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Old 02-06-2009, 07:37 PM   #1
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1973 31' Sovereign
Maryville , Tennessee
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31' Sovereign's black tank size?

Hello out there. This may be a no brainer, but i'm fairly new and trying to find out the capacity of my black tank. I've got a 73 Sovereign 31 with a rear bath, and unfortunately, it's a model with only a black tank. Does anyone know the black tank size or capacity?

thanks
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Old 07-17-2009, 08:15 PM   #2
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1973 31' Sovereign
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here you go

I just replaced mine as part of a renovation. Here are the details of the exact replacement part, from the tank manufacturer:

Inca Plastics Inc. - 20 gal

Others may know for sure, but I believe this is a 25 gallon tank.

Cheers -
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Old 07-17-2009, 09:30 PM   #3
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Can't guarantee a '73 black tank is the same size, but I can tell you that the '74 tank is 20 US gallons.

Look at your link - it says 20 gal.
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Old 07-17-2009, 09:57 PM   #4
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Maybe it's a Canada thing...

Quote:
Originally Posted by Aage View Post
Can't guarantee a '73 black tank is the same size, but I can tell you that the '74 tank is 20 US gallons.

Look at your link - it says 20 gal.
Our black tank is 25 gallons (listed), closer to 30 gallons actual. The grey tank is supposed to be 10 gallons (wheee!), but is really 15.
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Old 07-18-2009, 03:24 AM   #5
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20 gallons, in my '72.
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Old 07-18-2009, 06:53 AM   #6
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1973 31' Sovereign
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Aage-
yes, I know the link says 20 gallons, but Inca has their website divided drawings for holding tanks divided into 10 gallon increments. There are multiple cases of tanks in the 30 gallon section being 33, 35, and 37 gallons for instance.

That said, the specific drawing for the H4 does say 20 gallons...

My manual says 25 gallons from the factory. The new tank from Inca was dimensionally identical to the original factory tank I took out. It had two exceptions:
1. the "pilot" marks for the 2" vent line and the 3" inlet connection were in slightly different positions. I ordered the tank without holes or fittings in case this was the case, so it wasnt a problem. The 3" slip outlet was perfect.
2. The Inca tank was black; the original was white.
3. The Inca tank was half the cost (including shipping across country) than buying it from any Airstream dealer who had the tank in stock.

A quick calc of the "rectangular" dimensions provided in the drawing yields an internal volume of 25.8 gallons. From this dimension we need to remove the effect of the slope inside the tank, which is roughly 3.5 gallons, and the volume of the "notch out" by the slip fitting which is about 2 gal. I've rounded and not done super precise geometry, but I think its close enough for our purposes.

So in the end, somewhere around 20 gallons is probably about right. Looks like the manual is stretching the truth a bit!
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Old 07-18-2009, 07:34 AM   #7
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Quote:
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Our black tank is 25 gallons (listed), closer to 30 gallons actual. The grey tank is supposed to be 10 gallons (wheee!), but is really 15.
Terry,

Are your two tanks original, or replacements?

How do you know the actual capacity? Have you done some measurements?

In other words, curious why the Owner's Manual would list different numbers than you have...
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Old 07-18-2009, 07:36 AM   #8
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Math, schmath...

I checked the capacity of ours the hard way--a gallon milk jug of water at a time. I've experienced the difference between "official" and "actual" figures many times, and wanted to know for sure what we had, especially being the third owners (you never know what they may have done).
So, if you really, really, REALLY want to know, you can find out--one gallon at a time.
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