What issue may arise if a contact ultrasonic instrument is set with an excessively high pulse repetition rate?

When a contact ultrasonic instrument is set with an excessively high pulse repetition rate, the transmission of ultrasonic pulses occurs more frequently than the instrument can effectively process. This can lead to a situation where the instrument does not have enough time to distinguish between echoes from previous pulses and the new echoes that are arriving. As a result, this can cause what's known as phantom indications, which are false signals or readings that appear on the display due to overlapping echo signals. These false readings can mislead the operator into thinking that there is an indication when there is not, which can severely impact the accuracy and reliability of the ultrasonic testing.

Adjusting the pulse repetition rate is crucial to ensure that the instrument can adequately capture and analyze each echo without confusion caused by the overlap of successive signals. Ensuring that the pulse repetition rate is set appropriately allows for clearer separation of echoes and more accurate readings.