To measure MDPE pipe accurately, the key dimension you need is the outside diameter (OD). Measurements of standard MDPE are always taken from the outside diameter of the pipe.
This is the most critical measurement because it determines compatibility with fittings like couplers, elbows, and tees, which are designed to grip the outer surface of the pipe.
Why Measure the Outside Diameter?
Unlike some other pipe systems where the nominal size might refer to the inside diameter or a general size category, MDPE pipe sizing is based on its outside diameter.
- Fittings Compatibility: All standard MDPE pipe fittings are designed to connect specifically to the outside diameter of the pipe. A fitting labelled '25mm' is designed for a pipe with a 25mm outside diameter. Using a pipe with a different OD will result in an improper, potentially leaking, or failed connection.
- Standardization: Using OD as the standard measurement ensures consistency across manufacturers and simplifies the selection of the correct fittings.
How to Measure the Outside Diameter
While you might know the nominal size (e.g., 20mm, 25mm), verifying the actual OD is good practice, especially if dealing with unmarked pipe or critical connections.
- Tools: The most accurate tool for measuring OD is a set of calipers (vernier or digital). A flexible measuring tape can give a rough idea but is less accurate for precise fitting.
- Technique:
- Wrap a flexible measuring tape around the circumference of the pipe. Divide the circumference measurement by π (approximately 3.14159) to get the OD.
- For a more precise measurement, use calipers. Open the jaws of the calipers and place them around the pipe. Close the jaws until they are snug against the pipe's outer surface, ensuring they are perpendicular to the pipe axis. Read the measurement displayed on the calipers.
- Multiple Readings: Take measurements at a couple of different points around the pipe's circumference to account for any minor variations.
Other Important MDPE Pipe Measurements
While the outside diameter is paramount for compatibility, other measurements provide essential information:
- Nominal Size: This is the common size designation (e.g., 20mm, 25mm, 32mm). It typically corresponds directly to the pipe's outside diameter.
- Wall Thickness (SDR): The thickness of the pipe wall is related to its pressure rating. This is often indicated by the SDR (Standard Dimension Ratio) rating. A lower SDR number indicates a thicker wall and a higher pressure rating (e.g., SDR 11 vs. SDR 17). Wall thickness can be measured using calipers on the cut end of the pipe.
- Length: The total length of the pipe run needed for your project. MDPE is often supplied in coils (e.g., 25m, 50m, 100m) or straight lengths.
Common MDPE Pipe Sizes and OD
Here is a table showing common nominal MDPE pipe sizes and their corresponding outside diameters:
| Nominal Size | Outside Diameter (OD) |
|---|---|
| 20mm | 20mm |
| 25mm | 25mm |
| 32mm | 32mm |
| 50mm | 50mm |
| 63mm | 63mm |
Always confirm the exact OD, wall thickness, and pressure rating marked on the pipe itself if possible.
Understanding that MDPE pipe is sized by its outside diameter is crucial for selecting the correct fittings and ensuring a reliable installation.
