Waldo Posted November 28, 2016 Share Posted November 28, 2016 Hi, I have a cycle pump (upright, when a foot-plate you can stand on, and also has an air pressure gauge). If I pump up my car tyres, to say 32psi; then take my car to the petrol station to use their air machine; when I connect their nozzle to my tyres, it's showing as 37psi... Which one is the most accurate? Link to comment Share on other sites More sharing options...
Alcoblog Posted November 28, 2016 Share Posted November 28, 2016 Hi, I have a cycle pump (upright, when a foot-plate you can stand on, and also has an air pressure gauge). If I pump up my car tyres, to say 32psi; then take my car to the petrol station to use their air machine; when I connect their nozzle to my tyres, it's showing as 37psi... Which one is the most accurate? A mere human would require mental capabilities beyond present comprehension to answer a question such as that, categorically, without any further information. Are both pumps at the same height above sea level, is one better than the other, is one pump faulty, does the tyre leak in the journey between one pump and the other, is one pump made in China, are both pumps faulty, do little elves go and change the tyre pressure without you noticing, just for a giggle … stuff like that. The only possible clue you've given is the fact that you're uncertain as to what pressure you pump the tyres with regard to the cycle pump, as in your use of the word 'say' which indicates vagueness, or not always 32psi, whereas you state it reads specifically as 37psi at the garage. If this is the case, I'd suggest there's fairly strong evidence for the garage pump to be faulty. Tricky one this. Link to comment Share on other sites More sharing options...
Bargepole23 Posted November 28, 2016 Share Posted November 28, 2016 Hi, I have a cycle pump (upright, when a foot-plate you can stand on, and also has an air pressure gauge). If I pump up my car tyres, to say 32psi; then take my car to the petrol station to use their air machine; when I connect their nozzle to my tyres, it's showing as 37psi... Which one is the most accurate? Is there any requirement for garage forecourt pumps to be regularly calibrated? If so, then I would suggest that the garage one is more accurate....assuming of course, that you don't get your cycle pump calibrated annually. Link to comment Share on other sites More sharing options...
Gamston Posted November 28, 2016 Share Posted November 28, 2016 Hi, I have a cycle pump (upright, when a foot-plate you can stand on, and also has an air pressure gauge). If I pump up my car tyres, to say 32psi; then take my car to the petrol station to use their air machine; when I connect their nozzle to my tyres, it's showing as 37psi... Which one is the most accurate? Petrol station is my guess. If you have any doubts, then check the pressure at more than one petrol station. Those upright pumps are good. A few months ago I used one on my spare wheel and discovered it was over inflated, when I checked the pressure at my local petrol station. Link to comment Share on other sites More sharing options...
smithy266 Posted November 28, 2016 Share Posted November 28, 2016 Maybe neither. Try a tnird gauge! Link to comment Share on other sites More sharing options...
Mr Bloke Posted November 28, 2016 Share Posted November 28, 2016 Hi, I have a cycle pump (upright, when a foot-plate you can stand on, and also has an air pressure gauge). If I pump up my car tyres, to say 32psi; then take my car to the petrol station to use their air machine; when I connect their nozzle to my tyres, it's showing as 37psi... Which one is the most accurate? Hmmm... ... well, being one of the more observant forum posters, I happened to spot a clue in the thread title and decided to do a bit of googling in-depth research into the air retention properties of pneumatic wheel coverings... so I may be able to help you. A cycle pump, as the more astute of you may realise, is obviously calibrated to be accurate over the range of air pressures normally associated with a cycle tyre. That could be anything from 50psi to well over 100psi depending on the tyre width and weight of the rider. Even a low pressure mountain bike tyre will rarely be below 35psi... punctures expected excepted (subject to citation from cyclists). Now I realise psi might be Greek to a lot of you, but it is just a scale of pounds per square inch that we armchair scientists like to use to measure how hard something is. As fellow scientists will know, most pressure gauges work by the air in your tyre pushing a piston which pushes a spring which turns the pointy thing that you read on the scale. I've obviously simplified that bit to cater for all forum proficiencies. Anyway my point is, a gauge that is calibrated for accurate readings in the 50psi to 100psi range might not be as accurate at reading pressures outside that range due to its spring not being up to the job. I therefore conclude that an air pressure gauge at a petrol station which was specifically designed for use in inflating car tyres is the more accurate... ... unless it's broken? Link to comment Share on other sites More sharing options...
