iPhone 6 Plus barometer in NYC Subway

[Javier]

I wanted to see the elevation of the City Hall Loop using the 6's new barometer on the iPhone 6 Plus. When it entered the loop, it started showing me that we were going up for some reason, and when we entered city hall, the barometer immediately dropped down and then slowly fell back into place when we reached the station.

 

Does anyone know if this elevation is true or not?

[Kamen Rider]

A barometer is not an altimeter!

[Javier]

Oh s*it. Dam I'm so stupid.

[Tokkemon]

Barometer measures air pressure, not altitude. Yes, they can be roughly correlated, but not at the level you're talking about.

[YankeesPwnMets]

Well, as altitude goes up , atmospheric pressure decreases, and as altitude goes down, atmospheric pressure goes up. However, when you are only talking about a couple of feet, it isn't really that accurate to use a barometer to try and estimate altitude.

[itmaybeokay]

I actually have a barometer app on my 6 that displays the output as a graph of "relative change" in feet over time. I thought it would be fun to try to use one day going through one of the under-river tubes to see if I could generate a graph of its upgrade and downgrade. 

 

Needless to say - it don't work like that. 

 

As was mentioned, a barometer is not an altimeter - not on it's own at least. You can use a barometer as an altimeter if you're in an area of relatively stable air pressure and you know the barometric pressure at sea level. 

 

As anyone who's rode in the head car through an under-river tube can tell you - the tube is not an area of stable pressure. The train basically acts like a piston and generates a column of air in front of it - stand at the east end of the BMT platform at Lex/59 when a train is approaching to see what I mean - you can feel a strong unrelenting wind long before you can even see the lights of the train. From a physics perspective, the train is compressing the air in front of it as it goes through the tube, and the higher pressure air created is rushing into the lower pressure air in front of it and so it continues all the way down the tube until there's a vent-point, at a station, ventilation facility, or even emergency exit. 

 

My attempt to use the altimeter for that trip through the canarsie tube didn't plot the grade of the tunnel, but it was interesting to see the swings of air pressure!

 

I'm guessing you were in the back of the train going through the loop. Same way the piston effect generates compression of air at the front of the train, the back of the train creates low pressure as air rushes in to fill the space where the train was. Running through the relatively constrained tunnel - you probabaly had a small piston effect going on accounting for it showing the altitude rising. Enter city hall loop, and now the space is too big for the train to act like a piston - the air pressure normalizes. 

 

Science!

[Kamen Rider]

Adding to the SCIENCE! Since I'm stuck waiting with my Grandmother at the eye doctor...

 

Barometers and Altimeters are related devices, much in the same way a cell phone is related to a Walkie-talkie.

 

While both use pressure, the Barameter is not calibrated the same way. An altimeter displays feet above sea level in increments of 100, so a plane flying at "Flight level 350" is flying at 35,000 feet.

 

Barometers you usually hear about on the news display inches of mercury, forced up a vacuume tube by downward pressure of the atmosphere.

 

The real problem with your little experiment is you are underground (and therefore inside) and on top of that inside a climate controled train car. You are not going to get the correct readings.