DN, PN, glass dimensions and the view diameter: what they mean
Four different characteristics that are often confused: DN, PN, the free diameter and the glass outer diameter.
Short answer
DN is the nominal size of the connection; PN is the nominal pressure class; the free diameter is the actual view window; the outer diameter is the size of the glass disc. These are four different values.
PN is a connection class, not a guarantee of working pressure: the assembly at specific temperature and medium is confirmed by the manufacturer.
Definitions
| Characteristic | What it means | Frequent mistake |
|---|---|---|
| DN | nominal size of the fitting connection | taking it as the view diameter |
| PN | nominal pressure class of the connection | taking it as permissible working pressure at any temperature |
| Free diameter | the visible zone inside the fitting | substituting it with DN or the outer diameter |
| Glass outer diameter | the actual size of the disc | ordering glass “by DN” without measuring |
How to measure at replacement
Record all four values from the fitting: DN (from the marking or documentation), free diameter (through the window), outer diameter and disc thickness (from the removed glass, if it exists).
Relation to the view area
The view area grows proportionally to the square of the free diameter: doubling the diameter quadruples the zone. Estimate it with the area calculator before ordering.