Journeyman Plumbers Isometric Analysis and Math Questions and Answers — Questions and Answers
Question 1: A plumber needs to install a 45-degree offset. The horizontal distance between the centerlines of the two parallel pipes, known as the 'set', is 20 inches. What is the center-to-center length of the 'travel' piece of pipe?
- 20 inches
- 28 1/4 inches (Correct answer)
- 34 5/8 inches
- 24 1/2 inches
Correct answer: 28 1/4 inches
To find the travel for a 45-degree offset, you multiply the 'set' by the constant 1.414. In this case, 20 inches (set) x 1.414 = 28.28 inches. The closest fractional answer is 28 1/4 inches (28.25).
Question 2: A horizontal drainage line that is 48 feet long must be installed with a uniform grade of 1/8 inch per foot. What is the total fall of the pipe from its starting point to its end point?
- 4 inches
- 8 inches
- 12 inches
- 6 inches (Correct answer)
Correct answer: 6 inches
To calculate the total fall, multiply the length of the pipe run by the required grade per foot. Calculation: 48 feet * (1/8 inch/foot) = 6 inches.
Question 3: When fabricating a rolling offset, a pipe must shift 15 inches horizontally (run) and 20 inches vertically (rise). What is the 'true offset' of this configuration?
- 35 inches
- 28 inches
- 25 inches (Correct answer)
- 22 inches
Correct answer: 25 inches
The true offset of a rolling offset is the hypotenuse of the right triangle formed by the run and the rise. Using the Pythagorean theorem (a² + b² = c²), the calculation is 15² + 20² = c². This becomes 225 + 400 = 625. The square root of 625 is 25. Therefore, the true offset is 25 inches.
Question 4: In pipe measurement, the dimension that must be subtracted from a center-to-center measurement to determine the exact cut length of the pipe is known as:
- Fitting allowance (Correct answer)
- Developed length
- Travel
- Throw
Correct answer: Fitting allowance
The fitting allowance, or take-off, is the length from the center of a fitting to the face of the fitting's socket or hub. This amount is subtracted from the center-to-center measurement for each fitting used to find the precise cut length of the pipe.
Question 5: Which of the following is the correct mathematical constant used to calculate the 'travel' piece when the 'set' of a 60-degree offset is known?
- 1.414
- 2.613
- 1.732
- 1.155 (Correct answer)
Correct answer: 1.155
The constant for calculating the travel of a 60-degree offset is 1.155. The formula is: Travel = Set x 1.155. The constant 1.414 is for 45-degree offsets, and 2.613 is for 22.5-degree offsets.
Question 6: An isometric drawing shows a vertical drain pipe rising from the floor. The line then turns to run horizontally toward the back and to the left. Which combination of fittings would be required to make this change of direction?
- One 45-degree elbow
- One 90-degree elbow and one 45-degree elbow (Correct answer)
- Two 90-degree elbows
- One tee and one 45-degree elbow
Correct answer: One 90-degree elbow and one 45-degree elbow
To change the pipe's direction from vertical to horizontal, a 90-degree elbow is needed. To then create the horizontal roll to the back and left, a 45-degree elbow is used in combination with the 90. This creates a three-dimensional change in direction that is common in plumbing systems.
Journeyman Plumbers Isometric Analysis and Math Questions and Answers
A plumber needs to install a 45-degree offset.
The horizontal distance between the centerlines of the two parallel pipes, known as the 'set', is 20 inches.
What is the center-to-center length of the 'travel' piece of pipe?