# Gasoline/diesel dispenser: how much pressure? **URL:** \ **Category:** Factual Questions\ **Created:** [September 19, 2022, 10:50am UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807 "2022-09-19T10:50:26Z")\ **Posts on this page:** 9\ **Page:** 1
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [September 19, 2022, 10:50am UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/1 "2022-09-19T10:50:26Z")
Inspired by this video, which explains how a gasoline dispenser nozzle “knows” to shut off when your tank is full: [![](https://img.youtube.com/vi/fT2KhJ8W-Kg/maxresdefault.jpg "How petrol pumps know when to turn themselves off") ](https://www.youtube.com/watch?v=fT2KhJ8W-Kg) When you turn the dispenser on (i.e. put your credit/debit card in, select a fuel grade), a pump starts supplying pressure to the handheld nozzle, and at that point you can dispense fuel whenever you pull the trigger lever. How much pressure is being supplied to the handheld nozzle? IME you can deliver a gallon of fuel in about ten seconds, but the flow area at the venturi throat doesn’t look like very much. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 19, 2022, 2:46pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/2 "2022-09-19T14:46:05Z")
> [@Machine\_Elf](#): > > How much pressure is being supplied to the handheld nozzle? Approximately 30 to 45 psi. Veeder Root Red jacket is a popular gas station pump, and you can look at the pump curves here > **[Red Jacket® Maxxum Submersible Turbine Pump (STP) | Veeder-Root](https://www.veeder.com/us/red-jacket-submersible-pumping-systems/red-jacket-maxxum-submersible-turbine-pump)** > > The Red Jacket® Maxxum Submersible Turbine Pump (STP) is intended for 6” high flow applications and provides the industry’s highest flow rates. You can look at other pump types too with their pump curves. You can covert pump head in ft to psi using the equation : ([Pumps - Head vs. Pressure](https://www.engineeringtoolbox.com/pump-head-pressure-d_663.html)) _p = 0.433 h SG (1)_ _where_ _p = pressure (psi)_ _h = head (ft)_ _SG = [specific gravity](https://www.engineeringtoolbox.com/specific-gravity-liquid-fluids-d_294.html) of the fluid_ For about 100ft, and Diesel specific gravity of 0.85, the pressure will be 0.433_100_.85 ~ 37 psi ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 19, 2022, 2:52pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/3 "2022-09-19T14:52:38Z")
> [@Machine\_Elf](#): > > IME you can deliver a gallon of fuel in about ten seconds, but the flow area at the venturi throat doesn’t look like very much. You can use this simple calculator for orifice / venturi calculation [https://experttoolsonline.com/danfoss/orifice\_calculator](https://experttoolsonline.com/danfoss/orifice_calculator) A high Cd in a orifice is just like a venturi If you select Cd = 0.98, SG = 0.85, Hydraulic Diameter = **5 mm** (obtained by trial and error) , Pressure drop = 2 bar (30 psi), you get the approximate flow to match your number (24 liters per minute or 4 liters per 10s or a gallon per 10s) ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [September 19, 2022, 3:21pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/4 "2022-09-19T15:21:39Z")
Could you explain this from first principles? Ignoring the extra pressure needed to pipe the fluid through the pipe and focusing on the nozzle, let’s say the diameter is 20mm and the flow rate 0.4 l/s; that makes a velocity of 1.27 m/s. If the density is 750 kg/m3, a simple Bernoulli principle shows that the extra pressure required to shoot out the fluid is only 750 x (1.27)^2/2 = 608 Pa. Your orifice calculator more or less seems to agree? ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 19, 2022, 3:48pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/5 "2022-09-19T15:48:17Z")
@DPRK - Let me try this ( I am not a physicist but an engineer) The contraction Diameter = 5 mm (its an annulus and the hydraulic diameter for a annulus is defined as the wetted Cross section divided by the wetted perimeter. For an annulus, it works out to Do-Di ) 0.4 l/s = 0.0004 m3/s 5mm = 0.005 m (Do - Di) So area of cross section = 0.0000196 m2 (pi()\*(Do-Di)/4) So velocity = 20.4 m/s ( 0.0004 m3/s / 0.0000196 m2) Rho x v2/2 = 750 \* (20.4 \* 20.4) /2 = 155,629 Pa ~ 23 psi 23 versus 30 psi is close enough for this level of calc. Also, although it is called a venturi, it behaves more like an orifice. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [September 19, 2022, 4:08pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/6 "2022-09-19T16:08:22Z")
Of course, this doesn’t take into account the much higher pressure in the pumps used to fill truck tanks. IME they are around twice as fast and woe betide any car driver who tries to use one. ---
**Author:** ![am77494](https://avatars.discourse-cdn.com/v4/letter/a/d2c977/32.png) [@am77494](https://boards.straightdope.com/u/am77494)\ **Post date:** [September 19, 2022, 4:43pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/7 "2022-09-19T16:43:33Z")
> [@bob\_2](#): > > …this doesn’t take into account the much higher pressure in the pumps used to fill truck tanks. You can get higher flow by having a larger flow area or higher pressure. Since you are asserting that its higher pressure, I request a cite please. ---
**Author:** ![jnglmassiv](https://avatars.discourse-cdn.com/v4/letter/j/f07891/32.png) [@jnglmassiv](https://boards.straightdope.com/u/jnglmassiv)\ **Post date:** [September 19, 2022, 5:22pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/8 "2022-09-19T17:22:19Z")
> [@bob\_2](#): > > Of course, this doesn’t take into account the much higher pressure in the pumps used to fill truck tanks. They also have larger diameter nozzles. We used to have this crummy dock level truck at an old job of mine. I always hated driving it but the quick fill-ups was one nice thing. It had dual tanks and stations for trucks had the high speed pumps on both sides so you weren’t standing in the cold for terribly long. ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [September 19, 2022, 7:36pm UTC](https://boards.straightdope.com/t/gasoline-diesel-dispenser-how-much-pressure/971807/9 "2022-09-19T19:36:10Z")
> [@am77494](#): > > You can get higher flow by having a larger flow area or higher pressure. Since you are asserting that its higher pressure, I request a cite please. I only know from personal observation, so you may well be correct that the higher flow rate is due to a larger nozzle. This answer from Quora seems to be authoritative: > **[Do diesel pumps pump faster than other pumps?](https://www.quora.com/Do-diesel-pumps-pump-faster-than-other-pumps/answer/Mark-Harris-60?ch=10&oid=176564868&share=9ee1676c&srid=hOsYO&target_type=answer)** > > Answer (1 of 6): That depends, It all has to do with displacement, and if the gas engine is running on basically free natural gas, it might be a 702 cubic inch monster motor, and will pump hundreds of gallons per minute of water onto a large 1/2 mile...