Conductivity -the factors of affecting conductivity testing accuracy and attention of actually test process

Conductivity -the factors of affecting conductivity testing accuracy and attention of actual test process.

The factors of affecting conductivity testing accuracy and attention of actually test process。

When testing conductivity ,the accuracy is going to be affected by a lot of external factors ,during testing process,there are mainly aspects that we should pay attention to 

1.the technology preparation before test: before doing specifically conductivity measurement  ,we should refer to relevant information to know about the parameter conditions of measuring area,such as conductive coating thickness、the detected aluminum alloy thickness,the distribution of fastener.these are very important to judge and assess the final measurement results.

2.The influence of position

2.1When correcting device on the sample,the size of sample should not be small, boeing manual requires that the size of sample should not be lower than 1.2in*1.2;the position of probe setting should not be close to boundary or it will reduce conductivity value.boeing NDT manual requires that the change of conductivity caused by boundary should be ≤0.2%IACS

2.2 Under testing area ,the space between fastener should not be close,at leastsatisfy boeing NDT manual for change and requirements of conductivity caused by boundary or it will reduce conductivity sharply or it will reduce conductivity value sharply。

2.3The bump change on the surface of detected parts will affect the result of conductivity measurement;to make sure probe touch the surface adequately,the roughness of detected parts surface should be at least 150Pa

2.4the curvature influence of detected area。If the curvature is high,then the conductivity value will be reduced。According to requirements from boeing manual,the conductivity change caused by curvature should be ≤0.2%IACS .

3.the influence of thickness

3.1 Measured on the surface of the conductivity coating thickness can’t be too thick ,otherwise it will affect the accuracy of the measurement results。Can be determined by measuring the highest thickness of equipment, namely compared with smooth reference block, when the paste in the standard test block and lead to the conductivity value make the change plus or minus 0.3%IACS , the conductive film thickness value is measured on the surface of the maximum allowable of conductive coating thickness。

3.2 Structure thickness changes affect conductivity value accuracy。Boeing NDT manual requires the minimum thickness of reference block should be 3Times than standard penetration depth 。At the same time ,Boeing NDT manual thinks that if structure thickness is less 2.66 times than the standard penetration depth of the δ,it will reduce the conductivity value ,The images below given by Boeing NDT manual are the lowest penetration depth of various kinds of frequency。It is important to note that in our practical work ,the parts we measured especially the thickness of aircraft skin will be changed caused by chemical milling,some thickness even less than the lowest thickness required by Boeing NDT manual,the measurements value maybe exceed scope value of given from NDT manual ，we should be in accordance with the requirements of the Boeing manual and refer to the content of  BAC5946 ,for conductivity value given from Boeing NDT manual is just a  conductivity value temperature influence with wide material scope。

4.1Structure /regional temperature influence

Boeing manual requires that conductivity measurement should be in 4.5℃ to 32.2℃ environment temperature range,and it can do measurement in direct sunlight and cold outdoor environment ,In the process of measuring temperature cannot have change、equipment、probe and test block and the temperature of the measured piece should be the same ,the temperature difference is less than 2℃,all this is in order to eliminate the influence of temperature and material temperature increase will cause a decline in the conductivity ,in our actual work,the influence of temperature is larger。

4.2 Hand-probe the influence of the temperature difference

Boeing manual for conductivity measurement probe should match the equipment ,access to the plane of the measured piece,it is not sensitive to contact the pressure of the measured surface，rivals and probe as well as probe and the heat transfer of measured surface is not sensitive,but in practical work, temperature difference between the probe and hand has big influence on conductivity ,general conductivity probe has two kinds,one is with a pressure spring because the hand contacts spring set and not directly contacts with the probe ,so it can eliminate the influence of the temperature difference,the other one is without the spring set probes ,hand directly contacts with the probe ,the temperature of the hand will affect the conductivity value measurements,especially fingers is closer to measuring coil ,the effect is more obvious,therefore we need to appropriately add a thin layer of insulating material between fingers and probe to eliminate the impact。

The above factors will cause the accuracy of conductivity measurement values  ,the reason is  :according to vortex forming principle ,when the probe is placed on the surface of the structure / regional area,it will stimulate   vortex ring within the certain depth 。