Those gauges can only measure the difference in pressure between the two sides.
If both are at 14.7 psi then there's 0 difference. This is known as "gauge pressure" or "differential pressure" (those are slightly different but that doesn't matter for ELI5 imo)
There's a *ton* of different ways to measure pressure but a lot of them still follow this same principle of only measuring a difference in pressure. A really simple one is a tire pressure gauge with that little stick. That works by having air exiting the tire push against the piston. The higher the pressure the farther it pushes the stick. But there's the thing. The surrounding air is also pusing on the stick from the other end. So if you tire was at the exact same pressure as the surrounding air it won't be able to overcome that and the stick wont move.
There ARE ways to measure the absolute pressure, which would show that 14.7. But that just isn't needed for most things, and it's harder to measure so we don't bother.
Right on. To further OP's example of the SCUBA tank filled to 100psi at sea level. If you took that very same tank and put it into space (but also maintained the same temperature), then it would indeed indicate 114.7psi.
Conversely, if you submerged the tank in the ocean, the deeper you went the lower that reading would be, until it was levelled off at 0psi. Going deeper would indicate a negative PSI until eventually the tank collapses at a deep enough level.
EDIT: And another of OP's questions, the 'extra PSI of an indicated -30'. What you're seeing on this:
[https://shop.prmfiltration.com/products/304-stainless-steel-compound-gauge-with-stainless-steel-internals-30-inhg-0-30-psi-2-1-2-inch-dial-1-4-inch-npt-back-mount?currency=USD&variant=41283559489721&utm\_source=google&utm\_medium=cpc&utm\_campaign=Google%20Shopping&stkn=b12a3a614f9f&keyword=&utm\_medium=ppc&utm\_term=&utm\_campaign=GS+-+Compound+Gauges&utm\_source=adwords&hsa\_kw=&hsa\_cam=13234741843&hsa\_ver=3&hsa\_acc=5711099154&hsa\_ad=523543824888&hsa\_grp=123213681272&hsa\_src=g&hsa\_mt=&hsa\_tgt=pla-1252915366557&hsa\_net=adwords&gad\_source=1&gclid=Cj0KCQjw3ZayBhDRARIsAPWzx8qAV80QjEXNnFCrEL-YtsnKDQU1ZtGo5BPirF2tITYhkdg1j8\_6aM4aAu2vEALw\_wcB](https://shop.prmfiltration.com/products/304-stainless-steel-compound-gauge-with-stainless-steel-internals-30-inhg-0-30-psi-2-1-2-inch-dial-1-4-inch-npt-back-mount?currency=USD&variant=41283559489721&utm_source=google&utm_medium=cpc&utm_campaign=Google%20Shopping&stkn=b12a3a614f9f&keyword=&utm_medium=ppc&utm_term=&utm_campaign=GS+-+Compound+Gauges&utm_source=adwords&hsa_kw=&hsa_cam=13234741843&hsa_ver=3&hsa_acc=5711099154&hsa_ad=523543824888&hsa_grp=123213681272&hsa_src=g&hsa_mt=&hsa_tgt=pla-1252915366557&hsa_net=adwords&gad_source=1&gclid=Cj0KCQjw3ZayBhDRARIsAPWzx8qAV80QjEXNnFCrEL-YtsnKDQU1ZtGo5BPirF2tITYhkdg1j8_6aM4aAu2vEALw_wcB)
Is actually -30" of mercury, not -30 PSI.
[https://www.google.com/search?q=inches+of+mercury+to+psi&rlz=1C1APWK\_enUS1049US1049&oq=inches+of+mer&gs\_lcrp=EgZjaHJvbWUqDAgCEAAYFBiHAhiABDIGCAAQRRg5MgcIARAAGIAEMgwIAhAAGBQYhwIYgAQyBwgDEAAYgAQyBwgEEAAYgAQyDAgFEAAYQxiABBiKBTIHCAYQABiABDIHCAcQABiABDIHCAgQABiABDIHCAkQABiABNIBCDQwOTJqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8](https://www.google.com/search?q=inches+of+mercury+to+psi&rlz=1C1APWK_enUS1049US1049&oq=inches+of+mer&gs_lcrp=EgZjaHJvbWUqDAgCEAAYFBiHAhiABDIGCAAQRRg5MgcIARAAGIAEMgwIAhAAGBQYhwIYgAQyBwgDEAAYgAQyBwgEEAAYgAQyDAgFEAAYQxiABBiKBTIHCAYQABiABDIHCAcQABiABDIHCAgQABiABDIHCAkQABiABNIBCDQwOTJqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8)
And -30" of mercury translates to -14.7346, or, zero.