Alcoblog Posted November 28, 2016 Share Posted November 28, 2016 Hmmm... ... well, being one of the more observant forum posters, I happened to spot a clue in the thread title and decided to do a bit of googling in-depth research into the air retention properties of pneumatic wheel coverings... so I may be able to help you. A cycle pump, as the more astute of you may realise, is obviously calibrated to be accurate over the range of air pressures normally associated with a cycle tyre. That could be anything from 50psi to well over 100psi depending on the tyre width and weight of the rider. Even a low pressure mountain bike tyre will rarely be below 35psi... punctures expected excepted (subject to citation from cyclists). Now I realise psi might be Greek to a lot of you, but it is just a scale of pounds per square inch that we armchair scientists like to use to measure how hard something is. As fellow scientists will know, most pressure gauges work by the air in your tyre pushing a piston which pushes a spring which turns the pointy thing that you read on the scale. I've obviously simplified that bit to cater for all forum proficiencies. Anyway my point is, a gauge that is calibrated for accurate readings in the 50psi to 100psi range might not be as accurate at reading pressures outside that range due to its spring not being up to the job. I therefore conclude that an air pressure gauge at a petrol station which was specifically designed for use in inflating car tyres is the more accurate... ... unless it's broken? A reasonable hypothesis, but fundamentally flawed as you seem to have failed to take into consideration the ambient air temperature of both pump locations, as well as relative humidity (I'm not talking about someone's uncle having just got out of the bath here, before you try and throw confusion into the mix). Link to comment Share on other sites More sharing options...
Mr Bloke Posted November 28, 2016 Share Posted November 28, 2016 A reasonable hypothesis, but fundamentally flawed as you seem to have failed to take into consideration the ambient air temperature of both pump locations, as well as relative humidity (I'm not talking about someone's uncle having just got out of the bath here, before you try and throw confusion into the mix). Hmmm... ... you make a valid point Mr Alco. As we know, you should always check your tyre pressures when your tyres are cold. Once you've been driving for any length of time the temperature of your tyres soon increases and the heated air inside the tyres expands and as a result its pressure rises quickly. As the OP fails to tell us how far he drives to get to the petrol station then I suppose it's just possible that both tyre gauges are in fact correct, but by the time the OP gets to the petrol station the tyre pressure has increased by 5psi. Link to comment Share on other sites More sharing options...
Obelix Posted November 28, 2016 Share Posted November 28, 2016 Hmmm... ... you make a valid point Mr Alco. As we know, you should always check your tyre pressures when your tyres are cold. Once you've been driving for any length of time the temperature of your tyres soon increases and the heated air inside the tyres expands and as a result its pressure rises quickly. As the OP fails to tell us how far he drives to get to the petrol station then I suppose it's just possible that both tyre gauges are in fact correct, but by the time the OP gets to the petrol station the tyre pressure has increased by 5psi. I'd have said that's entirely likely. The tyre pressure can rise by as much as 8psi after a fast drive which is why you should check them of course when cold as you note. Link to comment Share on other sites More sharing options...
buddysbuddy Posted November 28, 2016 Share Posted November 28, 2016 Hi, I have a cycle pump (upright, when a foot-plate you can stand on, and also has an air pressure gauge). If I pump up my car tyres, to say 32psi; then take my car to the petrol station to use their air machine; when I connect their nozzle to my tyres, it's showing as 37psi... Which one is the most accurate? I was told years ago that if you run your car this will affect tyre pressure readings as the heat generated on the tyres causes the air in the tyres to expand therefore increasing the pressure reading. I was told that as far as possible the pressure should be checked when the tyres were cool. 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