In my previous article, I discussed the frequency range of a sensitive bridge probe (impedance probe). From a sensitivity standpoint, the take is that bridge probes have a relatively narrow frequency range, about equal to the resistance of the bridge, which has the effect of binding bridge probes to the applications they’re tuned for.
In the field, users may need probes a little more flexible than bridge probes. Transmit-receive (T-R) probes are such an alternative. The coil configuration in transmit-receive probes (also known as driver-pickup probes and reflection probes) have a relatively better sensitivity to defects, up to a certain point.
Advantages of T-R Probes
Physically, transmit-receive probes have two coils: one coil to generate eddy currents in the part under test, the other to sense the variations in eddy currents in the test material. In theory, the transmitter and receiver coils can be optimized separately to maximize their intended purpose. For example, the transmitter coil may produce a strong, uniform field in the vicinity of the receiver coil. At the same time, the receiver coil can be small enough to be sensitive to very small defects.
Limitations of T-R Probes
Furthermore, transmit-receive probes have a theoretically infinite frequency response because, having two coils, the relative sensitivity does not vary according to the frequency like in bridge probes. The sensitivity response of transmit-receive probes is still restricted to some extent, but despite these restrictions, the relative sensitivity range of the probes is still better than bridge probes.
From a sensitivity standpoint, however, transmit-receive probes are not always better than bridge probes. If that were the case, no one would ever use bridge probes. Transmit-receive probes have two major limitations when it comes to sensitivity:
Low frequency = Thermal drift
At low frequencies (better probe penetration), the transmitter coil has a low impedance that increases its current. This leads the transmitter coil to generate a lot of heat, which has negative effects on the quality of returned signals and the equipment itself.
High frequency = Resonance
At the high end of the frequency range, the transmitter coil’s inductance can resonate with the parasitical capacitance of coils, of the probe’s cable, and of the test instrument itself. This leads to over-amplified and distorted signals.
The following graphs illustrate these limitations. As you can see from the current curve of a transmitter coil in a transmit-receive probe, the current is very high at low frequencies, which may result in overheating and thermal drift. In this example, the minimum frequency of the transmit-receive probe is set to 100 kHz to avoid this.
Similarly, the transmit-receive probe’s cable and coil impedance has an incidence. Set the frequency too high and the coils resonate with the cable. The peak in the following graph corresponds to the resonance frequency where the eddy current signal signature cannot be used. Here, to avoid resonance, the maximum frequency of the transmit-receive probe is set to about 500 kHz for a 20 m cable.
As you can see, under most normal conditions, transmit-receive probes may have a wider range of relative sensitivity to defects (100–500 kHz for the 20 m cable probe above), which may make them better than bridge probes in many applications where better flexibility is an asset.
However, this is only one aspect — relative sensitivity — to be considered when deciding whether to use a bridge probe or a transmit-receive probe for an inspection project. Other considerations include the type of materials involved, the type of defects, the amount of liftoff, and many more.