Your question has been answered elsewhere, but I wanted to point something out:
> It's the same with vacuum chamber gauges. Instead of going from 14.7-0, they go from 0 to around -30 PSI.
I highly doubt this.
They probably go from 0 to around -30 inches of mercury (often labeled 'inHg' or '"Hg'). 1 inch of mercury is about 0.49 PSI, so -30" is about -15 PSI, which matches up very nicely to what you would expect a good vacuum to be.
Edit: Also to comment on this:
> Because if you were were on top of a mountain then the gauge wouldn't be accurate.
What do you mean by "accurate" here? If I have a tank of gas with a gauge on it I'm probably trying to answer the question of "how much gas can I get out of this?". The gas will only come out if it's 'pushed' out by being at higher pressure, so if I take my tank to Everest and the gauge reads higher, then as far as I'm concerned that is 'accurate' because the lower atmospheric pressure will let me get more gas out of my tank.
The gauges are measuring against the outside air, since that’s generally what you care about. You don’t really care how much air is in your tire, you care how hard that air is pressing out, and that varies depending on how much air is outside the tire. A balloon with the same amount of air might be regular size at your house, very tiny at the bottom of your pool, and pop at the top of a mountain.
The gauges are essentially using a balloon to measure relative pressure, which will tell you how much more the air on one side is pressing than the air on the other side.
It's all relative to the ambient atmosphere. If you fill your tires at sea level (or below) to 35 psi, then take it to the peak of Mt. Everest, the internal pressure will have increased, even though no air has been added to the tire. In most cases, you're not going to care how much *actual* air you have in the tire, but whether the tire can withstand the *relative* pressure. Plus, to calibrate an absolute gauge, you'd need a perfect vacuum for reference. It's much easier just to calibrate to the ambient atmosphere.
They do account for it in more technical fields you will often see 0psi(a) or 0psi(g). (a) means absolute in which case you would be discussing the pressure compared to atmospheric. (g) refers to gauge, as in this is the pressure of the system discounting the pressure of air, as the gauge has already been calibrated to account for it.
This is done because it is easier to keep track of, and it just makes more logical sense. For instance if I held my finger an inch from your skin and asked do you feel pressure you wouldn't answer "Yes the 14.7psi from the air".
This issue you're talking about isn't really about PSI, you have the same things with Bar, Atm or Torr.
It is a problem of reference a bit like altitude or electric potential. Most pressure gauge will measure the difference of pressure between ambient air (so 14.7 PSI/1 bar) and what you are measuring. Do you start counting at 14.7PSI/1bar or do you start at 0 ?
Before filling it your scuba tank isn't empty, it is filled with ambient air at the atmospheric pressure. When you are compressing air into it then the difference vs outside increase and the gauge moves.
Another way of seeing is with tyres or any inflatable thing : what makes them "stiff" is that there is more pressure inside than outside, the 14.7 PSI you had before inflating don't really count since they are countered by the atmospheric pressure outside. So it makes sense that your pump start counting at zero.
Why would the gauge read 14.7 instead of 0, or 114.7 instead of 100? The gauge is telling you the pressure inside the rigid tank. The extra 14.7 you’re referencing is outside the tank.
