Abstract: Using different types of ultrasonic detectors to make contrast experiments were described. In these experiments, when tests were made on the same concrete sample, the accuracy and reliability of the experiments may be directly affected by the time deviation caused by using different ultrasonic detectors. The prime reason of the time deviation was caused by the test method, in which the operator used to deduct the initial time value t0, and it had nothing to do with the types of the digital and analog detectors.