T.O. 33B-1-1
4-42
4.4.3.11
Ruggedness.
The instrumentation must be capable of operating in the test environment. This may include a variety of environmental
extremes of temperature, humidity, dust, and vibration.
4.4.3.12
Specific Instrumentation Requirements.
Choice of an eddy current test instrument must take into account the type of flaw to be detected, the permeability of the
material (nonferromagnetic or ferromagnetic), type of probe to be used, display method (meter, CRT, digital display,
recorders, etc.), test frequency, and signal processing requirements, portability, if needed, and any accessories to be
used.
4.4.3.13
Instrumentation components.
In general most eddy current instruments consist of an oscillator, a bridge circuit or similar null balancing system, and
a variety of other circuits for processing and display of the eddy current signal. Depending upon the complexity of the
instrumentation and the requirements of the test, the following components will be present.
4.4.3.14
Variable Grequency Oscillator.
A basic eddy current instrument, while operating at a single frequency during a particular test, usually has an operating
frequency range that is adjustable to meet a large variation of inspection situations. Low frequencies increase depth of
penetration and consequently would be used for subsurface flaw detection or in high conductivity materials. Higher
frequencies limit depth of penetration and thus are used for low conductivity materials as well as for detecting smaller
flaws. Some instruments also incorporate a fine adjustment of frequency as a mechanism for suppressing lift-off.
These instruments incorporate the probe coil in parallel with a capacitor as one leg of a bridge. The coil/capacitor
combination is resonant near the intended operating frequency. The frequency selected for operation is off-resonant
enough to where lift-off causes less of an impedance change than caused by a defect and the impedance change for
increasing lift-off is opposite to that for a defect.
4.4.3.15
Bridge Circuit.
A basic bridge circuit is shown in Figure 4-37. In this example, a voltage is applied at points E1 and E2 to the bridge
containing impedances Z1, Z2, Z3, and Z4. Z1 and Z4 are fixed impedances of the same value; Z3 is an adjustable
impedance; and Z2 the unknown or test probe impedance. Initially, Z3 is adjusted so that no current flows through the
amplifier. This means the voltage at points A and B is the same and the bridge is said to be balanced or nulled. Any
change in impedance of Z2, the test probe impedance, will result in a current change through the leg of the bridge and
consequently a change in the voltage at point B. A current will then flow through the amplifier, since a voltage or
potential difference exists between points A and B. The bridge is now said to be unbalanced. The bridge can again be
balanced by adjustment of Z3 and the change in the test probe impedance, Z2, may be determined by measuring the
change in Z3 required to rebalance the bridge. The bridge circuit in an eddy current test instrument is termed an
impedance bridge since the circuit contains both resistive and reactive elements. Impedance Z2 in Figure 4-37 would
consist of the eddy current test coil. Other reactive elements, inductors, and capacitors may be included in the
impedance bridge depending upon the specific design and function. However, the basic principle is that a change in
impedance of the test coil results in an unbalance of the bridge circuit. The output (unbalance) from the bridge circuit
can be amplified, processed and displayed.
