My 40 gallon waste tanks only hold 32 gallons

[Vibeman]

I recently installed Horst probes in the waste tanks on my 2020 Hurricane 29M. Before I installed them I hooked up the hose and my little water meter to determine exactly how many gallons were in the tank at each level. My Thor literature states my black and gray tank capacity is 40 gallons each. First I leveled the coach, then I put my little water meter in line with my garden hose and put the end of the hose up into the motor home kitchen sink. I routed the hose through the window near the sink so I could sit outside by the tanks and control the water flow from there. The new sensors were not yet installed but the new holes were in place next to each original sensor. I just started filling the tank and watched the lowest hole for water. I was surprised at the actual capacity of my “40 gallon” waste tank and at the actual capacity at each sensor level too, here’s what I found:
It took approximately 7.5 gallons to fill up to the lowest (right most) hole on the tank. The one used for the grounding probe. I then plugged that hole and it only took another 1/2 gallon to fill to the second (33%) sensor hole. I plugged that hole too, then added more water. It only took another 4 gallons to get up to the third (67%) sensor hole. I then plugged that hole and continued filling while I watched the MIRA Firefly app on my phone to monitor the original top sensor which was still in place and hooked up. Due to it’s location I can’t drill a new hole next to it for a replacement sensor so I had to use it as is. The top sensor is located very near the tank filler hole coming from the kitchen sink so some splashing was probable occurring when the water level got near the top sensor giving me a couple of early “Full” readings, but I just stopped filling for a minute or two and it went back to 67%. I finally got a steady and continuous “Full” reading from the top sensor after adding an additional 13 gallons.
RESULTS: (rounded to full gallons)
33% = 8 gallons
67% = 12 gallons
Full = 25 gallons
I then continued filling until water backed up into the kitchen sink, this took an additional 7 gallons. Surprisingly, at this point no water backed up into the shower pan! When I moved the coach to dump the tank, the water in the kitchen sink went down and was no longer visible. So before dumping the tank I re-leveled and started adding water directly into the shower drain, it took 2 more gallons before it too backed up. That all adds up to 34 gallons, but with some backup occurring.
I would say the maximum safe quantity of water in my gray tank would be 32 gallons, but only when parked and level. If we are traveling I would not want more than 30 gallons in the tank to allow room for sloshing without backup. Or maybe even 25 gallons, which, coincidentally, is “Full” on the Firefly panel.

[Judge]

While I have not measured my gray tank, I have used my digital flow meter on my black-tank flush port and another on my fresh water tank. My black and gray tanks are 40 gallons and I have had 36 gallons in the black tank without it backing up.

My fresh water tank is 60 gallons and I have measured that much with another flow meter when filling it.

The tank sensor readings are about the same and not surprising. My fresh water tank will read start to read full at about 42 gallons. The black tank can start reading full at under 30 gallons.

Perhaps you do have a smaller tank or there is another issue..... a significant dent in the tank or some debris in the tank.

[widdershins]

'specs subject to change'. I purchased a TT in 2013 that claimed 18 gallon black tank... the black tank held 6 gallons (including 2.5' of 3" drain)

in other words, marketing BS is marketing BS....

edit: have you ever checked your flow meter? I've got one, been meaning to test it out.. I'm not sure how accurate they can be on a 1/2" hose vs. 3/4" hose?

[Travelin' Texans]

You've just proven what everyone either suspected or already knew, the tank level sensors are worthless for any accurate information. If they are the typical sensors if you were to do that test again you'd most likely get different results, they are useless, always have been since my 1st 1978 TT.
Also as posted on all rvs, all things are subject to change without notice.
But now you know exactly what's what on your particular rv so you can go from there.

[Vibeman]

Yes, this is for my RV only. My results are from my testing methods. I am pretty confident in my numbers though. I tested the flow meter for a measured 5 gallons and it read within mfg’s +/- specs. Now that I know some real numbers I can more accurately predict when my tanks will get full and that is important to me when traveling and staying at non-hookup sites. I still wonder how my kitchen sink could back up and no water backed up in the shower, which is at least 2’ lower. Could there be a check valve somewhere? Could there be a bubble of air somewhere?
Judge said he filled and measured his black tank capacity through the tank flush port. Filling it from the bottom up under pressure may have an effect on the capacity compared to filling it with gravity from above. I can see a lot of my tank from the outside so I do not think it is dented or deformed. It is pretty heavy plastic anyway. According to the Thor schematics it is flat on top so there is not much room for hidden airspace. Someone in another thread said tank capacity is calculated from the outside measurements. If true that would probably explain my findings.

[The Gritz Carlton]

Quit dissecting fleas. When they're full, just dump them..You'll get a good sense of how much they will "take" the more you use them. Don't wait till turds are floating in the kitchen sink.

