Probe Types

T.O. 33B-1-1 4-51 4.5.2.4 Probe Types. The most common type of eddy current probe used in field applications is the absolute probe. The absolute probe contains a single coil that is placed in contact with or adjacent to the part being inspected. Since any changes in the area interrogated by the coil produce a response, absolute probes can be used to measure specific materials properties such as electrical conductivity and magnetic permeability. Differential probes contain two or more coils and are intentionally designed to produce a response when changes are sensed by the active coil only. Consequently, if the differential probe has two coils mounted side by side, gradual changes in electrical conductivity or magnetic permeability would be sensed by two coils simultaneously and no response would occur. On the other hand, if an abrupt change in conductivity should occur, localized to where it can be sensed by only one coil at a time, then there would be a response. Cracks cause a localized conductivity change and consequently can be readily detected by differential probes in the presence of slowly varying changes in electromagnetic properties or conditions that would cause interfering responses in absolute probes. 4.5.2.5 Sensitivity. The ability of an eddy current instrument to detect small variations in test coil impedance is a measure of its sensitivity. This quality is interrelated with the properties of the test coil and the operating frequency. Therefore, instrument sensitivity to a particular flaw condition or material property should be established from calibration standards representing this condition. 4.5.2.6 Frequency Requirements. As the eddy current test frequency is increased for a specific eddy current application, the eddy currents are confined to a smaller volume adjacent to the inspection probe coil. This concentration increases the proportion of generated eddy currents intercepted by a small crack or other defect. Higher frequencies should then provide better response to the smallest defects. This statement holds in general, but other conditions may limit the sensitivity when using higher frequencies. In some instruments, high induction losses limit instrument output at these higher frequencies. Lower frequencies may be required for increased penetration to detect subsurface or far surface flaws. Optimum sensitivity to cracks or other flaws generally occurs in specific frequency ranges for each combination of metal, flaw size and flaw depth. Operating frequency ranges can be established for each application by using the calculated depth of penetration using the conductivity and permeability of the material. These calculations should be confirmed with the use of calibration standards which simulate the anticipated flaws to be detected. 4.5.2.7 Resolving Power. The ability of a test system to separate the signals from two indications that are close together is defined as resolving power. This property plus sensitivity must be considered in every flaw evaluation situation. Probe design, test frequency, and instrumentation design are all factors in determining the resolution of an eddy current system. 4.5.2.8 Signal To Noise Ratio. As the gain of a test system is increased, a background of electrical noise will be observed. This may be represented by erratic meter movement, excessive background signals on a waveform display, or excessive, random patterns on a recorder. This "noise" can be the result of random variations in the electrical system of the test instrument, normal variations in material properties, or stray electrical signals from other electrical devices. Signal-to-noise ratio is not a function of the instrument alone, but is also dependent on lift-off, surface finish, and conductivity and permeability variations within the inspection part. In order for an eddy current test instrument or any other electrical test instrument to be useful, it must provide flaw signal information that is greater than the background noise of the test system. Otherwise the inspector could not see the difference between the flaw signal and the background noise. For maximum reliability in eddy current inspection, a high signal-to-noise ratio is desired. Unless the signal from the crack or other flaw for which inspection is performed is significantly greater than the signals from electronic noise and from material and test variables for which inspection is not being performed, the desired signals may be lost in the noise. No specific signal-to-noise ratio is mandatory, but a minimum of 3-to-1 is desirable for flaw detection. 4.5.2.9 Signal To Noise Ratio And Sensitivity. As the required crack size to be detected is decreased, the gain or sensitivity of the eddy current instrumentation must be increased to provide readable indication from small cracks. The higher gain results in greater indications from small cracks. The higher gain also results in greater response from variables other than cracks and the noise level