detecting winter rain gutter moisture?

I've got a downspout that dumps into an underground cistern.  Site issues during construction left the slope leading down to the pit a bit too high and a bit too shallow for the water to run effectively when it gets below freezing.  Basically, the line freezes up enough to start blocking water and then it backs up even more.
 
I'm looking to add a heater tape to it (they actually make such things).  I'd like to figure out a way to avoid running it just because it's below freezing outside.  I'd like to have it heat the line only when moisture is actually present.
 
The gutters are aluminum and the underground pipes are PVC.  I could potentially put a sensor anywhere along the length that's involved.
 
Any suggestions?

There are some capacitive water level sensors that you might be able to use.  They will work when mounted on the outside of a plastic water tank, so I'm thinking that maybe you could mount one on the outside of the PVC pipe, if that is where the backup starts.

I'm not sure if detecting it at the bottom is a good idea or not.  Because by the time it's pooled enough to be an issue there might already be freezing problems.  That and I'm not really looking to measure a 'level' of moisture but more like just the presence of it.  But I'm open to debate on how to best approach the problem.

Site issues during construction left the slope leading down to the pit a bit too high and a bit too shallow for the water to run effectively when it gets below freezing

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I had a similiar issue.  All of the infrastructure (small 100 home subdivision) planning though in our subdivision was done very mickey mouse willy nilly like.  The initial ideas  - methodologies were good.
 
I had a friend who was a land surveyor and personally bugged him about what I was dealing with prior to moving to the new subdivision.
 
That said over the years I regraded certain parts of it and redid the drainage.  Some major pieces related to having my landscaper bury drainage lines below frost lines.  One little project took him 2 weeks of digging (it was more than 200 feet and more than 5 feet deep in sections).  It was difficult due to the irrigation lines and my low voltage stuff etc already in place.  The initial "fix" was creating a large french well infrastructure; which did not work and was a waste of time and efforts and money.  It is though still being utilized for the gutter drains which go underground.
 
Might be easier to redo the infrastructure. 

pete_c said:

Might be easier to redo the infrastructure. 

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Yeah, I hear you.  But slope of the lot and the position of the various pieces makes it pretty much impossible to "fix".  They already put the cistern down at the very deepest the excavator would reach.  That and the slope of the lot ended up dropping more than the architect planned, leaving the bottom elbow from the downspout much lower than anticipated.  There's just enough slope to have the water run properly (and not backup from the cistern) provided it's not a torrential amount of flow.  But the pipe ends up being above the frost line.  A short cut may be to avoid running that line to the cistern entirely, and just use a pop-up drain across the lawn.  But it'll still end up likely having some freezing issues.
 
So my thought is to run some heater tape to help avoid the problem of it freezing and causing more blockage.  I'd just rather not have to manually interact with the tape or have it waste energy when it's not needed.

Yeah; what a PITA.  My sister's drains go into a cement slab and walkway adjacent to the house. 
 
Guessing then you cannot raise the bottom elbow; eh?
 
Here I have a wide, long and sloped driveway with one gutter drain in the middle which sometimes causing issues. 
 
Yeah the interaction of the sensing / heater tape and bringing electric there will a bit of a PITA.
 

 
Geez what a crazy morning...two online banking things and a splat of SQL logging errors in the last hour; then me telling CS just to write stuff on a piece of paper as I was doing in my home office....then while speaking to wife (at a bank) line dropped and got a real person message saying that there are phone problems...and concurrently wife got an message broadcast to everybody there relating to a state wide phone outage?
 
What the heck?

The elbow is only a few inches below grade.  Raising it would put it above ground, along with the lateral leading to the drainage cistern.  The pit and pipes needed to be deeper.  Or the drainage should have gone to a pit out in the front yard, but then THAT would've needed to be deeper too.   I can get electric to either the top end near the gutters (via the attic) or at the bottom from the adjacent main service and generator equipment.
 
So for now I'm entertaining the idea that if I keep the pipe warm it'll be able to drain effectively enough to be useful.  I'd just like to have a way to automate it in a way that avoids wasting more electricity than necessary.

So where are you hoping to detect moisture?  Is the idea to only have the heater on if there's a reason for water to be flowing into it?  
 
We're doing one now where a davis weather station detects wind to control aspects of a pool; I wonder if you could detect rain fall in a similar fashion; or put a moisture sensor in the pipes.  Or if you only needed to thaw in the event that it actually froze, then you could even put a sort of pressure sensor in the pipe at the bottom - if it froze and water accumulated in the pipe, it'd build pressure which you could use to kick on the heater and defrost the ice.

Can you use a water sensor to start a timer that if a freeze temperature sensor activates at the same time water is present then start the heater? Then you can shut it off if freeze sensor is off more than 15 minutes or water timer shuts off.

The timer for the water should be so that any water triggering it has enough time to drain through your system.

Hope I explained what I mean correctly.

