Which parameter primarily governs the maximum scanning speed of ultrasonic equipment?

• A. The frequency of the search unit
• B. Viscous drag problems
• C. The pulse repetition rate of the test instrument
• D. The persistency of the CRT screen

Answer: (D)

When you move the probe to scan a surface, the rate at which you can perceive and interpret the trace on display is what mainly limits how fast you can scan. The CRT screen’s persistence—the length of time a trace stays visible after being drawn—directly affects this. If persistence is high, traces linger long enough for you to follow rapid sweeps; if persistence is too low, the trace fades too quickly and fast scans become hard to read. So, the display hardware’s ability to retain the image governs the practical maximum scanning speed.

Other factors influence signal quality and data rate, such as the transducer frequency (affects resolution and penetration), the mechanical effort of moving the probe (viscous drag), and how often echoes can be produced and sampled (pulse repetition rate). But for how fast you can actually scan while still seeing a coherent trace, the CRT screen’s persistency is the controlling factor.