The Importance of Witness Marks
My neighbor learned about the importance of witness marks this week. He had to replace his pool pump and of course this involved replumbing piping from the new pump to the filter. The project also expanded into replacing some short turn elbows with 45º fittings on the filter return side.
What went wrong?
He started building the connection from the pump to the filter and glued each piece as he got closer to the filter. Unfortunately, when he got to the union the two sections of the piping did not line up. One section was too high and also at an angle to the matching side of the union. He had big leaks to contend with due to the angle after tightening the union as tight as he could. That’s when I got called in.
Shortening, lengthening, setting an angle
After looking over the installation, it was clear that we would need to redo some of his work. We would need to reduce the height of the rise from the pump to the filter. This could be fixed in the vertical pipe section by cutting out part of the pipe and installing a coupling. Also, we could use the cut to twist the pipe so that the two sections of pipe at the union would line up parallel.
Once we had accomplished those tasks the horizontal section from the filter to union needed to be extended about half an inch. Another cut on the horizontal piping and a coupling would take care of this extension problem.
Witness marks to the rescue
So how do you fix all these errors and have the sections come together perfectly? You use witness marks.
witnessmark (plural witness marks)
Dry Fitting
Witness marks work especially well when you are building a section of pipe. Dry fitting the sections together and verifying that each section is perfectly lined up and is not stressing to the connections as you build each section. This is what we did, cutting the section, twisting the pipe into position. We hand tightened all connections. Once all the dry fitting of all the parts was completed it was time for witness marks.
Two types of witness marks
Two different marks
The first mark was about keeping the angle we needed so that the two pipe sections would line up parallel at the union. But not just a strike across the joining pieces but several marks close to 90º apart so that we could eliminate or minimize any twist or angle change from our dry fit. One mark by itself and another two marks together placed 90º from the first single mark. All marks were made with a permanent marker striking across the fitting and the pipe(see example above).
The second mark
The second type of witness mark was a mark on the pipe at each edge of the coupling. This type mark was all about the extension/length of the section. This mark was placed on the vertical pipe(in addition to the angle witness marks) and kept our height at the correct extension. The second time we used this type mark was on the horizontal section which needed to be extended about half of an inch. The mark was placed again at each end of the fitting after dry fitting that section.
We removed the piping from the pump to the union. We left the pipe section from the filter to the union in place. As we prepared the pipe and fittings with cleaner and medium PVC glue we were careful not to obliterate our witness marks. We lined up all of our witness marks. We reinstalled the pipe section from the pump to the union once the glue had set up. When it was all done everything lined up perfectly. A testament to the importance of witness marks.
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