I went camping earlier this week.  Fired up the stove for dinner and, yup,
was out of propane.  Had to drive to town and fill up [?, it took 1.75
gals...very slowly.  Something wrong?] so I'd definitely be interested in
some way to tell what the level is.

Mike

----- Original Message -----
From: "Ezra Hall" <ehall@TOGETHER.NET>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Friday, April 21, 2000 5:40 AM
Subject: LP tank level detection proposal, its Friday..

> Given that it is Friday now, I will start another thread..
>
> I searched the archives for info on detecting Liquid Propane level in the
> Manchester tank. It seems there are no elegant solutions. Pressure is not
> an indication of propane level until there is no liquefied propane left in
> the tank, too late unless you happen to be near a filling station, plus
> pressure is temperature dependant. A mechanical float would require
welding
> and re-certification of the tank, I am not interested plus I couldn't find
> a sensor to weld on! That leaves thermal methods. Detection of difference
> in temperature along the outer surface of  the tank requires high flow
> rates to cool the liquid through adiabatic expansion, we don't flow a lot
> of propane with Westy appliances, so this doesn't work well. The only
> practical method remaining (sure you could use acoustical waves or RF, but
> that gets expensive and complicated) is detection of thermal resistance
and
> mass. Presence of LP on the other side of the canister wall decreases
> thermal resistance and increases thermal mass to absorb heat whereas
> portions of the tank with only gas have a relatively higher thermal
> resistance and smaller thermal mass. Liquid crystal level gauges take
> advantage of this by measuring temperature at the surface of the tank when
> heat is applied to the surface. This elegant solution is described in the
> archives as the "fling the boiling water" method. Ok, as you can guess by
> now, I want some sort of electronic warning that tells me the tank is low.
> Unlike my recent thread on towing a wheelbarrow sized motor to power
you're
> A/C system while camping, I think this concept is actually fairly straight
> forward to implement, the question is, how much interest would there be
for
> such a system? Well, here goes:
>
> - To apply heat to the tank, an electrical heating element will be used (a
> flat thin film strip)
> - To detect temperature, inexpensive thermistors will be used
> - A minimum of two sensors is required to detect that LP is below a
> threshold, more sensors could be used for finer granularity
> - In the two sensor scenario, one thermistor is placed at the desired
> liquid level threshold, the second would be used to set the compare
> temperature of a gaseous portion of the tank (near the top)
> - A simple op-amp window comparetor would be used to compare voltage from
> the level sense to the voltage from the reference sense. The window will
be
> set large enough to prevent false readings, ie. A certain temp delta will
> be required. Of course, measurements are only taken during or after heat
> application via the heating strip.
> - Additional level sensors and comparetors could be added for additional
> levels of granularity
>
> This really is a simple solution. The only difficult part will be affixing
> the sensors to the tank, perhaps a stainless steel band around the tank
> could hold them in place vs. an adhesive that could fail. The electronics
> and LED(s) could be placed in the same housing with the batt and water
> level sense. When you flip the switch, you would also get an LP reading.
If
> there is enough interest, I can develop this into a product and sell to
> list members at a decent price. The above should be enough info for anyone
> with the skills to fabricate and build such a device. Anyone interested?
> Thanks,
> Ezra (going camping this weekend, with full LP tank)
> '88 Westy, 94K
> '86 944, 260K