He was already on the right answer. Code stuff is simple enough, have to be able to interpret the signals.
ASME V 2007 Fig T434.3 calibration block for pipe (notes)
"(e) Notch lengths shall be sufficient to provide for calibration with a minimum 3 to 1 signal-to-noise ratio."
Any electronic device is prone to EMF interference, this is why faraday cages are built around welders when they have to be in radio sensitive areas.
Hi freq units are especially prone to this.
Your S:N ratio is the key.
Regards,
Gerald