Which two items are essential for thickness measurement during gunning operations?

In gunning operations, accurately measuring the thickness of the refractory material is crucial for ensuring that the application meets design specifications and provides optimal performance. The correct answer, which includes shot board height and depth gauges, makes sense because these tools are specifically designed to measure the thickness of the applied material directly and accurately.

Shot board height gauges help assess the level of the refractory material being applied, allowing the operator to understand how much material is being deposited at any given moment. Depth gauges work similarly, providing a precise measurement of how thick the material is in various locations, ensuring uniform application.

The other options do not offer the same precision or relevance to the specific context of gunning operations. For instance, while measuring tapes and rulers can be used in general measurements, they lack the specialized design needed for the accuracy required during refractory applications. Similarly, calipers and micrometers, while precise, are more suited for measuring small objects rather than the thickness of applied refractory materials over an expansive area. A laser level and depth gauge might provide useful level references but do not specifically address the thickness measurement required in this scenario. Therefore, the choice of shot board height and depth gauges is the most effective and relevant for thickness measurement during gunning operations.