New smoke laboratory buyers often ask whether they need "the ISO chamber" or "the ASTM chamber". The reassuring answer is that the hardware is the same family - the NBS smoke density chamber - and both methods run on it. The real differences are the exposure arrangement, the quantities reported and the markets that ask for each document. This post separates them.
The shared principle
In both methods a specimen is mounted inside a sealed chamber (the long-standing NBS design, internal volume defined by its roughly 914 mm cube dimensions) and exposed to radiant heat. Smoke accumulates in the closed volume instead of being swept away, and a vertical light beam with a photometric system measures how much light survives. From transmittance comes specific optical density, the dimensionless number both standards report. Both also run in two modes: non-flaming (radiant exposure alone) and flaming (with pilot burners).
Where they differ: exposure
ASTM E662 works with its own radiant furnace arrangement - in the Motis SDB documentation, an ASTM E662 furnace delivering 25 kW per square metre, with a six-burner unit for the flaming mode, and the specimen in the chamber's standard orientation. ISO 5659-2 specifies its cone-type radiant heater arrangement and its own specimen configuration, and is the method written into the railway standard EN 45545-2 and the IMO smoke procedures. The differences are in the heater geometry, specimen mounting and the prescribed test conditions - which is why a chamber is configured and calibrated per method rather than per brochure.
Where they differ: reporting
E662 reports specific optical density over time, with maximum specific optical density (Dm) and the values at the time points the calling specification cares about - aviation work under FAR 25.853 classically reads the density at four minutes against its cap. ISO 5659-2 reports Ds values at defined points and, for railway use, the VOF4 accumulation figure that EN 45545-2 limits are written around, and it is the chamber whose gases are sampled for the toxicity CIT index. Same physics, different arithmetic on the report.
Which markets ask for which
ASTM E662 dominates North American specifications: aviation interiors under the FAA regime, US building and transit work, and naval NES-influenced specifications (NES 711 is a close relative). ISO 5659-2 dominates European and international work: railways under EN 45545-2, marine under the IMO FTP Code, and European product standards. Laboratories serving export customers usually hold both methods on one chamber - the practical question is not either/or but configuration and calibration scope.
And the toxicity connection
Both chambers increasingly work with gas analysis attached. The Motis SDB documentation lists Drager tube options for aviation smoke toxicity sampling and an FTIR interface for qualitative and quantitative analysis of the smoke gases - the route by which a density chamber becomes a toxicity station for ISO 5659-2-based railway work or BSS-family aviation methods. If your specification includes a toxicity index, plan the sampling interface when you buy the chamber, not after.
Running both methods on one chamber: what changes in practice
Laboratories that hold both methods on a single chamber learn that the switch is procedural, not mechanical. The heater arrangement and specimen orientation for the called method are fitted and the chamber's calibration for that arrangement is verified - the foil-backed copper-disc calorimeter reading for the E662 furnace setting being the classic example. The software template changes with the method, because the reported landmarks differ: E662 reports lean on Dm and the specification's time points, while ISO 5659-2 railway reports need Ds values at the method's points and the VOF4 accumulation, calculated from the first four minutes. Conditioning and specimen geometry follow the called document as well. None of this is difficult, but it must be written into the laboratory's procedures method by method; auditors test the paper trail per method, not per chamber.
A note on specimen behaviour the numbers hide
Both methods share a blind spot worth knowing before you specify limits from a single curve. Materials that melt and pool, that intumesce against the heater, or that delaminate and expose fresh surface mid-test can produce density curves that owe as much to the artefact as to the chemistry. The methods handle the common cases with prescribed holders and retainers, but an experienced smoke laboratory reads the curve shape - a sudden second peak, a collapse in transmittance late in the test - as a prompt to look at the specimen remains, not just the number. When two materials tie on Ds, the one whose smoke arrives late and slowly is usually the better engineering choice for escape time, whatever the single figure says; the full transmittance record, which both methods produce, is where that judgement lives.
Common specification mistakes in smoke work
Three errors recur in procurement documents. First, naming a chamber but not a method ("smoke density per NBS chamber") - the chamber runs several methods with different results. Second, quoting a competitor's Ds value without its mode, so a flaming-mode figure is compared against your non-flaming one. Third, setting a limit tighter than the method's reproducibility between laboratories, which manufactures disputes no instrument can settle. The cure is the same in all three: cite method, mode, landmark and limit together, or expect the laboratory to ask before testing.
Conditioning and repeatability, briefly
Smoke output is among the most conditioning-sensitive properties in fire testing: a specimen that has absorbed moisture smokes differently from the same specimen dried to constant mass, and chamber methods prescribe conditioning for exactly this reason. Repeatability between laboratories for optical density is respectable but not unlimited, which is why well-drafted specifications set limits with the method's known scatter in mind and why borderline results deserve a repeat run rather than an argument. Budget the conditioning time - days, not hours - into any smoke programme with a deadline.
One chamber, honestly specified
The buyer's summary is simple. The chamber is common; the method configuration, calibration and report template are the purchase. Specify them by name - method, modes, sampling options for toxicity, and the landmarks your reports must print - and the instrument that arrives will be the one your specifications meant. Leave them generic, and you will own an excellent box that produces numbers nobody specified. Ask for the method list in writing on the quotation itself, so the configuration you approved is the configuration that is commissioned, calibrated and handed over.
Choosing, in one paragraph
Name your market first: US-facing work, E662; European rail/marine work, ISO 5659-2; both markets, one chamber with both method configurations and calibrations. The NBS smoke density chamber page documents the chamber's construction and options, including the 914 x 914 x 610 mm Teflon-coated chamber and the photomultiplier optical system.