Mystery of tire air pumps at gas stations....

[Trujillo]

If you go to a gas (petrol) station and put air in your tires, the first reading seems to indicate what pressure is currently in the tire (before the pump starts). 

 

So, when you hit your target air pressure -- let's say 32 psi -- the pump beeps and you remove the air chuck from the tire stem. Okay. The tire ought to have the pressure now that you just put in and was recorded on the digital display, right? 

But if you immediately put the chuck back on the tire stem, the display reads far less (like 27 psi) than it just did when the tire was "full" to the pressure you programed in. Shouldn't the tire read the same as what it just registered immediately after you finished "filling" it? 

 

Mystery....

[ExpatOilWorker]

9 minutes ago, Trujillo said:

If you go to a gas (petrol) station and put air in your tires, the first reading seems to indicate what pressure is currently in the tire (before the pump starts). 

 

So, when you hit your target air pressure -- let's say 32 psi -- the pump beeps and you remove the air chuck from the tire stem. Okay. The tire ought to have the pressure now that you just put in and was recorded on the digital display, right? 

But if you immediately put the chuck back on the tire stem, the display reads far less (like 27 psi) than it just did when the tire was "full" to the pressure you programed in. Shouldn't the tire read the same as what it just registered immediately after you finished "filling" it? 

 

Mystery....

 

That is a heavy mystery to solve on a Sunday morning. 

I have checked with a digital pressure gauge and always get close to the final 32 psi.

Have u observed this at more than one filling station?

 

[ChakaKhan]

I would trust those thai pumps as much as I would trust a roomful of bargirls with my new iphone....

 

always use your own gauge after....and double up with regards to the bargirls ????

[Dante99]

You are missing the point by focusing on what the machine says.  The actual pressure in your tire is what is important.

[pseudorabies]

Maybe there is a choke point between where the pressure is being measured (upstream of the chuck) and the inside of the tire?  Maybe the chuck isn't depressing the valve stem pin all the way?  This would cause pressure to build upstream and give an artificially high pressure during filling.

 

[Dante99]

4 minutes ago, pseudorabies said:

Maybe there is a choke point between where the pressure is being measured (upstream of the chuck) and the inside of the tire?  Maybe the chuck isn't depressing the valve stem pin all the way?  This would cause pressure to build upstream and give an artificially high pressure during filling.

 

Or maybe the dodo put the chuck on sloppily and lost 5 pounds of pressure in the process.

 

It might be informative to know what happened after the 27psi reading.  Did the machine proceed to add air until reaching a reading of 32 or did the machine just immediately beep indicating that it was already at 32?  Or was the chuck removed quickly so Tujii did not find out?

[beau thai]

I always fill beyond what I need then check with my own good manual gauge and release some air if necessary. Not only in Thailand.

[Trujillo]

Of course with external gauges, you have to wonder if they themselves are accurate. 

 

 

[Thailand]

24 minutes ago, Trujillo said:

Of course with external gauges, you have to wonder if they themselves are accurate. 

 

 

Couldn't you just buy a gauge checker to check the gauge you use to check the pressure from the suspect air pump?

[HashBrownHarry]

42 minutes ago, Trujillo said:

Of course with external gauges, you have to wonder if they themselves are accurate. 

 

 

you mean the 99 baht ones of Lazada are not reliable?

[Trujillo]

I think people are misunderstanding...

 

What is happening is that the pump fills the tire to 32. And then when you remove the chuck from the tire and immediately put it back on the stem, the reading is less than 32. Regardless whether the gauge itself is accurate, it should read the same as you filled it immediately afterwards as well. But it doesn't. No filling station does. So there's some design thing happening I am not aware of. 

 

"You mean the 99 baht ones (tire pressure gauges) of Lazada are not reliable?"

 

Of course I don't mean that. Anything that costs 99 baht is 100% reliable and accurate. 

Edited December 14, 2020 by Trujillo

[amexpat]

On 12/13/2020 at 8:45 AM, ChakaKhan said:

I would trust those thai pumps as much as I would trust a roomful of bargirls with my new iphone....

 

always use your own gauge after....and double up with regards to the bargirls ????

Yawn, barstool wisdom.  I use the pumps at multiple stations and check it with my gauge. Always consistent all around. 

Can't help you with the bargirls. 

[richard_smith237]

Its obviously due to the thermal expansion of the air you are pumping in your tyres.... 

 

Are there any of those air-pumps which inflate your tyres with nitrogen ????

 

Trans... over to you... 

[Techno Viking]

On 12/13/2020 at 4:33 PM, beau thai said:

I always fill beyond what I need then check with my own good manual gauge and release some air if necessary. Not only in Thailand.

 

Same here, I inflate to 40 psi and then bring them back down to what I desire with my own gauge.

[Techno Viking]

12 minutes ago, richard_smith237 said:

 

 

Are there any of those air-pumps which inflate your tyres with nitrogen ????

 

 

Typically not at petrol stations, B-Quick and Tyreplus have them if thats your kink.

