okay, I'm trying to design a trap for a modern/futuristic setting that implements a room that gradually gets colder and colder and colder. I've got a good idea of stats, but I have little idea of how to rate its CR.

here's what I've got

Freezing Room Trap CR 12
Type mechanical; Perception DC 25; Disable Device DC 30;
EFFECTS
Trigger location Reset automatic reset
Effect onset delay (1 round); (1d6 cold damage, plus an extra 1d6 cold damage every additional round to a max of 10d6 cold damage by round 10); never miss; multiple targets (all targets in 30-ft. square chamber)

...and here's the break down of it's CR

Percep 25 CR+1
DisDvc 30 CR+3
Auto reset CR +1

...where i get tied is the damage. Should I just use the max damage as its calculating factor?

Dmg 10d6 (average of 35); multiple targets (35 x 2 = 70; 70/10 = 7)
CR +7

total CR 12?

does that look about right? thanks in advance for any help.

If you have it, check out Frostburn it has a trap just like this.

At most, I would use the average damage, since they targets only take the maximum damage once. I say at most, because it's likely that the party will leave the room (if they can) once they realize that the damage is getting worse as time passes. However, the rules as state that, if the damage is repeated over multiple rounds, that the average damage should be multiplied by the average number of rounds. Converting that to numbers:

(35+3.5)/2 * 2 * 10 = 385, or CR +38.

I think you also missed the +2 since it never misses, but if the +38 is at all accurate, that's not going to matter much. :-P

On one hand, a CR in the 40s sounds reasonable, since 385 damage will kill just about any 20th level character if they stay the full 10 rounds, although I'd expect various cold protections and cure spells would be cast quickly to mitigate the effect. OTOH, once it's clear that the room is getting colder, I'd expect the party to retreat before they take the full damage. Based on that, I'd calculate the damage for a shorter duration. If you assume they'll only stay for 6 rounds, the total damage is

(21+3.5)/2 * 2 * 6 = 147, or CR +14

Or 3 rounds, for which

(10.5 + 3.5)/2 * 2 * 3 = 42, or CR +4

I feel like I'd go with +6-8 on the CR. After three rounds, it might not be clear that the damage is increasing, especially if you low roll on the extra dice. There should probably be at least a perception check to allow them to notice that the room is getting colder.

Part of the problem is that the damage dealt if roughly proportional to the square of the time spent in the trap, so it quickly gets worse if they don't leave.

On a related note, if they can't leave and don't have access to protection from cold spells, the trap is much more dangerous.

(21+3.5)/2 * 2 * 6 = 147, or CR +14

Or 3 rounds, for which

(10.5 + 3.5)/2 * 2 * 3 = 42, or CR +4

I feel like I'd go with +6-8 on the CR. After three rounds, it might not be clear that the damage is increasing, especially if you low roll on the extra dice. There should probably be at least a perception check to allow them to notice that the room is getting colder.

Part of the problem is that the damage dealt if roughly proportional to the square of the time spent in the trap, so it quickly gets worse if they don't leave.

On a related note, if they can't leave and don't have access to protection from cold spells, the trap is much more dangerous.

so lets see that's (3.5 + 7 + 10.5 + 14 + 17.5 = 52.5) * 2 = CR + 10

CR 7 + 10 = 17 hmmm...not significant enough for a CR 17 trap.

I like your original CR 12 estimate, this is not CR 17 or CR 40. For a CR 12 trap it is easily detectable by a Rogue at that level and disable device isn't all that high either so the potential high damage output is mitigated by those factors.

So keep it at CR 12 and when you are done go to Kobold Quarterly and submit it as a Tuesday trap!

I like your original CR 12 estimate, this is not CR 17 or CR 40. For a CR 12 trap it is easily detectable by a Rogue at that level and disable device isn't all that high either so the potential high damage output is mitigated by those factors.

So keep it at CR 12 and when you are done go to Kobold Quarterly and submit it as a Tuesday trap!

The average damage over 10 rounds is 195, average damage per round would then be 20 points which doubled is 40 which is a CR 4 modifier.

That and the CR 5 modifier due to automatic reset + perception etc gives you a CR 9.

One other element to consider is how long it will take the rogue (or any one with disable device, as it is mechanical) to disable, anywhere from 3 to 9 rounds. 1 round to find the mechanism and 2 to 8 to actually disable. Assuming they don't suspect a trap until they get hit by it once that's 4 to 10 rounds of exposure if they stick it out to disable. Average time in the trap would come out to 5 or 6 rounds in my view.

Should this count as ongoing damage? I ask because it impacts the spell casters ability to get critical protection spells up.

I guess in my mind it comes down to if the PCs can openly leave the trap area . If they can a lower CR is reasonable, but if they are locked in the danger jumps up.