The formula to calculate the desired time fuse length (DL) is:
\[ DL = \left( \frac{TL}{TT} \right) \times DT \]
Where:
Let's say the test length is 10 inches, the test time is 5 seconds, and the desired time is 20 seconds. Using the formula:
\[ DL = \left( \frac{10}{5} \right) \times 20 \]
We get:
\[ DL = 40 \]
So, the desired time fuse length (\( DL \)) is 40 inches.