[elektrified]

I've double-checked numerous times with a decent quality gauge I have, and it is always very close - if not spot on - with the reading at the filling stations.

[Dante99]

1 hour ago, elektrified said:

I've double-checked numerous times with a decent quality gauge I have, and it is always very close - if not spot on - with the reading at the filling stations.

+1 

[Andrew Dwyer]

People still not getting what the op is saying :

 

He pressurised the tyre to 32 psi then, for some reason, stuck the hose back on and the reading was back down to 27 psi !!

What happened next we don’t know:

Did it pump a fair amount of air in again to get to 32 psi ?

or

Did it jump back up rapidly to get to 32 psi ?

 

The former would be somewhat of a mystery and the latter, although strange, acceptable.

 

I personally bought a gauge from Lotus and check the temp before replacing the dust cap, always found them to be pretty accurate despite the connection on the end of the hose looking like it has been run over a few times !

[richard_smith237]

9 minutes ago, Andrew Dwyer said:

People still not getting what the op is saying :

 

He pressurised the tyre to 32 psi then, for some reason, stuck the hose back on and the reading was back down to 27 psi !!

What happened next we don’t know:

Did it pump a fair amount of air in again to get to 32 psi ?

or

Did it jump back up rapidly to get to 32 psi ?

 

The former would be somewhat of a mystery and the latter, although strange, acceptable.

 

I personally bought a gauge from Lotus and check the temp before replacing the dust cap, always found them to be pretty accurate despite the connection on the end of the hose looking like it has been run over a few times !

 

The Ops initial explanation was perfectly understandable. I’m not sure how some have become confused or misunderstood. 

 

The Op also indicated that this anomaly also occurs with air-pumps at other filling stations [no filling station does], which is why he is wondering if this is a design issue or technical issue or possibly even a calibration flaw. 

 

 

Personally: I’m no so sure. I pump up my tires, do so a month later etc and the pressures are similar, having lost just a couple of psi. That said - its definitely something worth checking and wouldn’t do any harm to perform a ‘double check’.

 

I’ll  go with the ‘double tap’ next time I put air in the tires - just to see if this happens. 

 

 

 

[Trujillo]

What does happen in all these stations is that the pump usually will fill the tires higher than what you programmed in by a few pounds, and then it releases air to the designated number. You can hear it (and see this on the display). 

 

A: When I immediately put the chuck back on the tire that was reading 2 seconds ago 32psi, and it shows 27, it does kick in to fill with more air up to 32. 

[Thailand]

Thought I had a puncture but the tyre/tire was only flat at the bottom so I guess it's OK!

[ChipButty]

My local gas station has a airline but no gauge, so think yourselves lucky

[MadMac]

And then you have the lady parking at the pump, looking in the mirror. lipstick, messing around with the car she lives in....

[weelegs]

I love reading you guys on subjects like automotive repair, light bulbs, home renovation. You are experts! 

 

On the epidemiology of SARS-CoV-2 (the Covid-19 virus). not so much.

                                                                                                                                           ????

[boonchu]

I filled my tyres with nitrogen, 400bt first time after that free top ups. I just drive to the tyre shop where I filled the tyres and get the free tyre pressure check and free top up of nitrogen. Don't even have to get out of the car.

[Russell17au]

Ok, this is what happens when you use the tyre inflators at the service stations. You set the pressure that you want the tyre and when you connect the hose to your tyre the pump will inflate your tyre to the recorded pressure. But when you disconnect the hose, the air in the hose depressurises because the shut off valve is where the gauge is. This means that if you reconnect the hose to your tyre then the air pressure will equalise back through the hose to the gauge valve, so you get a lose of air pressure in your tyre. If the pressure valve is at the end of the hose like in many western countries then the hose is continually at the right pressure and will not re-pressurise from the tyres pressure. The first figure that you see when you connect the hose to your tyre is not the correct pressure that is in your tyre, it is lower by a couple of PSI because your tyre has pressurised the hose back to the valve in the machine which could be 6 metres of hose that your tyre has pressurised. 

[Jaxxper]

I don’t use petrol station air pumps because it’s more convenient to do at home with my electric tyre air compressor. Always found the gauge on the compressor to be a bit hit and miss though,  so I overfill by a couple of PSI, and then bleed down to the correct pressure verified by an analogue pressure gauge.

[Dante99]

19 hours ago, Andrew Dwyer said:

I personally bought a gauge from Lotus and check the temp before replacing the dust cap,

It is great you check the temp but why is that releveant to this thread about pressure?

[Dumbastheycome]

Is it not possible that if tyres /tires are under  optimal pressure will be  running hot so inflating them may provide some cooling which would result in a  small pressure drop? Recommended pressures provided often give hot and cold figures.

[Thujone]

I use a footpump and a gauge.

(I've got two gauges, different models, which both give the same result, so I assume they are accurate).

I might be a dinosaur, but I'm a fit dinosaur!

 

But this isn't relevant to the OP...