In the USA, the EPA mandates certain spill prevention and leak detection methods. One of these is, if the nozzle valve is open when the pump starts ( or manifold valve in the case of many dispensers on one pump). The unrestricted hose end will be interpreted as a broken hose leaking and the mechanical (no electrical required) leak detector before the dispenser hose will shut the flow off to a trickle. Reseting requires fixing the leak (close the nozzle valve) and shutting off the source (pump usually) briefly. Here is a good description of the entire fuel dispensing system, above described is "MLD" http://www.acetank.com/Public/Solutions/ServiceStations/index.cfm?requesttimeout=100 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