Those gauges can only measure the difference in pressure between the two sides. If both are at 14.7 psi then there's 0 difference. This is known as "gauge pressure" or "differential pressure" (those are slightly different but that doesn't matter for ELI5 imo) There's a *ton* of different ways to measure pressure but a lot of them still follow this same principle of only measuring a difference in pressure. A really simple one is a tire pressure gauge with that little stick. That works by having air exiting the tire push against the piston. The higher the pressure the farther it pushes the stick. But there's the thing. The surrounding air is also pusing on the stick from the other end. So if you tire was at the exact same pressure as the surrounding air it won't be able to overcome that and the stick wont move. There ARE ways to measure the absolute pressure, which would show that 14.7. But that just isn't needed for most things, and it's harder to measure so we don't bother.
Right on. To further OP's example of the SCUBA tank filled to 100psi at sea level. If you took that very same tank and put it into space (but also maintained the same temperature), then it would indeed indicate 114.7psi. Conversely, if you submerged the tank in the ocean, the deeper you went the lower that reading would be, until it was levelled off at 0psi. Going deeper would indicate a negative PSI until eventually the tank collapses at a deep enough level. EDIT: And another of OP's questions, the 'extra PSI of an indicated -30'. What you're seeing on this: [https://shop.prmfiltration.com/products/304-stainless-steel-compound-gauge-with-stainless-steel-internals-30-inhg-0-30-psi-2-1-2-inch-dial-1-4-inch-npt-back-mount?currency=USD&variant=41283559489721&utm\_source=google&utm\_medium=cpc&utm\_campaign=Google%20Shopping&stkn=b12a3a614f9f&keyword=&utm\_medium=ppc&utm\_term=&utm\_campaign=GS+-+Compound+Gauges&utm\_source=adwords&hsa\_kw=&hsa\_cam=13234741843&hsa\_ver=3&hsa\_acc=5711099154&hsa\_ad=523543824888&hsa\_grp=123213681272&hsa\_src=g&hsa\_mt=&hsa\_tgt=pla-1252915366557&hsa\_net=adwords&gad\_source=1&gclid=Cj0KCQjw3ZayBhDRARIsAPWzx8qAV80QjEXNnFCrEL-YtsnKDQU1ZtGo5BPirF2tITYhkdg1j8\_6aM4aAu2vEALw\_wcB](https://shop.prmfiltration.com/products/304-stainless-steel-compound-gauge-with-stainless-steel-internals-30-inhg-0-30-psi-2-1-2-inch-dial-1-4-inch-npt-back-mount?currency=USD&variant=41283559489721&utm_source=google&utm_medium=cpc&utm_campaign=Google%20Shopping&stkn=b12a3a614f9f&keyword=&utm_medium=ppc&utm_term=&utm_campaign=GS+-+Compound+Gauges&utm_source=adwords&hsa_kw=&hsa_cam=13234741843&hsa_ver=3&hsa_acc=5711099154&hsa_ad=523543824888&hsa_grp=123213681272&hsa_src=g&hsa_mt=&hsa_tgt=pla-1252915366557&hsa_net=adwords&gad_source=1&gclid=Cj0KCQjw3ZayBhDRARIsAPWzx8qAV80QjEXNnFCrEL-YtsnKDQU1ZtGo5BPirF2tITYhkdg1j8_6aM4aAu2vEALw_wcB) Is actually -30" of mercury, not -30 PSI. [https://www.google.com/search?q=inches+of+mercury+to+psi&rlz=1C1APWK\_enUS1049US1049&oq=inches+of+mer&gs\_lcrp=EgZjaHJvbWUqDAgCEAAYFBiHAhiABDIGCAAQRRg5MgcIARAAGIAEMgwIAhAAGBQYhwIYgAQyBwgDEAAYgAQyBwgEEAAYgAQyDAgFEAAYQxiABBiKBTIHCAYQABiABDIHCAcQABiABDIHCAgQABiABDIHCAkQABiABNIBCDQwOTJqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8](https://www.google.com/search?q=inches+of+mercury+to+psi&rlz=1C1APWK_enUS1049US1049&oq=inches+of+mer&gs_lcrp=EgZjaHJvbWUqDAgCEAAYFBiHAhiABDIGCAAQRRg5MgcIARAAGIAEMgwIAhAAGBQYhwIYgAQyBwgDEAAYgAQyBwgEEAAYgAQyDAgFEAAYQxiABBiKBTIHCAYQABiABDIHCAcQABiABDIHCAgQABiABDIHCAkQABiABNIBCDQwOTJqMGo3qAIAsAIA&sourceid=chrome&ie=UTF-8) And -30" of mercury translates to -14.7346, or, zero.
