I have a new watch that has a barometric pressure function and it says to set it properly you need to reference off of another barometric pressure setting that’s accurate. If I don’t go out and buy a specific purpose barometer but instead Google up the local weather and use the barometric inches from that would that be the same inside my house as outside if I’m trying to set a reference point?
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I would imagine it’s at least close, as long as you don’t have running an AC, (central) heat, bathroom exhaust, attic/roof fan, kitchen/stove exhaust etc. All of those things will lower the pressure in your house. I don’t know how much, but they will lower it. Similarly, if it’s really windy out, I the pressure outside should be lower than the pressure inside (right?).
But either way, you’ll be setting not to “outside” but to wherever the local station is. I don’t know enough about barometric pressure to know how much it changes over a given distance, but, for example, I wouldn’t calibrate a thermometer like that.
ETA, you might be able to go to your local news station’s website and get a ball park idea as to where a nearby weather station is and drive there. That should be better than sitting inside your house some random amount of miles away from it.
ETA2, you might even hit up their facebook page and ask, some news station’s weather team’s have their own FB page. If you check it out, you’ll be able to see, pretty quickly, if they interact with their ‘fans’. If they do, you could probably just ask them for advice. Even if it’s just something as simple as where you could drive to and what website to look at to calibrate it.
Without wanting to overstate the obvious - look up the pressure reading, them move outside to calibrate your watch?
It would be a very unusually airtight house that could maintain more than a tiny difference between inside & outside pressure.
If you want to check, see what altitude change your watch reports when you open your front door.
At industrial facilities I’ve worked in we had motor control centers that were kept at positive pressure to keep out corrosive gases. Using airlocks and multiple gas seals it required great effort to maintain a positive pressure of 0.1 inch water column, approximately 0.2 mm Hg. It would shock me if you could maintain even a tenth of this differential in a leaky house not designed for this purpose.
To get the pressure at a closer location, go to WeatherUnderground and input your zip/town/whatever. Scroll down to see a list of nearby personal weather stations. Find some that are close to you and at a similar altitude. You have to click on a station to see its barometric pressure. Check out a few to verify they are similar.
(Looking at a list of PWSes you can easily see that a few are outliers based on temp or humidity. Skip those.)
Regarding my above post (and this one), I don’t (didn’t) know what a normal pressure is or how much it changes or even what it would take to change it. Inputting some random zip codes for the east coast, west coast and mid west I get pressures of 30.04, 29.81 and 30.02. I’m getting about a hundredth of a degree change moving around my county (actually, one just actively dropped from 30.03 to 29.99).
Having said that, it probably is easiest to just look it up and set it based on that. But I’d still recommend going outside.
My post above was mostly based on exterior doors being noticeably harder to open and close on a windy day and interior doors doing the same thing when a central HVAC unit is running. It might not make any difference, but if it were my watch, I’d go outside and do it (and probably on a calm day so there isn’t a difference between you and the weather station).
Altitude is critical … I suggest actually driving to your nearest airport that has a weather station and set your watch to that “station pressure” (actual barometric pressure) … don’t use the “altimeter setting” (adjusted to sea level) …
If you know you’re at the same elevation as that airport and within a reasonable distance … then that may be close enough, not sure how accurate a wristwatch needs to be … I’m checking my local airport which is at 360 feet above sea level and that’s about 0.4" of Hg lower than what it would be at sea level …
Yes, altimeters were just barometers with different face plates … one didn’t fly at a specific altitude, rather one flies along a pressure shell …
IMO watchwolf49 pretty well nailed it. I’d much rather use NWS’s reports of pressure at their officially calibrated stations than Wunderground’s random assortment of whatever uncalibrated stuff some random somebodies cobbled together and put online.
The OP could look at NWS reports from airports and other official stations within a, say, 30 mile radius of him and note how much variation there is at any given time. Then ballpark an average of those and declare that to be his local outdoor pressure.
Note that official NWS readings are taken at 50 minutes after each hour and published sometime between 55 after and on the next hour. So for best precision the OP will want to do his calibration when the readings are fresh and not on a day of highly dynamic weather.
As a ballpark in aviation we figure that on mild days pressure changes 0.01" Hg to 0.03" Hg per hour and about the same per 100 miles. During aggressive weather or frontal passages it can be 10x that much.
As to indoors or out: I’d expect wind to be the biggest factor. On a 0 to low wind day the differential inside vs. outside of a typical house would be undetectable. On a blustery day the difference between windward outside, leeward outside, inside, and from moment to moment in any of those places, could easily be 0.02" Hg.
As to the gross range in the sea-level atmosphere: 30.60" Hg is about the max in an Arctic high, and 28.00" Hg is about the min in the center of a monster hurricane. WAG 29.75" Hg to 30.20" Hg would cover probably 95% of the hourly readings over a year in almost any US city. Again that’s local pressure corrected to sea-level. The actual local pressures depend on your local terrain elevation. The good news is that in any given locale that correction is a constant.
I don’t want to be too pedantic, but shouldn’t you guys be talking about Pascals or millibars rather than inches of mercury in a tube?
One can use whichever units are the local standard. You’re 100% right that Pascals have the best claim to capital-p Physics legitimacy.
OTOH, the OP liveth in the Unyted Ftates. That Realme moste strangye where the Benighted Fooles in charge persyst withall in using the Antique and Venerable Unyts of ye Olde Inchys of Quicksylver.
Confusesth them not with thy new-fangled Notions. For they be conservatyf of Minde and Foule of Moode.
Units are interconvertible. Aviation apparently uses mercury inches, despite sealed tube barometers being no longer in style. They also use feet for altitude and knots (a bastardized version of mph) for speed and pounds for fuel. And the best part is, since the USA was an early leader in aviation, it gets to force all the other country’s pilots to use the units and speak english on the radio.
Knot quite… I gather that Russia and China are metric, so pilots who don’t have modern switchable instrumentation have to do sums in their heads while coming in to land there.
Knots go back to the days when sail ruled the waves and are one minute of latitude per hour, which made good sense back then when you measured it by dropping a lump of wood over the side attached to a rope with knots in it and counting them over a set time.
Miles are a lot better of course…
After the Romans measured it as a thousand paces, QE1 decided that it was 5,280 feet in 1593.
I actually had a tube of mercury tied to a ruler in my lab.
If I mathed correctly, every difference of 0.01 torr will result in 2.2 pounds of force on an 8’ x 10’ wall.
I was wondering if a tiny difference like that would blow the house. It seems not.
Wow, someone actually posted using torrs … makes me feel like a youngster …
All that takes time. Pressure differences propagate at the speed of sound (duh) which is about 1 foot per millisecond. So if, say, a puff of wind blows in a crack in the house and creates a small (say couple cubic inch) region of higher pressure, that expands and dissipates across as 12x12x8 foot room in a mere 12 milliseconds.
Strong winds can apply large direct forces to flat plate areas like exterior walls or garage doors. The incremental contribution of interior/exterior pressure differences is negligible by comparison.
Posts 5 and 7.
Something practical missing from this thread. Why is the OP concerned? What does he want to do?
Barometric pressure measurements are most commonly used comparatively. That is, the person taking the measurements is usually only concerned about which way they are trending, not with exactly how accurately they are being measured.
If the OP is trying to monitor shifts, then it doesn’t matter how perfectly his watch barometer is calibrated. Not to mention, how accurate the watch barometer is designed to be, plays a part in what to do as well. If the barometer you are trying to calibrate isn’t all that sensitive and accurate to begin with, getting a precise calibration measurement isn’t really useful.
1 torr = 0.945 finkles per furlong per microfortnight per microfortnight