1.25" (32 mm) HDPE pipe in N. America is not equivalent of 32 mm metric pipe

Much of my career in the geothermal industry has been based in North America. The principles the geothermal industry works on in North America is the same as industry in Europe, Austral-Asia, South America and the rest of the world works on...but quite a few of the little details about how systems are deployed are different in the different regions. One of the biggest differences is...

The use of the metric system in most of the world outside of North America, and the distribution systems connected to the heat pumps in North America and the rest of the world. 

How HDPE pipe is described in Imperial units is different than how it is described in the metric system. In North America, if you are using 1.25" pipe, the 1.25" refers to the nominal inside diameter of the pipe...approximately 32 mm in diameter if you're speaking metric, right? Not quite so simple. In metric, if you are designing a system using 32 mm pipe in the boreholes, you are referring to the outside diameter of the pipe...and the inside diameter of 32 mm HDPE pipe is about the same size as the inside diameter of what a U.S. or Canadian designer or installer would call 1.00" pipe. 

To make it more confusing, PEX-a pipe is referred to in North America and the rest of the world by the outside diameter of the pipe. In metric, PEX-a pipe with an outside diameter of 25 mm would be called 25 mm pipe. In North America it would be called 1.00" PEX-a pipe, but if it was 25 mm HDPE pipe it would be called 0.75" pipe in North America.

Does this matter? If you're designing a GHX in North America, it is confusing to label with the imperial units followed by the metric equivalent in brackets, as in 1.25" (32 mm). The equivalent metric pipe would be 40 mm, but many would do a conversion and wondering they should be ordering 1.50" pipe. In fact, in the recently released CSA-448-13, the pipe is referred to in only Imperial units specifically to avoid any confusion. The remainder of the Standard is written in metric with Imperial units in brackets (since Canada is officially metric)

Pipe dimensions make a difference when determining the pressure drop of the GHX and if the flow is laminar, transitional or turbulent, and can make a significant difference in the performance of a GCHP system.