Why do we use percent deviation rather than simply expressing the size of the error itself?

the percentage we have the fraction between the error and the magnitude,

Explanation:

When absolute error is used, it is a value that remains constant regardless of the magnitude of the measurement, for large measurements it can represent great accuracy, but for small measurements the accuracy may be low, for example> if we use a tape measure with error of Dx = 0.1 cm if the measurement is 1 meter this error is acceptable, but if the measurement is 1 cm the error is too big.

When we use the percentage we have the fraction between the error and the magnitude, so if this relationship is small the measure is very tight and but if the relationship is high the measure has little pressure, for example

           Δx = 1 cm = 0.01 m

          % = 0.01 / 1   100 = 1%

          % = 0.01 / 0.01 100 = 100%

therefore the precision of the measurement is known the percentage error

electromagnetic shielding is the practice of reducing the electromagnetic field in a space by blocking the field with barriers made of conductive or magnetic materials. shielding is typically applied to enclosures to isolate electrical devices from their surroundings, and to cables to isolate wires from the environment through which the cable runs. electromagnetic shielding that blocks radio frequency electromagnetic radiation is also known as rf shielding.

the shielding can reduce the coupling of radio waves, electromagnetic fields and electrostatic fields. a conductive enclosure used to block electrostatic fields is also known as a faraday cage. the amount of reduction depends very much upon the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic

from wikipedia