Consulting the Oracle (google) we find this example: http://www.opw-fc.com/product_category.aspx?cid=7 apparently some fuel nozzles (pump handles to us, fuel nozzle to the industry) do allow one to dispense product from an unpressurized hose. There are fuel nozzles, however, that will not dispense product until the line is under pressure, preventing this sort of theft. These pressure sensitive nozzles are marketed more as spill and 'loss' prevention. -Adam On 7/11/08, Sean Breheny wrote: > Hi Olin, > > I may be relatively young (28) but I've been driving for 10 years and > certainly have filled up a car before :) > > Let me try to rephrase my question by giving an example. > > Let's say I go to fill up. First, I slide my credit card and select > the type of gas I want. Then I remove the nozzle and place it into the > filler tube in the car. Next I squeeze the handle and the pump starts > pumping fuel. > > When the tank fills and the nozzle detects gas next to the small holes > on the outside of the nozzle, it closes the valve at the nozzle and > the gas stops flowing. I could "top it off" by continuing to cycle the > handle through open and closed. > > Now, I put the nozzle back into the holder on the pump and complete > the transaction. At this point, the entire tube from the pump to the > nozzle is still full of gas, right? > > Now if I remove the nozzle from the holder again, without swiping my > card again, and then squeeze the handle, how come I don't get the > contents of the tube spilling out? Sure, the pump will not deliver > more gas into the tube, but I would think that the gas inside the tube > could still come out. > > I can only think of a few ways this could be prevented: > > 1) suck the fuel back out of the tube into the pump > 2) somehow enable the pump to signal the valve at the nozzle to remain > closed when there isn't an active transaction going on. > 3) Have the entire connection from pump through to nozzle be so air > tight that no air could get back in to displace the gasoline so that > it will not come out unless the pump is pumping (I'm having trouble > trying to determine whether this would work as it seems that normally > even a tube which is sealed at one end and filled with liquid can > still be emptied from the other end). > 4) have the valve at the nozzle end require a minimum pressure to open > (which is greater than the pressure developed by gravity on the > vertical height of gasoline in the tube) > > My question is which of these is done (or what possibility did I not consider) > > Thanks, > > Sean > > > On Fri, Jul 11, 2008 at 8:48 AM, Olin Lathrop wrote: > > > > I'm not sure what you're asking as it's not making much sense. > > > > The valve for starting and stopping gas flow when filling your car is in the > > nozzle. This valve is usually activated by a grip that you squeeze. When > > your tank is full this valve shuts off by itself, or you manually release > > the grip to shut it off. If you were to squeeze the grip again, more gas > > would flow, but you'd also get charged for that. > > > > The pump senses when you return the nozzle to the holder in the pump and > > completes the billing. On some pumps this is done by swinging a lever which > > phisically locks the nozzle in place. It won't be released until the next > > billing has started. Some pumps shut down the internal pump, but of course > > it is turned on again when the nozzle is removed by the next customer. > > > > I'm not really sure what you're asking. Haven't you ever filled up a car > > before? If not, go with someone next time they do it. It's really not > > complicated, and it's pretty obvious how the system works when you look at > > it. > > > > > > ******************************************************************** > > Embed Inc, Littleton Massachusetts, http://www.embedinc.com/products > > (978) 742-9014. Gold level PIC consultants since 2000. > > -- > > http://www.piclist.com PIC/SX FAQ & list archive > > View/change your membership options at > > http://mailman.mit.edu/mailman/listinfo/piclist > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- EARTH DAY 2008 Tuesday April 22 Save Money * Save Oil * Save Lives * Save the Planet http://www.driveslowly.org -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist