frank condelli has sourced something that works:
http://members.aol.com/Fkc43/ptgauge.htm

On May 15, 2005, at 8:16 PM, Rocket J Squirrel wrote:

> You know, Mark's question does bring up something I have been wondering
> about for some time: How is it that, here in the 21st century, no one
> has yet been able to figure out a "gas gauge" for propane tanks? I know
> you can weigh a tank to know how full it is (if you know its empty
> weight), but that's kind of hard to do with a Westy tank. I always feel
> a little stupid wandering around without a clue about how much propane
> I'm carrying.
>
> --
>
> Mike "Rocket J Squirrel" Elliott
> 71 Type 2: the Wonderbus
> 83.5 Westfalia: Mellow Yellow ("The Electrical Banana")
> KG6RCR
>
>
>
>
> On 5/15/2005 3:23 PM Mark Cumnock wrote:
>
>> i went for my first fill up, I had the time, never had filled it up,
>> and I
>> have know idea how much was or is in it. The P.O. had propane in it.
>>
>> We hooked the hose up, turn the pump on. opened the overfill/check
>> valve,
>> liquid propane imed. came out.
>>
>> Does that mean it was/is already full? Or did I/we do something wrong?
>>
>> Looking forward to your answers
>>
>> Mark
>>
>>
>>