[Lt Keefer]

We just use our RV until the chance comes around to dump. For us, that's almost every night. Since we don't boondock, the size of the tanks is nor relevant. Neither is the accuracy of the sensors. We stayed in a park with water and electricity only one time. We dumped our tanks going in and dumped going out. We made it the whole week without needing to dump, so that is our baseline.

[dkoldman]

Originally Posted by The Gritz Carlton

Quit dissecting fleas. When they're full, just dump them..You'll get a good sense of how much they will "take" the more you use them. Don't wait till turds are floating in the kitchen sink.

We wait until we see yellow on the flea's teeth. We can go a week on 70 gal fresh water and 61 gallon gray tank and 40 gallon black tank. Never had a problem but if we start to see flea poop in the kitchen sink we dump immediately.

3 1/2 years and are gauges are still not only working, but very accurate. Truth be told the Gray tank gauge is only one we really need

[Bill87]

Vibeman, thanks for your measurements and posting. This explains why my tanks on my 2022 Windsport 29m seem to be filling way too soon. We do boondock and this motorhome doesn’t last as long as or 20 foot trailer even though they report the same size tanks! I’m going to do a similar study on our coach.

[dkoldman]

May I suggest given the doubts / concern, get with the tank mfg and they will send you the specifications. You only need to verify part number actually installed.

I had similar beef with my sales literature telling me the capacity of my LPG tank was 22 gallons. When empty (or very close to empty) and I filled it full, it was never close to 22 gallons. I called Manchester tank and they verified it was 22 gallons tank; but I would only be able to add 17.6 because of the bleed off at 80%.

My guess is that you will have a 40 gallon tank and if anything the way it is mounted / oriented or similar may not allow you to fill 100%? Seeing the tank's shape may be relevant.

[MJC62]

Originally Posted by The Gritz Carlton

Quit dissecting fleas. When they're full, just dump them..You'll get a good sense of how much they will "take" the more you use them. Don't wait till turds are floating in the kitchen sink.

Gritz you definitely have a way with words!!

[Vibeman]

Originally Posted by Travelin' Texans

You've just proven what everyone either suspected or already knew, the tank level sensors are worthless for any accurate information. If they are the typical sensors if you were to do that test again you'd most likely get different results, they are useless, always have been since my 1st 1978 TT.
Also as posted on all rvs, all things are subject to change without notice.
But now you know exactly what's what on your particular rv so you can go from there.

My post was not about sensor accuracy. It was not about the sensors at all. It was about waste tank capacity, which I verified without using any sensors other than my eyes and ears. Even though I left the original top sensor in place during the test, I was still able to verify when the tank was actually full by listening to the fill water splashing onto the surface of the water in the tank, since the tank fill port is mounted on the top surface of the tank. When the splashing stopped, the tank was truly full.
BTW, I did the test twice, and got the exact same results both times.

[Vibeman]

Originally Posted by The Gritz Carlton

Quit dissecting fleas. When they're full, just dump them..You'll get a good sense of how much they will "take" the more you use them. Don't wait till turds are floating in the kitchen sink.

Dissecting fleas is a hobby of mine, I don’t want to quit.

[Vibeman]

Originally Posted by dkoldman

May I suggest given the doubts / concern, get with the tank mfg and they will send you the specifications. You only need to verify part number actually installed.

I had similar beef with my sales literature telling me the capacity of my LPG tank was 22 gallons. When empty (or very close to empty) and I filled it full, it was never close to 22 gallons. I called Manchester tank and they verified it was 22 gallons tank; but I would only be able to add 17.6 because of the bleed off at 80%.

My guess is that you will have a 40 gallon tank and if anything the way it is mounted / oriented or similar may not allow you to fill 100%? Seeing the tank's shape may be relevant.

dkoldman,
According to Thor schematics my waste tanks have a flat top and the flat top is mounted close underneath, and parallel with, the coach sub-floor. No hidden upper air spaces. The kitchen sink and the shower drain both dump directly into the top of the tank.
The outside dimensions (actual measurements taken by me) of the flat, rectangular top of my gray tank is 18” x 91”, giving a surface area of 1,638 square inches. At 231 cubic inches/gallon, 1” of water at the top of the tank would be approximately 7 gallons. This is using the outside dimensions of the tank, actual inside capacity would be a little less, maybe 6 gallons. So only 1” of airspace across the entire tank surface would account for the missing 6 gallons. I suppose it is possible to still have some airspace at the top of the tank, even when the vertical fill pipe is backed up into the sink. I have actually seen the bottom of the sink drain inlet using my borescope and, of course, there is a flange and gasket on it, lowering the effective height of the outlet by 1/2” or so. Maybe if I kept the sink full for a long enough time the water would continue to slowly go down until all air was displaced from the tank, at which point it would also back up into the shower. Also, as I mentioned in my original post, there is a 3/16” - 1/4” thick coating of slime on the bottom half of the tank walls, and probably a deeper layer of slime stuck to the bottom. This would also reduce the effective capacity.