A few years back after a long cold winter (with much snow and ice) I noticed a problem relating to an upper gutter drain to a lower roof section.  It related also to the position of the roof and the sun during the day.  It was a corner lower section of the roof with the gutter drain right in the corner to the lower roof.  Over the winter a small pond of snow and ice had formed building up and sitting for long periods of time.  It was correctly done with the right flashing (it was a combo of brick / cement board).  I ended up working with a roofing company to redesign the drainage of the upper gutter removing and changing the pitch of the gutter and capping the old gutter drain and moving the drain to the other side.  It was a pita to do because it was a pitched roof on another pitched roof where the roofer had to be on the pitched roof with a ladder working on the upper roof.  Today I do not notice the change in the position of gutter drain; it is the same color as the trim on the house and the problem is gone.
 
I know I am back to the fixing of the infrastructure rather than a automating a bandaid solution. 
 
In the 90's had a building which I had to keep heat tape on few water pipes after a cold snap which burst the pipes and water dumped into a parking lot turning it into a lake of sorts.  I did end up just leaving the water flowing to a trickle in the area.  One way or another it wasn't automated back then and a real PITA to have to pay attention to the fix (so automating it would have been a neato thing).

Yes, I was thinking along those lines.  Detect that water is present, confirm the outside temp is low enough to warrant using the heat and then turn the tape on.  For how long is a good question.  I'd want to avoid a race condition between detection and switch activation.
 
The main problem I have is that water along the bottom section freezes up and blocks the line.  This then leads to water backing up into the downspout where it also freezes.  Eventually this leads to most of the upward section being frozen, and that causes overflow problems at the top (something to avoid due for a whole other set of issues).  So I'm not entirely sure where would be best to detect the water.
 
I don't have a 3rd party automation system active yet, that's a decision long overdue.  Between now and the winter I'll have to choose something.  But meanwhile I'll have to get going on putting AC wiring in to accommodate connecting the tape.  
 
It may be more useful to have it set up without detection and automation until we actually get a situation requiring the heat.  Then I'll be able to get some real-world numbers on the duration necessary to clear the line from a blockage.  

I think the tricky part with this problem is reliably detecting when moisture or ice is present at the same time the temperature is near/below freezing.   A normal water/flood detector like you would use in a basement doesn't work particularly well with ice, which led me to suggest the capacitive detector.   But unfortunately, that won't work on an aluminum gutter.
 
My gutters often have small amounts of water in them for days, even after it stops raining or all the snow has melted.   The last bit of water doesn't drain out completely, and with the cold temperatures, is slow to evaporate.   So what I'd really want is something that can detect more than just a trace of moisture.
 
In searching around, I did come across this gutter ice sensor.  It is designed to work with this rather expensive controller.
 
Sensor data sheet:  http://networketi.com/wp/wp-content/uploads/ds_git119037A.pdf
Controller data sheet: http://networketi.com/wp/wp-content/uploads/ds_aps3c_22687B.pdf
 
The data sheet for the sensor says it has a heated moisture sensing grid, so that would solve the problem of a normal sensor not being able to detect ice.  It has only a 3 wire interface, so it looks like it might be easy to use with something other than the APS-3C controller.

I think that once the temperature goes above 35 to 40 and its daylight out that you could shut the heater off.  Probably have to turn it on in the evening again.
 
Most heater cables can be run down into the downspout and in your case the pipe leading away. 
 
I wonder if you could use a photo transistor and an LED on opposite sides of the pipe.  If you loose the path then its clogged or frozen.  Probably need two pairs to minimize false trips from dirt accumulation and clean it once or twice a year. 

Digger said:

I think that once the temperature goes above 35 to 40 and its daylight out that you could shut the heater off.  Probably have to turn it on in the evening again.

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Depending on how frozen the ground is, and where you are measuring the temperature, you could still have problems.  You can have air temps well above freezing, but the ground will remain frozen, especially if there is a good layer of snow on the ground.

Excellent point regarding ground temp vs air.  Since my problem basically starts with a backup underground (due to freezing) I was thinking I'd have to focus some monitoring in that area.  
 
I think I'm heading toward rigging up an in-ground box, like one of these:
http://www.amazon.com/gp/product/B003MZ1EX2

 That'd give me some options on maintaining access to a section of the pipe likely to be involved in the problem.  PVC is pretty easy to work with so I could probably experiment with a couple of different ways to monitor it.
 
It does seem like a combination of a photo-diode & LED and temperature would give the best range of options.  A sensor pair across the top portion of the 4" pipe might provide an effective area to watch for obstructions.  That, combined with a temp sensor would let me better choose when the tape needed to be activated.  As in, a torrential rain in non-freezing weather wouldn't be an issue.  Nor, perhaps, would a heavy rain even if it was below freezing.  But a blockage for 'period of time' and also during sub-freezing temps would.
 
Then again, K.I.S.S. is certainly worth remembering.  
 
I think I'll have the gutter guy come out to talk with me about options.  It's possible there's another way to approach this, including, perhaps rearranging where the downspouts descend from the roof.  I can't easily correct the in-ground situation, but I'd be willing to entertain re-jiggering the gutter slopes and such...