ECR Posted June 18, 2013 Share Posted June 18, 2013 How on earth do they work ? My (rudimentary) electrical knowledge tells me that submerging electrical contacts in water causes a short circuit (problem). How is it that the connections to the fuel pump can be submerged in petrol and not short circuit ? Is petrol non conductive ? I also assume that any sparking from a partialy submerged fuel pump is of insufficient temperature to ignite the fuel vapour ? Just intrigued ....... Edited by - ecr on 18 Jun 2013 09:44:22 Link to comment Share on other sites More sharing options...
Budjuggler Posted June 18, 2013 Share Posted June 18, 2013 Don't know - but my water feature has a submersible pump in water. And thatworks Link to comment Share on other sites More sharing options...
Roger Ford Posted June 18, 2013 Share Posted June 18, 2013 Water isn't very conductive - mains will go through it but I doubt you get significant current with 12v. In fact water itself isn't really conductive at all, it's only the impurities which pass a current. Don't know about petrol - or I didn't until I googled: Clean, dry, non contaminated gasoline has a conductivity of ~330pS/M, so is rated as a low partial conductor. I've often wondered about the possibilities of sparks, though. Link to comment Share on other sites More sharing options...
Claire Voyant Posted June 18, 2013 Share Posted June 18, 2013 How would any sparks which may occur within a submerged pump reach any vapour, which by definition be in air? Link to comment Share on other sites More sharing options...
Roger Ford Posted June 18, 2013 Share Posted June 18, 2013 When you run the tank dry and it's no longer submerged? Or when the car is upside down in a ditch? Edited by - Roger Ford on 18 Jun 2013 11:27:52 Link to comment Share on other sites More sharing options...
ECR Posted June 18, 2013 Author Share Posted June 18, 2013 So, if I dropped say a wiper motor into a bucket of water would that continue to work ? Why do we experience "water in the electrics" problems with our cars ? Link to comment Share on other sites More sharing options...
Barbarossa. Posted June 18, 2013 Share Posted June 18, 2013 I think that they are just very well sealed. The only reason that you need air running through an electric motor is for cooling. Being submerged gives much better cooling through the casing there would be if it was in air. Link to comment Share on other sites More sharing options...
ECR Posted June 18, 2013 Author Share Posted June 18, 2013 But the two connections on the pump (both fastons) are not sealed in any way and are covered with fuel/vapour ..... I can't believe that sealing is a factor. It would only need a couple of seal failures worldwide and the Naderesque witch hunt would start. Any seals must fail to a safe mode .... Edited by - ECR on 18 Jun 2013 12:04:22 Link to comment Share on other sites More sharing options...
rj Posted June 18, 2013 Share Posted June 18, 2013 If dropped into water corrosion would occur on the wires if not properly isolated - due to the electricity. If dropped in fuel there is no oxygen present, hence the fumes can not burn. Link to comment Share on other sites More sharing options...
ECR Posted June 18, 2013 Author Share Posted June 18, 2013 If dropped into water, wouldn't a dead short happen ? The pump is very often not completely submerged. The pick up is at the bottom and the wiring is 75/90 mm above that and so would often be in fumes. I still haven't had an explanation that convices me. I know they work, I believe they must be safe, but why/how ? Link to comment Share on other sites More sharing options...
keybaud Posted June 18, 2013 Share Posted June 18, 2013 Quoting ECR: If dropped into water, wouldn't a dead short happen ? The pump is very often not completely submerged. The pick up is at the bottom and the wiring is 75/90 mm above that and so would often be in fumes. I still haven't had an explanation that convices me. I know they work, I believe they must be safe, but why/how ? Fuel conducts less than water and acts as a coolant. As a child, I used to drive battery-powered, toy cars along the bottom of a swimming pool. I just had to add some weights to stop them floating. The water issue is more about corrosion due to dissolved air (oxygen), but this is a slow process and won't stop an electric motor working, unless the potential differnce (voltage) acrosss the terminals is sufficient to short circuit in water, but not in air. Link to comment Share on other sites More sharing options...
tbird Posted June 18, 2013 Share Posted June 18, 2013 Why do we experience "water in the electrics" problems with our cars ? Pure distilled de ionised water is a very poor conductor of electricity, the muckier it gets the more the impurities conduct. "damp electrics" are usual on HT side of things, and a very thin film of water, HV will track to earth through a thin film of dirty water. Techy answer here 😬 Actualy thinking about it the senders have always been in the tank and open to fuel, far more likely to spark than a sealed motor and I have never heard of one going bang Tim Edited by - tbird on 18 Jun 2013 18:11:01 Link to comment Share on other sites More sharing options...
