PMMC Instrument Magnetic Decay Cause

A Permanent Magnet Moving Coil (PMMC) instrument relies on a uniform magnetic field produced by permanent magnets. Magnetic decay refers to the weakening of this magnetic field over time.

Analyzing Causes of Magnetic Decay

Let's examine the potential causes for magnetic decay in PMMC instruments:

• Variation in resistance: Changes in the resistance of the moving coil affect the current flow and coil deflection, but not the strength of the permanent magnets.
• Quality/Aging of spring: The control spring provides controlling torque. While its quality and aging affect instrument accuracy by altering the torque, they do not cause the magnetic field itself to weaken.
• Aging of magnets: Permanent magnets naturally lose their magnetic strength over time due to factors like temperature fluctuations, mechanical shocks, and inherent material properties. This phenomenon is known as aging.

Main Cause Identification

The weakening of the permanent magnets directly reduces the magnetic field strength. This reduction affects the torque produced by the instrument, leading to inaccurate readings. Therefore, the aging of the magnets is the primary reason for magnetic decay in PMMC instruments.