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The scale of a draft gauge is measured in inches of what substance?

  1. Mercury.

  2. Steam.

  3. Water.

  4. Oil.

The correct answer is: Water.

The scale of a draft gauge is measured in inches of water because draft gauges are specifically designed to measure the pressure or vacuum in a system in terms of water column height. This measurement reflects how much pressure is being exerted against the water in a vertical tube, represented in inches. Using water is advantageous due to its density and properties, which allow for sensitive readings in relatively small pressure changes that are common in drafting applications. Measurements can vary depending on the specifics of the system, but water is the standard reference medium because it provides a practical balance between sensitivity and the need for physical space in the measuring device.