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Date:         Fri, 21 Apr 2000 08:40:01 -0400
Reply-To:     Ezra Hall <ehall@TOGETHER.NET>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Ezra Hall <ehall@TOGETHER.NET>
Subject:      LP tank level detection proposal, its Friday..
Content-Type: text/plain; charset="us-ascii"

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


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