Taking all this into consideration, it might be possible to get a nominal 40 gallons of water into a (clean) tank, at least close enough to 40 gallons to use for marketing purposes.

I have attached a couple of schematics showing the general shape and layout of the tanks.

[The Gritz Carlton]

Originally Posted by Vibeman

Dissecting fleas is a hobby of mine, I don’t want to quit.

Excellent eyesight is priceless.

[Travelin' Texans]

Originally Posted by Vibeman

My post was not about sensor accuracy. It was not about the sensors at all. It was about waste tank capacity, which I verified without using any sensors other than my eyes and ears. Even though I left the original top sensor in place during the test, I was still able to verify when the tank was actually full by listening to the fill water splashing onto the surface of the water in the tank, since the tank fill port is mounted on the top surface of the tank. When the splashing stopped, the tank was truly full.
BTW, I did the test twice, and got the exact same results both times.

Well alrighty then, you can go back to dissecting fleas!

[Vibeman]

Originally Posted by dkoldman

May I suggest given the doubts / concern, get with the tank mfg and they will send you the specifications. You only need to verify part number actually installed.

I had similar beef with my sales literature telling me the capacity of my LPG tank was 22 gallons. When empty (or very close to empty) and I filled it full, it was never close to 22 gallons. I called Manchester tank and they verified it was 22 gallons tank; but I would only be able to add 17.6 because of the bleed off at 80%.

My guess is that you will have a 40 gallon tank and if anything the way it is mounted / oriented or similar may not allow you to fill 100%? Seeing the tank's shape may be relevant.

Good guess dkoldman, thanks!

Flea autopsy report:
According to Thor schematics my waste tanks have a flat rectangular top and the flat top is mounted close underneath, and parallel with, the coach sub-floor. No hidden upper air spaces. If the coach is level, then the tank is level. The kitchen sink and the shower drain both dump directly into the top of the tank. According to my measurements, 1” of trapped airspace at the top of the tank would hold approximately 6 gallons and mostly account for my missing capacity.
Taking this into consideration, it might be possible to get a nominal 40 gallons of water into the tank, at least close enough to 40 gallons to be used for marketing purposes.
Thor never made a claim on the tank’s usable capacity.

[Jimbo56]

Originally Posted by Vibeman

Yes, this is for my RV only. My results are from my testing methods. I am pretty confident in my numbers though. I tested the flow meter for a measured 5 gallons and it read within mfg’s +/- specs. Now that I know some real numbers I can more accurately predict when my tanks will get full and that is important to me when traveling and staying at non-hookup sites. I still wonder how my kitchen sink could back up and no water backed up in the shower, which is at least 2’ lower. Could there be a check valve somewhere? Could there be a bubble of air somewhere?
Judge said he filled and measured his black tank capacity through the tank flush port. Filling it from the bottom up under pressure may have an effect on the capacity compared to filling it with gravity from above. I can see a lot of my tank from the outside so I do not think it is dented or deformed. It is pretty heavy plastic anyway. According to the Thor schematics it is flat on top so there is not much room for hidden airspace. Someone in another thread said tank capacity is calculated from the outside measurements. If true that would probably explain my findings.

You may want to open the storage compartment under the shower and see if your drain pipe is broken. Mine broke. Other posters have had similar. Replaced with rubber hose and clamps.

[Bill Johnson]

I have a 40 gallon black tank in my motorhome and I went to visit my son, I don't remember how many days maybe 3 it said it was full. I asked my son where we could dump and he said there is a Wisconsin state park about 45 minutes away. I called a few campgrounds in the area and they all said no dumping unless we were campers. I had my blue boy I think it was the 25 gallon one. It was only half full when we emptied it. We went to the Wisconsin state park and I can't remember what they charged I think it was 10 or 15 and that was okay because in Wisconsin you can't even drive into a park without paying to get in. Now any time it shows full I shine a light down there and check, I bet I could go another week before it would be full. I could see the bottom of the tank when it says full. Never believe the gauges except for my fresh water tank that when it says full I have only a few inches to spare

[Bob Denman]

Originally Posted by The Gritz Carlton

Quit dissecting fleas. When they're full, just dump them..You'll get a good sense of how much they will "take" the more you use them. Don't wait till turds are floating in the kitchen sink.

https://www.thorforums.com/forums/at...1&d=1666279453