How to know when fresh water tank is full?

[uwskier]

I use that water meter exactly the same way. Only I put an anti-backflow adapter between it and the black tank flush hose. Want to make sure nothing in the black tank flush hose goes backwards into the meter.

There should already be an anti-siphon valve or some form of siphon break on that circuit I thought.

[jguzek]

I also have a flow meter on both the fresh water inlet and the black tank flush. Works great.

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[ajg617]

I used a meter for the spring flush of the fresh water tank. Capacity is 81 gallons and meter showed 78 when it started coming out the overflow. 2/3 light came on at 55 gallons. Pretty close. I don't think the meter is meant for potable water though if you use what's in your tank for that. Garden hose variety.

[more tired since retired]

There should already be an anti-siphon valve or some form of siphon break on that circuit I thought.

My understanding is there is an anti-siphon value with the black tank flush, but if it failed (which happens) I may or may not pick up on it. Adding an anti-siphon valve at the meter is safety backup.

[IBEW Sparky]

My understanding is there is an anti-siphon value with the black tank flush, but if it failed (which happens) I may or may not pick up on it. Adding an anti-siphon valve at the meter is safety backup.

You are correct, as someone else mentioned there is already an anti-siphon valve and it's usually located higher up in a wall with plumbing access, usually behind the shower.
It's located higher for multiple reasons one of which is in case of valve failure the water that remains in the line drains down both ways. Since you need some way of shutting off the water supply to the hose feeding the black tank flushing port there really isn't any way of cross contaminating the fresh water supply. Adding an additional backflow prevention device will only serve to slightly restrict the water pressure going into the black tank, which will intern reduce the effectiveness of the tank rinse.

[pwalton]

So ... my memory served me wrong (as usual) and y'all were right. The port on the camp side is a gravity fill. The campground I stayed at this past weekend did not have a potable fill near the dump station (like most I've been too) but potable water fills throughout the camp where you had to stop in front of other campers while you filled ... weird. Since I felt like I was interrupting someone's dinner I was in a hurry and forgot to use the new water flow meter! But ... I now know where the overflow is ... yay!

First "real" trip in the books and it can marked as a success!!

At the completion of the weekend my husband even towed it from the camping spot to the dump station which was his first time driving a trailer ... ... he did great, but it was like being in the passenger seat with a 16 year old at the helm again!

[sande005]

Late to this thread, but I'll share yesterdays story. Note that I have never seen this before with either my old Jayco Fifth, or my current Imagine. But the internet tells me it can be a "thing" that many encounter (even Grand Design):

We partially filled my buddy's new trailer for the first time (Some Other Brand). Added bleach, continued filling and shut it off after it came out of the overflow. Ran the bleach water out of all the faucets, etc. Hmmm...a steady stream still coming out of the overflow. Tank read full. Watched it for over 20 min. and it still kept flowing, and the tank still read full. What???? Finally it stopped. On a whim, I started fulling 1 gal buckets in the kitchen sink, until the pump could pump no more. Only 4 gallons, from a 45 gallon tank! OMG.
Apparently we had overfilled the tank (faster in than it could go out), and the overflow turned into a siphon. Then as it drained, outside air pressure was enough to collapse the walls of the tank to keep the flow going. Not till the collapse is enough that the tank resists and pulls air back in will it stop. Buddy says today at the dealer that there is no serious damage, but was strongly cautioned not to use the overflow as the gauge to when it was full. Which leaves watching the idiot lights, and running to the hose bib when it gets close.

I think I'll buy a garden hose flow meter at the local hardware store, to measure what goes in. Even if they tend to not last very long.

As I said, not an issue with the trailers that I have owned...but at least something to watch for. There are bazillions of discussions about possible modifications to prevent this, but all involve dropping the bottom and a moderate bit of replumbing.

[pwalton]

Late to this thread, but I'll share yesterdays story.

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Apparently we had overfilled the tank (faster in than it could go out), and the overflow turned into a siphon. Then as it drained, outside air pressure was enough to collapse the walls of the tank to keep the flow going. Not till the collapse is enough that the tank resists and pulls air back in will it stop. Buddy says today at the dealer that there is no serious damage, but was strongly cautioned not to use the overflow as the gauge to when it was full. Which leaves watching the idiot lights, and running to the hose bib when it gets close.

.

I wouldn't think this would be an issue when filling via gravity fill ... because the air pressure would always be equal inside and outside the tank (definitely not an air tight seal when shoving a hose into a hole). Maybe a safety would be to remove the gravity fill cap even if filling from the dock in the storage area? Thanks something else I have to think about!

And now I'm imagining how the tank / tank fill system is constructed just to figure out how something so simple could end up being so complicated! XD

[sande005]

I wouldn't think this would be an issue when filling via gravity fill ... because the air pressure would always be equal inside and outside the tank (definitely not an air tight seal when shoving a hose into a hole). Maybe a safety would be to remove the gravity fill cap even if filling from the dock in the storage area? Thanks something else I have to think about!

And now I'm imagining how the tank / tank fill system is constructed just to figure out how something so simple could end up being so complicated! XD

From my internet searching, I think the answer is "it depends". Manufacturers have 2 or 3 ports on the tank - a vent near (or on) the top that is also the overflow, a low down output line (to the pump), and sometimes an input (gravity or not). Some don't have the last, and just Tee into the lower output - so the tank fills from the bottom. Filling from the top seems to allow air to get into the tank when disconnected, so less of a problem of getting a siphon going. Filling from the bottom doesn't have that advantage.

[ajg617]

I was really surprised at the dribble coming out of the overflow when I filled the tank to flush it. Took forever to empty with the fresh water dump valve open also. I expected the water to pour out. It didn't.