ECR Posted June 18, 2013 Author Share Posted June 18, 2013 The battery powered toy example is interesting. Am I right in surmising that there is just not nough oomph at 12v for the current to flow through the liquid between the two spade terminals ? Problems with cars happen on the high tension side because there is more oomph available. If you threw a 240 volt motor into a bucket of water then we know what would happen ..... Is voltage level the explanation ? Link to comment Share on other sites More sharing options...
tbird Posted June 18, 2013 Share Posted June 18, 2013 technicaly Potential difference between the two points, but yes the higher voltage the more likely the insulator is to break down and allow current to flow. Tim Link to comment Share on other sites More sharing options...
ECR Posted June 18, 2013 Author Share Posted June 18, 2013 So at 12v the fuel is enough of an insulator to stop any shorting out .... What about sparks igniting vapour ? Why is that not a problem Link to comment Share on other sites More sharing options...
mav Posted June 18, 2013 Share Posted June 18, 2013 Incorrect fuel / air ratio. Link to comment Share on other sites More sharing options...
tbird Posted June 18, 2013 Share Posted June 18, 2013 we are presuming that the pump motor is surrounded by either petrol or vapour, I would guess all the electrical bits are within a sealed chamber possibly oil filled, there is no great pressure so O rings would do, drive to the pump itself could be via, a sealed shaft or across a membrane. Submersible pumps are very common technology, (ponds, sump pumps etc.) so I guess they are pretty good at sealing them. Tim Link to comment Share on other sites More sharing options...
BBL Posted June 18, 2013 Share Posted June 18, 2013 Quoting ECR: What about sparks igniting vapour ? Why is that not a problem Not enough oxygen in the tank, I believe. Link to comment Share on other sites More sharing options...
tbird Posted June 18, 2013 Share Posted June 18, 2013 another guess would be that any small sparks generated are no where near energetic enough to ignite the mix of gasses in the tank. If you think how difficult it can, sometimes, be to get the fuel air mix to explode in the cylinders where you want it to with the correct mix and a high energy long(is) duration spark. probably a combination of all the things mentioned so far, or we are all just really lucky 😬 Tim Edited by - tbird on 18 Jun 2013 20:14:33 Link to comment Share on other sites More sharing options...
ECR Posted June 18, 2013 Author Share Posted June 18, 2013 tbird there are absolutely exposed connections on the pump assembly so there's no sealed chamber involved (my earlier observations about seal failure are relevant I think). Incorrect fuel air mix : it would need to be always incorrect which would mean that you could chuck a cigarette in there safely at any time .... I think it must have something to do with the temperature of a 12v spark perhaps Link to comment Share on other sites More sharing options...
OliverSedlacek Posted June 18, 2013 Share Posted June 18, 2013 If a suitable pump is submerged in petrol, the petrol acts as an insulator. Even if inverted, the air/fuel ratio in a petrol tank wont ignite as its too rich. Interestingly I know of a mechanic who got so fed up with a rough old mains powered electric drill that he taped the trigger down and dropped it in a plastic bucket of water. 20 minutes later it eventually stopped working and he could get his boss to buy a replacement. Link to comment Share on other sites More sharing options...
Member Jonathan Kay Posted June 18, 2013 Member Share Posted June 18, 2013 The movie trick of blowing up a car by throwing a lighted cigarette into the petrol tank has been myth-busted several times. Warning: the next warning is a health and safety warning. This does NOT mean that petrol vapour is not highly flammable and dangerous. (Nearly said "inflammable" there.) Jonathan Link to comment Share on other sites More sharing options...
Wrightpayne Posted June 18, 2013 Share Posted June 18, 2013 Natural Gas is only explosive at between 5 and 15% gas in air. In theory you could go into a room at 50% concentration light a match and it won't go bang, the big assumption is that the gas is evenly distributed. In a petrol tank the vapour must be of a high enough concentration for it not to be a problem. Ian Link to comment Share on other sites More sharing options...
BBL Posted June 19, 2013 Share Posted June 19, 2013 Quoting ECR: Incorrect fuel air mix : it would need to be always incorrect which would mean that you could chuck a cigarette in there safely at any time ... It should always be incorrect, unless it is "empty" - the tank will typically have far more petrol fumes than air. In theory, you could chuck a lit cigarette in safely, but the problem is that the fuel/air ratio is far more likely to be at a sufficient ratio to support combustion right at or very near the filler neck. edit -- An example , ignited by static charge. Edited by - BBL on 19 Jun 2013 01:36:53 Link to comment Share on other sites More sharing options...
diggerman Posted June 19, 2013 Share Posted June 19, 2013 I think I've found the solution to ending it all! Poof Peter Link to comment Share on other sites More sharing options...
Recommended Posts
Create an account or sign in to comment
You need to be a member in order to leave a comment
Create an account
Sign up for a new account in our community. It's easy!
Register a new accountSign in
Already have an account? Sign in here.
Sign In Now