Using Survey Equipment to find Pipe Grade
Although most residential plumbers will only need to know how to read a Philadelphia rod in order to pass the plumbing exam, those interested in working on industrial and commercial sites may have to find the grade of long pipe runs using a Philadelphia rod and transit (Survey Equipment).
The image below shows an imperial Philly rod that is being read to a height of 5.76 feet (the 5 foot marker is out of view). The top and bottom of each black mark is one hundredth of a foot; the top of each is even, while the bottom of each marking is odd.
Using a metric Philadelphia rod is very similar to an imperial philly rod, except each mark is one centimeter and has the even numbers starting from the bottom of each black mark. The large numbers in the image below show tenths of a meter and are marked by the line on which the numbers rest.
Grade the Pipe
To properly grade a piping run using a transit and Philly rod you will need to do some simple math, take proper distance measurements, and have a note pad to record the measurements.
- Know the grade you need on the pipe. 1:50= 1/4″ per foot, 1:100 = 1/8″ per foot
- Measure the distance of the pipe you are finding the grade, and calculate the drop (grade) it should have. For example 64 feet of pipe at 1/8″ per foot would need to drop 8 inches.
- To find the actual grade of the pipe, have the rod man place the philly rod on the pipe at the high end and take a reading with the transit, then have him move to the low end and take another reading. Subtract the high side reading from the low side reading, to calculate the actual drop of the pipe.
- To find the grade of a pipe when you know the drop and the distance, simply divide the distance by the drop using the same units of measurement. Example: the pipe run is 38 feet and the drop is 5.5 inches. 38 x 12 = 456 inches. 456 / 5.5 = 83 so the grade would be 1:83.
Knowing how to read a Philadelphia rod and calculate grade will help you with a whole section on the plumbing exam, as well as experience in the field.