Eddy Current
Applications

Introduction to Eddy
Current Inspection

Rotating probe(s)
bar & tube testing

'Quadent'
crack depth gauge

Eddy Current
Inspection Menu

Fatigue and other cracks, found either visually or with the aid of such techniques as magnetic particle inspection, in machinery and plant invariably pose the problem that whilst the position and length are easily ascertained, the depth to which the crack penetrates, is not immediately obvious. However, it is usually the depth of the crack, which determines the fate of the defective component; whether or not the item can continue in service under observation with planned remedial action by repair or replacement, or whether the item has to be immediately scrapped.

As an aid to gaining full information about the crack we have developed the QUADENT crack depth meter. With this portable instrument the depth of a surface breaking crack can be quickly and easily measured. Little skill is necessary and the information is presented in absolute terms of depth. The propagation rate can be determined and in longer cracks the profile can be plotted.

The QUADENT can be used on steel, iron, stainless steel or any metals of relatively low conductivity.

Uses the principle of AC potential drop

Measuring range 0 to 100mm

Accuracy ±10% of reading or 1mm whichever is the greater

Single switch, with Off/Charge, Metric and Inches settings

Display resolution Metric to 0.1mm or Inches to 0.01 Inch

Battery powered with an internal charger

Up to 1000 measurements from a fully charged battery

Probes have four hardened steel, spring loaded, electrodes
in a housing incorporating an automatic zero button

Optional calibration block for calibration verification

Optional spare sensor probes and contact pins

The measurement of crack depth uses the principle of electrical AC potential drop. In this method the increase in potential drop in a current passing between two electrodes in contact with a sound surface adjacent to the crack, is compared to the voltage drop when the crack is situated between the two electrodes. Since the current traveling around the crack effectively increases the distance between the measuring electrodes the voltage drop will be greater. The increase is proportional to the crack depth.

Current passed between two outer electrodes A & D
Voltage measured between electroded B & C

The AC current between electrodes A & D is approximately 0.5 amp at a frequency of 1500Hz. At this frequency the current flows preferentially close to the surface (skin effect) and therefore closely follows the contours of the crack. Even in a very tight crack with fracture faces in contact, the path of least resistance is still around the bottom of the crack.

The application of the electrodes to the material surface is by means of a hand held probe. This houses four spring-loaded electrodes so that they will conform to curved surfaces.

The mode of application is to apply the probe to the surface of the material adjacent to, but not affected by, the crack and press the zero button integral in the probe. This action compensates for different conductivity characteristics of the material so that in effect the increase in voltage drop is related directly to the unaffected surface. The display will indicate 0.0. Following this procedure the electrode probe is moved so two middle electrodes straddle the crack - one on each side. The path length being effectively extended by the presence of the crack the display will indicate the measured depth.

The QUADENT will not only measure crack depth but has the ability to discriminate between surface marks, scores etc., which have the appearance of cracks, and genuine cracks. If during the measurement process the LED displays 0.0 then there is not a crack.

Specifications:

The equipment is supplied, with comprehensive instructions, in a rigid carrying case with foam cut outs to accommodate the various items.

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