At any rate, my point was the unit is not designed with a specific safety feature it needs to be used by non-trained personnel. A novice is not going to recognize the inherent danger (see Nevada’s post). What the manufacturer failed to include is a check for leaking that more then likely will produce an error. It is not technically challenging to add this feature but does require some smarts in the unit. More than likely, they already have a microcontroller so adding this feature is not that difficult…
With a leak, the gauge in the pump does not match the tire pressure even when stopped. The volume in the pump reservoir/hose is much smaller than the tire so a leak will deplete the air on the pump side faster than the tire, leading to a mis-reading of the tire pressure. Then the pump cycles back on and the process repeats, creating a potentially dangerous situation. You can see this yourself with a manual gauge, just cock it on the valve stem to produce a leak and see if it reads accurately. Arguing the reason for the lower reading is a nice theoretical discussion but not germane to the issue. A leak causes a mis-reading of the pressure…
Very well stated. And, yes, this was a sophisticated microprocessor run unit. I agree that programming a pause would be important, and probably easily done. Now I am curious about what company manufactures this.
You guys are still confusing volume with pressure. The air in the tire cannot go above source pressure unless it was already there before the source was hooked up. You can play around with the Bernoulli effect all you want but air does not nor has ever moved from low pressure to a higher pressure. Thats simple physics and Bernoulli does not change that.
Edit: @oldnotdeadyet, I viewed the video you linked to and even it confirms that air flows from high pressure to low pressure. The scenario in the OP cannot have been caused by the pump at Costco and there is nothing dangerous in using those pumps unless you are a complete moron and set the air pressure WAY to high.
Keith. I am going to pull rank a bit and let you know I have a Ph.D. in physics and taught the Bernoulli principle for 18 years. Since it is counter-intuitive, it is difficult to teach it without pictures and words.
The thing you call “source pressure” would best be thought of as the pressure in the hose when the air is NOT moving. A big tire store wants to fill tires rapidly, so it fills a big tank plus the hose to 70-90 PSI. You use that hose with an on/off valve so that the pressure in the tire is correct. The valve is likely to also have a pressure gauge that is connected to the portion of the hose that is in-between the valve and the tire, and just as you say, this gauge will be at the tire pressure when there is no flow. When the valve is again opened, this gauge will have oddball readings depending on the pump pressure and the flow rate where it connects. I am recalling a tire valve/gauge where the gauge action was just like that of a pencil gauge, When the valve closed, the slider popped out. It was sucked back in when the flow was restarted. Obviously, it was connected to the “tire” side of the valve, not the side that is still connected to the 70 psi tank.
I realize that the Costco unit might be smart enough to measure the flow rate and have the pressure gauge calibrated to include the Bernoulli effect. But this would have the built-in assumption that the flow is going into the tire. If the connector is hissing, the unit would interpret that as a very low tire which needs lots of air to get up to pressure, and would keep pumping.
I used a pump at a Wawa a year ago, on the high pressure mini spare. Their pump would not go above 35 psi when I needed 60. They had set their tank pressure at 35 psi. That is how they got around the problem.
Then you should know two things here, Bernoulli has nothing to do with filling a tire. You should also know that air goes from high pressure to low pressure.
The valve stem in the tire has a one way valve in it called a Schrader valve and its cross sectional area is about half or less than that of the hose, the on/off valve and the section of the nozzle that contains the gauge. This is also past the point of the leak. There is no path for air to get around the Schrader valve so even though it kind of resembles a venturi, it does not act like one. That is why the Bernoulli affect does not apply here.
I kinda wonder what your Costco set up looks like. Our Costco does not have a public access air station, but they will fill your tires for free.
BTW, you won’t be the first Ph.D here to have made a mistake. I too make mistakes here but I own up to them, but so far I am not convinced that I am wrong. I don’t have a Ph.D but I do have an engineering degree and 20 ears experience working on aircraft so I am fully versed on the Bernoulli effect.
