Thanks to a previous post I read I decided to perform my own experiment this last weekend.
It seems that our black tank would always show full by about the second day of camping, after blaming the wife and mother-in-law for camping out in the small room I finally decided to get an actual measurement of the tanks capacity.
The first thing I did was measured the amount of water the toilet bowl held per flush (about 1/2 gallon).
Next I started dumping and counting 5 gallon pails of water into the toilet to see when, when was really when.
At 30 gallons the tank indicators said "no more." Mind you the specifications on my trailer said I had a 40 gallon black tank, so on I went. I had dumped 50 gallons in the tank and still didn't see any water at the bottom of the horizontal pipe.
Concerned I'd have a puddle outside somewhere, didn't have, I decided to leave well enough alone in the knowledge that the trailer met and exceeded specifications.
Oh and I am leaving it my secret as far as the wife and Mom-in-law go.
If you go underneath the RV and look at where the sensors are mounted yo will probably find the upper sensor which is the "Full" sensor just over half way up the tank. Every RV I have looked at from underneath I found this to be true, so they show full when really you are only just over half full. Even thou DW says I'm full.
Papa Bob
1* DW "Granny"
1* 2008 Brookside by Sunnybrook 32'
1* 2002 F250 Super Duty 7.3L PSD
Husky 16K hitch, Tekonsha P3,
Firestone Ride Rite Air Springs, Trailair Equa-Flex, Champion C46540
"A bad day camping is better than a good day at work!"
The only really PRACTICAL use for tank sensors is to give you a little time to LEARN how to tell whee your tanks are at. If you DEPEND on them you'll be in for a surprise sooner or later.
2004 F-250 SCREW Long Bed (new)
OR 2004 F-150 HD (85,000 towing miles) Rockwood 8314SS 34' travel trailer
We have enough YOUTH...how about a fountain of SMART
Is it all possible that the capacity of the tank is different from what it will actually hold for good reason?
Like propane cylinders not being filled all the way so they don't vent if you get them warmer than the climate in which they were filled, perhaps the manufacturer rates the tanks and installs the sensors so there is some overhead.
So if you drive with full tanks, the sloshing can be absorbed by the air on top, rather than bursting the floor flange and launching the toilet?
I suppose if you know you're going to dump on the way out of the campground, then that extra margin is well worth utilizing.
OP, did you calibrate the gauges while you were at it? On ours each indicator has a small adjustable "pot" just above each switch. I did ours when we first got the rig.
Our sensors are external to the tank and seem to be pretty accurate once adjusted.
Be careful as just an inch or two out of level will have the sensors read full early or late compared to what you are used to. My fresh water pick up is on the left side so I tend to level with the left slightly lower. This also tends to have the tank sensors read fuller sooner. As you found, once you know what the indicators are showing you can plan accordingly.
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