EN ISO 1716 determines the gross heat of combustion — PCS, from the French pouvoir calorifique supérieur — of a building product or its components. The current edition is ISO 1716:2018 (EN ISO 1716:2018). The result is expressed in MJ/kg, and in the Euroclass system it is needed for every A1 classification and is one of the two routes into A2.
What makes this test difficult is not the apparatus. It is that the numbers that matter are very small. A product aiming for A1 must show a PCS of the order of 2 MJ/kg. That is a small fraction of the energy released by a typical organic fuel, so every source of error that would be negligible in fuel testing becomes significant.
The principle in one paragraph
A weighed specimen is burned completely in a sealed steel bomb filled with oxygen under pressure. The bomb sits in a known mass of water inside an insulated jacket. The temperature rise of the water, corrected for heat exchange with the surroundings, multiplied by the calorimeter's energy equivalent, gives the heat released. Because most inorganic products will not burn on their own, a combustion aid of known calorific value — normally benzoic acid — is mixed with the specimen, and its contribution is subtracted.
Three determinations are normally made and averaged.
Where PCS fits in EN 13501-1
| Item | Class A1 | Class A2 |
|---|---|---|
| Homogeneous product, or substantial component | PCS ≤ 2.0 MJ/kg | PCS ≤ 3.0 MJ/kg |
| Internal non-substantial component | PCS ≤ 1.4 MJ/m² | PCS ≤ 4.0 MJ/m² |
| Product as a whole | PCS ≤ 2.0 MJ/kg | PCS ≤ 3.0 MJ/kg |
External non-substantial components — a facing, a thin coating — have their own limits per square metre; take them directly from the classification standard. The point to notice is the unit: for non-substantial components the criterion is per unit area, which means you also need an accurate mass per square metre of that layer. A good calorimeter result multiplied by a poor area-mass figure gives a poor classification.
For A1, EN ISO 1716 is always combined with EN ISO 1182. For A2, EN ISO 1716 can replace EN ISO 1182 in the non-combustibility part, but most products still need the single burning item test.
Calibration is the measurement
The energy equivalent of the calorimeter is determined by burning certified benzoic acid. Industry practice, and the summaries published by fire laboratories, describe at least five determinations with benzoic acid pellets in the 0.4–1.0 g range. Three practical rules follow:
- Recalibrate after any change that alters the heat capacity of the system: a new bomb, a replaced bucket, a different stirrer, a change in the mass of water.
- Track the energy equivalent as a control chart. A slow trend tells you something is wearing or leaking long before a single result looks wrong.
- Use benzoic acid from a certified lot, stored dry; the certificate value is the anchor of your traceability.
Specimen preparation for a product that does not want to burn
The standard sample size is small — a fraction of a gram of ground or chopped material mixed with an equal amount of benzoic acid. With so little material, representativeness is the main risk.
- Grind the whole sampled quantity, then sub-sample. Picking a "typical-looking" piece of a mineral wool board misses the binder distribution.
- For layered products, separate the components first and test each one; the classification is built from components, not from a blended powder.
- When the powder will not burn completely in the open crucible, the standard provides a wrapped specimen route using cigarette paper of known calorific value. Use it as described, and subtract the paper.
Incomplete combustion is the classic error
After every firing, open the bomb and look. Soot on the walls, unburned residue in the crucible or a glassy bead that has trapped carbon all mean the specimen did not burn completely, and the PCS is too low — in the direction that helps a classification. That is exactly why it must not be accepted.
Typical causes are too much inert mass relative to combustion aid, a crucible that is too deep, insufficient oxygen filling, or a fuse wire that does not touch the sample. Repeat the determination after fixing the cause; do not average a sooty result with good ones.
Corrections you must not forget
- Heat exchange. The temperature rise is corrected for heat lost to or gained from the jacket. An isoperibol jacket, like the one in our calorimeter, makes this correction systematic, but only if the jacket temperature is stable.
- Ignition energy from the fuse wire and any cotton thread.
- Combustion aids — benzoic acid and cigarette paper — subtracted at their certified or measured values.
- Acid and sulphur corrections apply to fuel testing under ISO 1928 and similar methods. If you use the same calorimeter for coal and for building products, keep the method settings separate so fuel corrections are not applied to a product test, or vice versa.
Scatter before averaging
Look at the spread of the three determinations before you average them. A wide scatter almost always points to incomplete combustion or a non-homogeneous sample, not to the product. Apply the validity rules in the standard and run further determinations where they require it.
Report checklist
- Edition of EN ISO 1716, and which components were tested.
- Energy equivalent used and the date of the last calibration.
- Individual PCS values, mean, and any rejected determinations with the reason.
- Combustion aid type and mass; method used (crucible or wrapped).
- Mass per unit area for each non-substantial component.
Our oxygen bomb calorimeter, with an isoperibol water jacket and PC software, is built in our own workshop in Istanbul for EN ISO 1716 as well as ISO 1928 and ASTM D5865 fuel testing. See the ISO 1716 page.