OK, I now understand why we are disagreeing, and I even suspect that your engineering experience beats my teaching experience. It would be nice if you would also recognize that the “symptoms” I saw at Costco were real, and that an explanation is needed. Since you have written in more detail about the problem, it is clear that you are focussed on the connection. That is not where the Bernoulli issue is. The issue is at the head of the machine. Since the pump might be working at 70 psi, measured when the air is static, the automatic machine needs to know when to shut off air flow so that the tire doesn’t go to 70 psi. When the air flow stops, the long hose air pressure equalizes with the tire, and this is the pressure that must be measured. So, a gauge must be connected to the piping in the machine, at a point after the shut-off valve. However, the piping and the hose itself are the venturi in question, and when the air is flowing, its pressure is reduced compared to it is stopped. So, the static air pressure can be measured at the pump end of the downstream hose, but the air needs to be static at this measurement time. So, the entire questions lies back at the head of the machine. Why did the hissing prevent the head from stopping the air flow, so that the pump kept running, allowing to 65 psi? Unless the air flow stops, the machine cannot measure the static pressure. And since the air flow is irregular, depending on how filled the tire is, whether the Schrader valve is fully open, or kinks in the hosing, whatever is going on at the head is dependent upon a closed connection at the other end of the hose so that a static reading is possible. Thus, it is the Bernoulli effect at the gauge in the head of the machine that prevents the machine from knowing what the static pressure is at the time that air is flowing. And, the hissing indicates that the air is always flowing, so pumping never stops.
Bernoulli in your carburetors: the air rushing through the throat of a carburetor, regulated by the throttle, reduces the pressure so that gas is sucked from the float bowl into the rushing air.
This is the same suction that pulls air out of a gauge attached to the piping in the tire pump head unit. Air flow must stop before the static pressure is known.
Think back over the years, 74 of them plus a few months, I have seen some stuff that did not make any sense, stuff that defied logic, so I can’t say you didn’t see what you think you saw. There is an explanation but I’m pretty confident that does not include Bernoulli.
BTW, most public access tire inflators that I have seen do not use an air tank, its a cheap AC powered pump much like the battery powered ones you can get and carry for yourself.
Edit: FWIW, when I was in the Navy, what you are describing as what happens to the gauge is called Motomotive force. I have not heard this term outside the Navy. Everyone in the Navy from the third butter cutter on the port chow line to the captain of a $20B Aircraft Carrier ($ includes the airwing) has to go through fire fighting school and basic damage control training. Part of that is the operation and maintenance of the P250 portable pump used for firefighting. It has a gas powered engine running a pump that pushes water through an (IIRC eductor) which uses the movement of a little water through a venturi to suck up a much greater volume of water, in this case 250 GPM at 125 psi. So I do understand what you are trying to say here, I just don’t think it applies to this case.
It is also the same principle as a jet pump. I had a house that used one of those for its water source and as soon as I bought it, the pump failed. Being in the Navy, I had very little money so I had to buy a rebuild kit and rebuild the pump myself.
Also, unless you were observing this guy from the time he pulled up to the pump, then you can’t be sure he didn’t do something else that would explain what you saw.
Your Navy example is perfect. It shows that the Bernoulli effect in a moving fluid can be quite strong. For the Navy case (or a similar unit a little less industrial in strength), think about the tube that is tee=ed off the venturi and runs down to the tank to suck up all that water. “Suck” is the critical word. If you took that tube out of the water and plugged the end with a pressure gauge, you would find suction that lowered the air pressure in the gauge. The pressure in the moving fluid is less than atmospheric, and the air in the gauge is sucked out until it matches that of the moving fluid.
For the tire pump example, the geometry would be the same. The gauge would be tee-ed off the piping and would experience the same suction, variably if the flow rate were changing. So, I need a gauge reading when the flow rate is zero.
Questions: How does the head unit on the tire pump know when to stop the pump? What happens if it doesn’t stop?
As far as the reliability of my observation, note that I stated that it was a Costco employee who stopped his truck to give the warning, a visual display about the tire pressure, and the employee knowing that the reduction of pressure was automatic.
Keith has gotten me to wonder about a different mechanism. Before I discuss it, you have to look at this short Bernoulli demo. It is very surprising to me.
This video seems to lead me to a different solution. Note that the Bernoulli effect in the video was basically due to a “leaky” connection that the professor used when blowing into the tube. The lowered air pressure in the high speed stream of air actually sucked in much, much more air, nearly filling the tube in one breath.
Is it possible that the leaking hiss at Costco was actually air going into the tire rather than air leaving the tire? Is that how it got to 65 psi? We need a cigarette smoker to smoke test the direction of the air flow at the hiss!
