes with time. On the other hand, deflection type of instruments can follow the variations of the measured quantity more rapidly and hence are more suitable for dynamic measurements on account of their faster response. However, there are commercially automatic control instruments (such as self balancing potentiometers) that maintain a continuous. null under rapidly changing conditions and thereby eliminate the need for manipulative operations.

Summarizing the above:

  1. Null type of instruments are more accurate than deflection type instruments.
  2. Null type instruments can be highly sensitive as compared with deflection type instruments.
  3. Deflection type of instruments are more suited for measurements under dynamic conditions than null type of instruments whose intrinsic response is slower.

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Santhakumar Raja

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