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[Gus Smedstad]

I have a couple of wells with reservoirs beneath them in my fortress.  They're fed by muddy ponds acting as cisterns.  Naturally, there's a problem with limiting the amount of water that flows into them, since left uncontrolled they'll overflow.

I can of course check the water levels manually and open the floodgates just long enough to let in the right amount of water, but where's the fun in that?  I set up a system where a pressure plate inside the reservoir would activate a drawbridge inside the water tunnel, blocking the flow of water.  The plate was set to trigger on 5-7 units of water.

The problem is, it worked once, and then stopped.  When the water level dropped it lowered the gate, but the next time it rose it didn't function.  Since I'd left the floodgate open, it ended up seriously flooding the jail, washing a prisoner and several food barrels into the reservoir, and drowning him.  To think the well was supposed to solve the last problem, where a prisoner died of thirst even though I had a well nearby and a tiny drink stockpile within reach.

Anyway, why didn't this setup work the second time?  Why won't the bridge rise now?

 - Gus

[Sphalerite]

Pressure plates can be tricky sometimes.  They don't send a continuous 'on' or 'off' signal all the time, but only when they change state according the the conditions they've been built to register.  When a bridge gets a signal from a pressure plate, it takes 100 ticks to raise or lower, and during that delay time any other signals that get sent to the bridge are ignored.  So if a pressure plate is sending fluctuating 'on' and 'off' signals (which can easily happen if the plate is in flowing water) it's possible that the signal to lower the bridge is sent, then the signal to raise the bridge is missed because the bridge was changing state.

This is why I don't use pressure plates to control water levels, and when I do have to I always have a backup lever to manually control the water flow.

[LordSlowpoke]

You need to make an automated way to stop overflows.
Hmm.
Here's an idea - connect the pressure plate to a floor hatch. That floor hatch will open when an overflow appears and dump the excess water right into some noble's room, caverns, obsidian farm, or whatever you want.
And your bridge problem appeared because bridges can't retract underwater/when submerged in magma.

[Sphalerite]

And your bridge problem appeared because bridges can't retract underwater/when submerged in magma.

Of course they can, what makes you think they can't?  I've used retracting bridges under water many times.

[LordSlowpoke]

And your bridge problem appeared because bridges can't retract underwater/when submerged in magma.

Of course they can, what makes you think they can't?  I've used retracting bridges under water many times.

Might be the fact that mine didn't even budge when they were underwater.

[xtank5]

If you use doors there won't be a delay.  That way you can use pressure plates to control the water flow. 

[Sphalerite]

Might be the fact that mine didn't even budge when they were underwater.

Then you had something wrong in the hooking up of the bridges, or the triggering mechanisms.  Water doesn't stop bridges from raising or retracting.

[lanceleoghauni]

I second the doors, I've found for small areas they're very effective.

[Gus Smedstad]

I used a bridge because the "on" signal meant "closed" in this case.  How does that work with a door?  Doesn't "on" mean open, like with floodgates?

 - Gus

[Sphalerite]

I used a bridge because the "on" signal meant "closed" in this case.  How does that work with a door?  Doesn't "on" mean open, like with floodgates?

 - Gus

Yep, 'on' means open with doors.  What you do is set the pressure plate to be on when the water is below, say, 4.  Then the door should close when the water rises to that level.  You will also need a lever to open the door to let water in to start the system.

[Gus Smedstad]

So, I assume the reason to recommend a door instead of a floodgate is that the door responds faster.  I've tried the 0-4 sensor + lever approach with floodgates, and the result was unreliable.  I thought it was because of the nature of the "on" state, which is why I tried the bridge instead.

 - Gus

[Marconius]

I've been setting up similar systems, based on info from the wiki.

Generally I set my pressure plate to trigger 0-4 and reset. Then I build a floodgate, connect it to the plate and a lever (in a safe place). After that I pull the lever to open the floodgate, then remove the lever. Once this is done, I let the water in (usually this involves either digging out a final square or opening a different floodgate). The water flows in and once there's 5 on the plate, the floodgate closes.

However, last time the water actually went under 5, the gate didn't open and refill automatically... even though it had done so in a previous case. I'm honestly not sure why, it's possible some debris or some plants somehow buggered my pressure plate.

[Altaree]

any reason there isn't a a diagonal pressure relief valve between you wells and the water source?