Why the measuring values are not correct sometimes? Herein the below lists should be considered:
1. The factors explanatory
A matrix metal magnetic properties
The thickness of magnetic method affected by substrate metal magnetic change (in practice, low carbon steel magnetic can be thought of as a slight change), in order to avoid the influence of heat treatment and cold working factors, should use standard pieces of metal with the same properties of specimen substrates for calibration instruments; May also use coated specimen for calibration.
B matrix metal electrical properties
Have influence on the conductivity of the matrix metal and base metal of conductivity is related to its material composition and heat treatment.Use metal specimen substrates with the same nature of the standard for instrument calibration.
C matrix metal thickness
Each kind of instrument has a critical thickness of base metal.If greater than the thickness, measurement is not affected by the thickness of base metal.
D edge effect
The instrument is sensitive to abrupt change in the shape of specimen surface.So close to the specimen within the edge or corner measure is not reliable.
The curvature of the specimen to measure.This influence is always as the radius of curvature decreases significantly increased.As a result, the bending specimen surface measurements is not reliable.
F deformation of the specimen
Measuring head can make soft coating specimen deformation, thus reliable measured data on the specimen.
G the surface roughness
The matrix metal and surface roughness of coating on measurement.Roughness increases, the effect increased.Rough surface will cause system error and random error, every time measurement, should increase the number of measurements in different positions, to overcome the accidental error.If matrix metal rough, must also be similar matrix without coating roughness on the metal specimen taken several zero position calibration instruments;Or with no corrosion to metal matrix solution of removing the cover, proofreading of zero again.
H magnetic field
Around all kinds of strong magnetic field generated by electric equipment, can seriously interfere with the magnetic method of thickness.
I attached material
The instrument is sensitive to the contact surface between the gauge head and the coating adhesion material, therefore, must remove the obstruction material, to ensure that the instrument measuring head direct contact on the surface of the test.
J head pressure measurement
Pressure imposed by the probe on the specimen size will affect the measurement readings, therefore, please keep the pressure constant.
K probe orientation
Have an effect on measuring probe placement way.In the measurement, should keep probe and the sample surface vertical.
2. Rules to be abide when using instruments
A base metal properties
For magnetic method, the magnetic properties and surface roughness of substrate metal, should be similar to that of a metal specimen substrates.
For eddy current method, the electrical properties of the metal matrix, and should be similar to that of the metal specimen substrates.
B base metal thickness
The thickness of base metal can’t be more than the critical thickness.
C edge effect
Should not be close to the deformation places of the specimens, such as edges, holes and angles.
Should not be measured in curved surface of the specimens.
E number of reading
Often due to instrument time readings are not completely the same, so you must take several readings in each measurement area. Local differences of layer thickness, also requires repeated measurements in a given area, especially for the rough surface.
F surface cleanliness
Before measurement, should remove any attachments on the surface of a material, such as dust, grease and corrosion products, but do not remove any coating material.