[william]

Yes you can do, but it depends. If you had selected correct frequency and you have saved the calibration tube data, then you can recalibrate as earlier.

In case, if you dont have the cal tube data, take a cal tube data at the same frequency at which you have inspected the exchanger and calibrate it. this is not technically right method but it will full-fill your requirement. We all know, that ECT is a comparative method. So, here you will get better reference.