Please help me to calculate CR for this trap

[Jon_Dahl]

Acid Pit Trap

CR ?; mechanical, location trigger; manual reset; DC 20 Reflex save avoids; 20 ft. deep (no damage from falling; special); Special: 10 ft. deep vat of acid (total immersion from falling; 10d6 damage per round). Search DC 21; Disable Device DC 25.

"Acid vat" is identical with the one mention here: http://www.d20srd.org/srd/environment.htm#acidEffects
Falling in to acid is identical as falling into water: http://www.d20srd.org/srd/environment.htm#falling

If someone could give me a solid answer I'd be happy.

Here's my opinion:
The average damage of the acid per round is 32 which is divided by 7 = 4.5 (rounded down). Disable device DC is so high, that it add +1 to the CR. The base CR is 1. So the CR is 1 + 4 +1 = 6.

[Diarmuid]

I think your CR6 is way off.

Look at another CR6 trap:

Lightning Bolt Trap: CR 6; magic device; proximity trigger (alarm); automatic reset; spell effect (lightning bolt, 10th-level wizard, 10d6 electricity, DC 14 Reflex save half damage); Search DC 28; Disable Device DC 28.

This one does 10d6 one time with a DC14 Reflex save for half, but has higher search/disable checks due to being a magical trap.

Now lets look at a CR7 trap:

Acid Fog Trap: CR 7; magic device; proximity trigger (alarm); automatic reset; spell effect (acid fog, 11th-level wizard, 2d6/round acid for 11 rounds); Search DC 31; Disable Device DC 31

Again much higher search/disable,. but we're only talking 2d6 dmg per round, and any creature should be able to get out in a maximum of 2 rounds.

The closest thing to yours from a mechanical perspective is this CR9 trap:

Wide-Mouth Pit Trap: CR 9; mechanical; location trigger; manual reset; DC 25 Reflex save avoids; 100 ft. deep (10d6, fall); multiple targets (all targets within a 10-ft.-by-10-ft. area); Search DC 25; Disable Device DC 25

Now, this is a slightly higher search DC, same DD DC, same damage, but higher Reflex DC...but the damage only happens once, but it's a 10x10 pit.

I'd probably put yours at CR9 from the comparables. It's really the 10d6 per round, no save that's doing it.

[Jon_Dahl]

Well I don't see it in the same way as you do, because your answer doesn't follow the internal logic of D&D.

Comparing magic traps with the mechanical ones is hard, since the magic trap CR is basically a flat-out 1+spell level (unless the damage is very high) but mechanical traps require a bit more calculation.

Also for some strange reason, mechanical trap CR is simply calculated by damage per round and it doesn't matter if it's round-to-round or a single shot.

However your analogy with acid fog is smart. But you still have to look that it's an area-effect (taking out possible numerous intruders) whereas the acid pit trap is a single-intruder trap. Comparison is hard... Persisting area effect with low damage vs. persisting single-creature trap with high damage...

If you wish to compare this trap to magic traps, I'm 100% that this is the best one:
Blade Barrier Trap
CR 7; magic device; proximity trigger (alarm); automatic reset; spell effect (blade barrier, 11th-level cleric, 11d6 slashing, DC 19 Reflex save half damage); Search DC 31; Disable Device DC 31. Cost: 33,000 gp, 2,640 XP.

- Almost the same reflex save DC (19 vs. 20).
- Almost the same damage, which persists (11d6 slashing for 11 minutes and as long you stay in the area vs. 10d6 acid for as long you stay in the pit).
- Blade barrier has much higher search and disable device DCs and also it's an automatic reset trap. Therefore actually acid pit trap should have a lower CR...

Just consider this:
10 ft. deep pit trap with perfectly smooth, flat, vertical walls. In the bottom there is a Blade Barrier-trap. How does it differ from this trap? This sort of trap would be a combination of CR2 deeper pit trap + CR7 blade barrier trap.

[teslas]

It matters because combining a blade barrier trap with that pit and those walls you mentioned would up its CR.

[Jeraa]

THe average damage of 10d6 is 35, not 32. Thats a base CR of 5. Disable Device DC is +1, and a liquid* trap adds another +5. Thats CR 11. The CR should probably be higher, as its damaging every round. But the rules don't account for that. I assume that the pit is only 5' wide. The 10' wide pit traps are multi-target traps (first targets in each of 2 adjacent 5' squares), which would add another +1 to the CR. You could also probably tack on an additional +1 to +3, as the fumes coming off a large body of acid are considered an inhaled poison.

