The idea was to dilute an alcohol-based extract made with the given pepper until it no longer tasted hot to a group of taste testers. The degree of dilution translates to the SHU. In other words, according to the Scoville scale, you would need as many as 5,000 cups of water to dilute 1 cup of tobacco sauce enough to no longer taste the heat.
While the Scoville scale is still in use today, scientists no longer rely solely on human taste buds to determine the heat rating of a hot pepper. (Understandably, receptors get "worn out or overused," according to Dr. Paul Bosland, professor of horticulture at New Mexico State University, making it difficult to taste things anymore.) Once they've determined how many parts per million of heat-causing alkaloids are present in a given chile pepper, scientists multiply that number by 16 to arrive at the pepper's Scoville rating.
For more about tasting chile peppers, their heat profile, and whether or not they can damage taste buds (hint: they can't!), read the full article below:
Read More: How Hot Is That Pepper? Unpacking the Scoville Scale | Smithsonian
Related: What Makes Chile Peppers Spicy?