Date:         Wed, 20 Dec 95 10:03:50 mdt
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         jonesl@tcplink.nrel.gov
Subject:      Propane Tank Gauge

     Merry Xmas Everybody!!
     
     Just so that everyone understands - - propane is a hydrocarbon 
     compound with a vapor pressure somewhat greater than atmospheric 
     pressure.  This means that it will stay liquid in its thin metal tank, 
     yet vaporize when the pressure drops to the regulator downstream 
     pressure (6 or 7 inches water column) to burn in stoves, furnaces, 
     water heaters, etc.  
     
     I have seen many large propane tanks that had either mechanical float 
     operated gauges or sight glass guages.  Most of these tanks were at 
     least as large as your VW bus.  Obviously not a practical application 
     for small tanks!
     
     There are several ways to tell propane tank level on a small tank.  
     These are based on the fact that propane liquid, like water, requires 
     heat to vaporize into a gas.
     
     -  if it is humid out, turn on your stove or furnace and watch the     
        tank.  The distribution tubing, the regulator and the top of the 
        tank will be colder than ambient temperature due to evaporation of 
        the propane being used.  So frost will start to form on the tank    
        down to the level of the propane liquid.  
     
     -  If it is not humid out, turn on an appliance for a few minutes, 
        then throw a cup of water on the tank.  Approximately same result 
        as above will occur.
     
     -  If you have a few bucks, stop by your propane or RV dealer and buy 
        a stick-on propane tank guage.  You may have to cut one of these to 
        fit on a VW camper propane tank.  These things are made of a 
        thermally sensitive material that changes color at different 
        temperatures.  If the temperature has recently changed (like when   
        sun comes up in the morning), the temperature of the liquid section 
        of the tank will lag behind the temperature of the vapor section 
        due to the diffence in heat capacity and density of the two       
        sections (Engineering students, please see Thermo 101).  At any    
        rate, the tank level will be at the point where the color of the    
        gauge material changes.  If the tank has been setting at an even 
        temperature for several hours and the liquid and the vapor are at 
        thermal equilibrium, you may have to turn on an appliance or vent a 
        little propane to generate enough temperature difference for it to 
        show on the gauge.
     
     please e-mail me if you have any questions