*Liquid: Any trap that involves a danger of drowning (such as a locked room filling with water or a patch of quicksand that characters can fall into) is in this category. Traps employing liquid usually have the never miss and onset delay features (see below).


Water-Filled Room: CR 7; mechanical; location trigger; manual reset; multiple targets (all targets in a 10-ft.-by-10-ft. room); never miss; onset delay (3 rounds); water; Search DC 20; Disable Device DC 25. Market Price: 21,000 gp.

Thats a trap in the book close to yours. Remove the onset delay (+1 CR), the never miss quality, the multiple targets (+0 or +1 CR). That drops the CR down to 5 or 6. Your trap does an average of 35 damage, so that adds 5 to the CR. The CR is 10 or 11. (Should be more, as the damage is every round, but the rules don't seem to care about that.)

[Jon_Dahl]

THe average damage of 10d6 is 35, not 32. Thats a base CR of 5. Disable Device DC is +1, and a liquid* trap adds another +5. Thats CR 11. The CR should probably be higher, as its damaging every round. But the rules don't account for that.

*Liquid: Any trap that involves a danger of drowning (such as a locked room filling with water or a patch of quicksand that characters can fall into) is in this category. Traps employing liquid usually have the never miss and onset delay features (see below).



Thats a trap in the book close to yours. Remove the onset delay (+1 CR), the never miss quality, the multiple targets (+0 or +1 CR). That drops the CR down to 5 or 6. Your trap does an average of 35 damage, so that adds 5 to the CR. The CR is 10 or 11. (Should be more, as the damage is every round, but the rules don't seem to care about that.)



Thank you, a very good answer. However, the water filled room is actually completely filled with water, thus forcing drowning. In the case of acid vat, you only need swim DC 6 to avoid drowning.


Calm water 10
If you fail by 5 or more, you go underwater.

You make an excellent point Jeraa, but I would feel that I'm giving out too much XP if you count that as a "liquid"-trap.


Liquid

Any trap that involves a danger of drowning is in this category. Traps employing liquid usually have the never miss and onset delay features (see below).

Yes, sure it involves risk of drowning, but really... You are technically right but I simply don't want to see it that way.

[Jon_Dahl]

It matters because combining a blade barrier trap with that pit and those walls you mentioned would up its CR.

Well, technically no. At least as far I know. But I do agree with you, but logically it couldn't up it more than +1.

Also consider this:
20 ft. deep pit trap with a fireball trap in the bottom. The floor is the touch trigger. It's an automatic reset trap, so it could fireball the intruder endlessly every round for 8d6 (reflex save half). It would be a CR 4, maybe 5 trap.

D&D traps are sometimes really confounding...

[Jeraa]

Thank you, a very good answer. However, the water filled room is actually completely filled with water, thus forcing drowning. In the case of acid vat, you only need swim DC 6 to avoid drowning.

A water filled room is only an example of a liquid trap. So is quicksand, which doesn't force drowning. Unlike the movies, quicksand doesn't drag you under. You can't even sink into it completely (its rather dense, so easy to float in). IF that counts as a liquid trap, then so does 10' of acid.


Yes, sure it involves risk of drowning, but really... You are technically right but I simply don't want to see it that way.

Doesn't matter how you want to see it. You asked for help, and we helped. The rules say its a CR 11 trap, at least.

Your trap would kill your typical 6th level fighter in 2 rounds (average hitpoints, 18 constitution = 61hp). It automatically kills a wizard or rogue character, if they fall in. There is no way this is a CR 6 trap*. Remember that a CR equal to your level is only supposed to take about 20% of your resources (like hit points) to overcome.

*I know. There are CR 6 10d6 damage traps in the book. Any trap that can automatically kill half the party (including killing the average 6th level wizard, who would usually only have 16hp, even if he succeeds on the Reflex save for half damage), and seriously injury the rest of a 6th level party with its average damage is not a CR 6, despite what the book says.

[Jon_Dahl]

Ok, I do agree with you. Rules aside, in your opinion Jeraa, what would be a fair CR for this trap? The pit is only 5' wide btw.

Edit: I decided to bypass the "liquid"-factor by the fact that the acid is only 7 ft. deep. Thus basically all characters can jump from the bottom (DC 0) to reach surface and breathe (round to round). Thanks, it was good that you pointed this out!

[Acanous]

you could just make it 5 feet of acid. Then they can breathe while standing.
I'd peg this trap as CR9 as it stands. Any higher and your Search/DD checks need to be a lot higher, any lower and your damage needs to scale down.