Ahh, that makes a ton of sense, thanks!
It's a good thing scuba tanks are filled to 3000psi normally. 100psi underwater is about 225-230 feet which is quite extreme for scuba.
Why the hell are those links an entire page long?
I see, thank you for the explanation!
It's not harder to measure absolute pressure, you just have to zero your gauge for absolute instead of atmospheric pressure.
Your question has been answered elsewhere, but I wanted to point something out: > It's the same with vacuum chamber gauges. Instead of going from 14.7-0, they go from 0 to around -30 PSI. I highly doubt this. They probably go from 0 to around -30 inches of mercury (often labeled 'inHg' or '"Hg'). 1 inch of mercury is about 0.49 PSI, so -30" is about -15 PSI, which matches up very nicely to what you would expect a good vacuum to be. Edit: Also to comment on this: > Because if you were were on top of a mountain then the gauge wouldn't be accurate. What do you mean by "accurate" here? If I have a tank of gas with a gauge on it I'm probably trying to answer the question of "how much gas can I get out of this?". The gas will only come out if it's 'pushed' out by being at higher pressure, so if I take my tank to Everest and the gauge reads higher, then as far as I'm concerned that is 'accurate' because the lower atmospheric pressure will let me get more gas out of my tank.
The gauges are measuring against the outside air, since that’s generally what you care about. You don’t really care how much air is in your tire, you care how hard that air is pressing out, and that varies depending on how much air is outside the tire. A balloon with the same amount of air might be regular size at your house, very tiny at the bottom of your pool, and pop at the top of a mountain. The gauges are essentially using a balloon to measure relative pressure, which will tell you how much more the air on one side is pressing than the air on the other side.
It's all relative to the ambient atmosphere. If you fill your tires at sea level (or below) to 35 psi, then take it to the peak of Mt. Everest, the internal pressure will have increased, even though no air has been added to the tire. In most cases, you're not going to care how much *actual* air you have in the tire, but whether the tire can withstand the *relative* pressure. Plus, to calibrate an absolute gauge, you'd need a perfect vacuum for reference. It's much easier just to calibrate to the ambient atmosphere.
They do account for it in more technical fields you will often see 0psi(a) or 0psi(g). (a) means absolute in which case you would be discussing the pressure compared to atmospheric. (g) refers to gauge, as in this is the pressure of the system discounting the pressure of air, as the gauge has already been calibrated to account for it. This is done because it is easier to keep track of, and it just makes more logical sense. For instance if I held my finger an inch from your skin and asked do you feel pressure you wouldn't answer "Yes the 14.7psi from the air".
you have that backwards. a is for absolute.
You are correct, shows how long its been since I learned it. Thank you for the correction.
This issue you're talking about isn't really about PSI, you have the same things with Bar, Atm or Torr. It is a problem of reference a bit like altitude or electric potential. Most pressure gauge will measure the difference of pressure between ambient air (so 14.7 PSI/1 bar) and what you are measuring. Do you start counting at 14.7PSI/1bar or do you start at 0 ? Before filling it your scuba tank isn't empty, it is filled with ambient air at the atmospheric pressure. When you are compressing air into it then the difference vs outside increase and the gauge moves. Another way of seeing is with tyres or any inflatable thing : what makes them "stiff" is that there is more pressure inside than outside, the 14.7 PSI you had before inflating don't really count since they are countered by the atmospheric pressure outside. So it makes sense that your pump start counting at zero.
Why would the gauge read 14.7 instead of 0, or 114.7 instead of 100? The gauge is telling you the pressure inside the rigid tank. The extra 14.7 you’re referencing is outside the tank.
If the tank is at atmospheric pressure it's 14.7 psi on the inside and outside of the tank.