I came across this in a patent search for something else. GB PATENT 339831A dated 18th December 1930. In the patent, a preamble refers to a tendency of existing silent burner caps to degrade (burn) over time, creating underburn as they get progressively hotter in use if they are not replaced soon enough. The patent does not specifically mention the phenomenon, but I think we’ve all noticed too that brass caps are inclined to glow red, giving the flames a red/orange tint, whereas steel csps do not. The patent states, Accompanying drawing. Presumably the alloy Hjorth refers to came to be known as Primus Metal. I see the French patent already appears in the Stove Reference Library/Patents HERE. John
Interesting patent John. I think this iron-nickel alloy (also known as Invar 36) mentioned by Hjorth above was the precursor to ‘Primus Metal’ but not the final composition. Some years ago I did some research into Primus Metal and while I could not determine the exact composition, the general opinion was that Primus Metal was an iron-based heat resistant alloy containing significant amounts of chromium as well as nickel - the chromium being a necessary addition to improve the high temperature oxidation resistance. A recent search gave the following comparison between Invar 36 and Primus Metal.
@Twoberth Thanks Duncan for that insight. So, a step towards the final composition. Judging by the state of this example of a Primus cap (not Primus Metal) there was certainly a need to get the formulation right.
Interesting detail on the caps thinning in use and therefore getting hotter. I'll have to look more closely at any which glow and see if they have degraded in this way- a possible answer to why some combinations of inner and outer caps just won't work together?
I’d go along with that up to a point Chris except I’ve had that incompatibility occur even when there was no apparent damage to either component. Astonishingly, the cap above created a usable output and must have got on so well with its new buddy inner cap that it didn’t drift into underburn. By way of an experiment, I tested this unhappy pairing of caps to near destruction, opening the air screw only when something like a supernova threatened.
It's remarkable isn't it, that the flame front still stays the correct side of the cap despite the gaping holes! There still seems to be a large proportion of art as well as science to silent caps, as much as they have been researched and discussed on these pages it certainly feels some things will remain a mystery, like why two near-identical caps will just not work in combination. I wonder if the original designers and developers shared this opinion as they devoted a big portion of their working lives trying to create the optimum product.
I somehow doubt it, they had to achieve commercial success by producing tantrum-free stoves. Someone who by the calibre of their research and fabrication skills achieved that measure of manufacturing consistency in producing silent burner caps that simply worked is Gary Adams (‘Bernie Dawg’) and in THIS post on their design and manufacture he made this reply to someone who questioned the design. In his response Gary effectively lays to rest the ‘mystery’ reputation for cap behaviour. At the end of his comments he provides a link to his ‘Frankies …’ post giving more details of cap design.
Poor explanation of my thoughts on my part- I recall Gary's work in developing his caps and agree that there are very tight known formulas involved to stay within the laws of physics and produce a working cap. To me, the mystery is why for example I own a Burmos stove with what appears to be the original Burmos branded burner and outer cap, all in good order without any visible thinning or wastage and clean inside, but it will simply not work with any inner cap I own, glowing red and triggering underburn. Change to another, dimensionally similar outer cap with the same number and diameter of holes in the same places and it is an incredibly reliable stove. There must be a scientific reason for it, but I can't work it out! I was wondering whether this type of anomaly plagued the original developers in their quest for stability and reproducibility.
@Blackdog That Burmos cap example is peculiar for sure. A reason for it, as yet undiscovered as you say, yet you have evidently covered all the possible causes. Burmos stoves have a reputation for stress-cracked pump tubes,suggesting their grasp of metallurgy was shaky. I wonder, is it possible that your Burmos cap suffers from a metal composition fault, absorbing heat too readily? The company evidently dabbled with a better formulation as in this example nade of Burmos Metal.
