How the water content of a pipe run is calculated geometrically, and what it is used for, e.g. with the expansion vessel and filling.
Introduction
The total water content of a system is useful to know when sizing an expansion vessel, calculating fill quantity, or judging how much liquid is shut off in a section of pipework.
It's pure geometry: the area of the circle multiplied by the length. A pipe with a 20 mm inner diameter holds about 0.314 l per metre, so 10 m gives a good 3.1 litres.
Assumptions
- 01A fully filled, straight pipe with a constant internal diameter.
- 02Bends and fittings contribute only marginally and are not included.
- 03Use the internal diameter — not the nominal DN or outer dimension.
Reference table: water content per metre
I praksis regner man sjældent cirkelarealet ud hver gang — man slår vandindholdet pr. meter op. Da det er ren geometri, kan tabellen udledes direkte af den indvendige diameter: V [l/m] = π · di² / 4000, med di i mm. Fx di = 20 mm giver π · 400 / 4000 = 0,314 l/m. Det afgørende er, at du bruger den RIGTIGE indvendige diameter for netop det materiale og den godstykkelse, du monterer — den nominelle betegnelse (DN eller ydermål) siger ikke i sig selv, hvor stort tværsnittet er.
| Copper (DS/EN 1057), outer × thickness | Int. diameter | Water content |
|---|---|---|
| 15 × 1,0 | 13 mm | 0,13 l/m |
| 18 × 1,0 | 16 mm | 0,20 l/m |
| 22 × 1,0 | 20 mm | 0,31 l/m |
| 28 × 1,5 | 25 mm | 0,49 l/m |
| 35 × 1,5 | 32 mm | 0,80 l/m |
| 42 × 1,5 | 39 mm | 1,19 l/m |
| 54 × 2,0 | 50 mm | 1,96 l/m |
The copper table uses the internal diameters that follow from EN 1057's standard thicknesses (outside diameter minus 2 × thickness); water content is the geometric consequence. Other thicknesses (e.g. heavy-wall for gas) give a different internal diameter and thus a different content.
| Threaded pipe, steel (DS/EN 10255, medium) | Approx. internal diameter | Water content |
|---|---|---|
| DN 15 (½") | ca. 16,0 mm | ca. 0,20 l/m |
| DN 20 (¾") | ca. 21,6 mm | ca. 0,37 l/m |
| DN 25 (1") | ca. 27,2 mm | ca. 0,58 l/m |
| DN 32 (1¼") | ca. 35,9 mm | ca. 1,01 l/m |
| DN 40 (1½") | ca. 41,8 mm | ca. 1,37 l/m |
| DN 50 (2") | ca. 53,0 mm | ca. 2,21 l/m |
Stål-tabellen bygger på EN 10255 medium-seriens indvendige diametre; galvaniseret udgave har samme mål. For PEX og PP kan indholdet ikke slås op alene på ydermålet, fordi godstykkelsen (SDR-klassen) varierer meget — fx har 16 mm PEX ofte 2,0 mm gods (di ≈ 12 mm → ca. 0,11 l/m) og 20 mm PEX ca. 16 mm indvendig (≈ 0,20 l/m). Brug altid producentens datablad (Wavin, Uponor) for den indvendige diameter, og indsæt i V = π · di² / 4000. Samme gælder rustfri pressrør (Geberit Mapress, Viega). (Kilde: DS/EN 1057; DS/EN 10255; producentdatablade — indvendige diametre.)
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