Actually, most of your results seem to suggest a 5* trinket is closer to a medium UV than a high; in fact, I suspect it's exactly a medium. There's one or two tests where a 5* was a half bar off from a medium, but since damage is probably tracked by the game in increments smaller than half-bars, that could easily be round-off error. I know I've had experiences in game where I didn't appear to take exactly the same damage from different ticks of the same burn.

Also: you list a "very high" bonus in several places, but it was my understanding that armor UVs didn't use that term. Is that a "maximum", or a combination of UVs, or what?

Based on checking a few arbitrary data points from your list, if we assume the numbers may be slightly off due to rounding, it looks like fire resistance probably causes exponential decay in the amount of damage you take. Going from "no resist" (13) to one volcanic piece (8) stops roughly 40% of the damage; if it's exponential, we'd predict two volcanic pieces to take about 4.7 damage. You list 4.5; that's equal within the available precision.

I suspect you could get better data if you allowed yourself to take burn damage several times in a row (without healing) and averaged the results. I hate to ask you to do more work after you've given us so much awesome data, but...I don't have the equipment to replicate your tests.

Also, is there any difference in the fire duration when you vary your resistance? I know there were some curse tests that said curse resistance affected the duration AND the number of items affected, so it seems plausible that other resistances could have multiple effects.

I got about 100 fov self inflicted fire uses at depth 28 (vanaduke) wearing Divine Veil and Vog Coat.
I started at full health, and healed using the healing pads, but never healed to full health so as not to cap whatever rounding value was accumulated so far.

We have to assume the healing pads always heal the same amount per tick: h.
We have to assume fov always does the same amount of damage to yourself: d. But I never paid attention if it always does the same number of ticks, so decide which is constant, the tick or the full set of damage.
We have to assume each full pip of health has the same value, which may mean the last one has less health than a full pip: p.

I have it recorded but I'm sure as hell I'm not analyzing it :P send me mail if you want the video to analyze yourself, it will be huge of course, you can tell me the encoding parameters you can live with so I reduce its size.

What to do:
Lets say I start with 28pips of health, then your health: hp is 27p < hp <= 28p or 27p+k

Analyzing the video you can define valid ranges for d, ie 4p < d <= 5p

h seems to always regenerate 3 pips, never seen it doing a fraction, as such it equals 3p. You can sort of validate now by making sure it never healed any different than 3p.

You may assume k is integer and not float. And maybe you can figure out how much health a pip has ;P

While not necessarily useful to your research I would like to point out something.

It's my belief that certain attacks by enemies have a different value of application, and that your resistance is both a resistance to application AND damage. When I do FSC, I carry the full divine set + a 5* fire pendant and certain attacks are completely unable to burn me (lava blocks, flame thrower puppies, zombies, vanaduke smash) while others are still able to burn me (oiler jelly puddles, flame vents) but when they do burn me, it was for an extremely short duration (it looked like 3 seconds initially, almost instantly ticking down to 2) I think part of the problem you guys may be having figuring this out is that not all applications of fire are equal; You'll need to find what appears to be the strongest source of flame application (say flame vents) and do the tests based off of those if you are searching for a perfect 'immunity' to flame. Seeing as I lack the tokens to get a second 5* pendant atm, I can't test divine + dual pendant so I can't help in that.

One other thing to consider, is I think that shields that list themselves as being naturally max against a condition (owlite) may actually have just infinite status resistance, because whenever flame zombies breath fire at me, they have a tendency to get bugged and breath it forever if I'm shielding, but my shield shows no signs of taking damage. I had a party stop attacking just to stare at me getting breathed on by 3 zombies and the zombies just kept on going never stopping, but neither my shield or I took any apparent damage.

Tantarian said his tests were performed at depth 23 (a clockwork terminal), so I assume they all involved self-igniting with the Fang of Vog (which might very well be the strongest source of fire in the game).

I'm sure that resistance to a given status has more than one effect. Tests with Curse resistance earlier in this thread indicate that it at least affects duration AND the number of items afflicted; I wouldn't be the least bit surprised if it also affects damage received and/or the probability of being cursed. These may or may not all follow a single numerical pattern.

There's another possible theory that I'm hesitant to suggest, because it seems excessively complicated and difficult to test, but it would explain the "plateaus" in Tantarian's tests (where damage doesn't seem to change across several resistance values) and also the "immunity" people report in some cases (e.g. to status bombs in Lockdown). It's that each status actually has several manually-defined "tiers" of power (perhaps corresponding to tooltip descriptions "minor", "moderate", "strong", etc.) and resistance causes the strength of a status you receive to be downgraded to a lower tier (perhaps even with randomized rounding or something).

If the status is weak enough, high resistance can downgrade it to tier 0, and you get to ignore it entirely (the status bombs all cause "minor" versions of statuses, so they'd be the first things you'd gain immunity against in Lockdown), but you'd still be partially vulnerable to stronger versions of the status. For example, if your resistance reduces fire by "three tiers", then when you get hit with a tier 2 fire attack, you're immune, but when you get hit with a tier 9 fire attack, you still suffer a tier 6 effect.

But the tiers wouldn't necessarily be linear; that is, tier 6 isn't necessarily 2/3 as strong as tier 9. They could follow whatever pattern the designers decided...for example, maybe each higher tier does exponentially more damage! That would explain why resistance seems to give exponential protection in Tantarian's tests, but "rounded off" to some threshold.

But if we're assuming "manually-defined tiers", then they could "just happen" to follow virtually any other pattern we could think of, so in order to have any chance of ruling them out we'd need to test several different status sources (with different strengths) AND several different resistance levels--enough that the "low resist vs. weak status" range of effectiveness overlaps with the "high resist vs. strong status" one.

And there's lots of possible variations on that idea. Maybe the number of tiers reduced isn't linear with the amount of resistance you have! Or maybe damage, duration, and other variables all follow completely separate systems! Or it could be that the total damage you take over the entire duration of fire follows some straightforward pattern, and then duration and damage/tick are fudged to make that work out. We need to step back and apply Occam's Razor somewhere, but I'm honestly not sure where.

Remember that damage is (we think) tracked in increments smaller than what the UI displays, so Tantarian's numbers are only approximate. Getting a 5 instead of 4.5 would not be conclusive.

A friend confirmed for me that equipping one piece of Mad Bomber gear plus a fire resist trinket caused their overall fire resistance/weakness bar to disappear, indicating that they exactly canceled out (or at least the UI thinks so). Haven't tested with a medium UV.

If we assume that the bar lengths are accurate, defense (not status resist) UVs are linear (with Max = 4*Low), and a medium is slightly less than a trinket. See here. I haven't measured resistance bars in the same way, though; anyone feeling inspired?

@Volebamus Thanks for that - I guess some of Tantarian numbers just confuse me now but at least that seems to confirm Wetstone is Med and Med is half of Max.

I'd still like to know why Tantarian numbers seem weird though maybe it's just that low is weird (not quite half of med) and wherever he's using low we get those strange numbers?

Also I wouldn't say that having bonus beyond a single Vog piece is worthless - while the damage decrease is minimal from that point I know that you gain immunity to the lower chances of fire at Vog + Vog + Wetstone. For example the zombie swing (Maybe a fair chance?) can no longer light you on fire at that point but can when just Vog + Vog. Decreasing chances is probably more important than decreasing damage anyway.

I might go buy a few magic pieces (assuming 2* resist is the same as low) and do some testing with a Fang of Vog myself with probabilities. Could someone confirm that resist from 2* gear is the same as a low UV? That would help a lot.