Author Topic: Atmospheric pressure: absolute to relative conversion  (Read 13343 times)

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Offline kickroot

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Atmospheric pressure: absolute to relative conversion
« on: April 29, 2013, 10:34:01 AM »
Hey guys,
I'm writing some software to read and post wx data via USB from my Ambient 2090.  According to the docs the barometer field reports absolute pressure. How can I convert this to relative pressure for my location? All I need is a formula, yet searching google seems to bring nothing.

Offline SoMDWx

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Re: Atmospheric pressure: absolute to relative conversion
« Reply #1 on: April 29, 2013, 11:51:35 AM »
Try these:

Station Pressure =  Ps = Altimeter in Inches * ((288 - 0.0065 * Elevation in Meters)/288)^5.2561
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Altimeter Setting =  As = (Station Pressure in MB - 0.3) * (1 + (((1013.25^0.190284 * 0.0065)/288) * (Elevation in Meters/(Station Pressure in MB -0.3)^0.190284)))^(1/0.190287)
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Sea Level Pressure =  SLP = Station Pressure & R-Factor
You most likely will have Altimeter Setting information that needs to first be converted to Station Pressure using the equation above.
You can get historical temperature and dew point information from here. Click 'History Data' --> Enter your location --> Click Custom.
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Pressure Altitude (Ft) =  Ap = (1-(Station Pressure in MB/1013.25)^0.190284)*145366.45 



Jim

Offline kickroot

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Re: Atmospheric pressure: absolute to relative conversion
« Reply #2 on: April 29, 2013, 12:49:51 PM »
Thanks SoMDWx.  I'll try to work through those. This site also looks helpful: http://www.meteo.tomkii.net/pressure_calc


Offline RPinCA

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Re: Atmospheric pressure: absolute to relative conversion
« Reply #4 on: March 22, 2016, 06:04:45 PM »
Resurrecting this old thread to keep my question in context. I'm writing software to convert the output stream from Gary's Wunder Weather Station to Cumulus, using its EasyWeather.dat file import mechanism. Steve posted http://sandaysoft.com/forum/viewtopic.php?f=10&t=5683, in which he says: "You should be reading and reporting relative pressure. This is the pressure corrected for your altitude, and it means that everyone reports values which correlate, regardless of their altitude." The import mechanism asks for both absolute and relative pressures, so the goal was to compute absolute pressure from the elevation-corrected pressure gather from the WWS.

I came upon this WXForum thread and tested the equation from @SoMDWx in Excel. I am a bit confused now, either by the definition of relative pressure from Steve, or by the discussion here. From the OP's question, the equation should give relative pressure from absolute, but it appears to provide absolute pressure from relative. Using a pressure of 29.92", here are some examples: at zero elevation, the equation result is 29.92". At 5,000' elevation, the equation results in 24.89", while 10,000' results in 20.57". Thus, I concluded the equation provides absolute pressure from relative pressure.

If anyone can help me understand my mistake, or clarify the definitions here and in Steve's Cumulus post, I would sure appreciate it.

Thanks... Randy

Offline CW2274

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Re: Atmospheric pressure: absolute to relative conversion
« Reply #5 on: March 22, 2016, 06:16:22 PM »
I know this doesn't answer your question, but if you use the altimeter you'll be accomplishing the same thing, if I'm getting what you're after.

Offline RPinCA

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Re: Atmospheric pressure: absolute to relative conversion
« Reply #6 on: March 22, 2016, 06:45:15 PM »
I wasn't clear in my earlier post. I am using the equation:

Station Pressure =  Ps = Altimeter in Inches * ((288 - 0.0065 * Elevation in Meters)/288)^5.2561

Now trying the Altimeter Setting in Excel.

Offline RPinCA

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Re: Atmospheric pressure: absolute to relative conversion
« Reply #7 on: March 22, 2016, 07:26:26 PM »
At this site, https://wahiduddin.net/calc/pressure.htm, the author states:

"To get a feeling for the magnitude of the absolute pressure, the absolute pressure in the idealized International Standard Atmosphere (ISA), on a standard day, is 29.92 in-hg at sea level. At 2000 ft the absolute pressure drops to 27.82 in-hg, at 4000 ft the absolute pressure is 25.84 in-hg, at 6000 ft the absolute pressure is 23.98 in-hg, and at 8000 ft the absolute pressure is 22.23 in-hg."

These conversions correspond exactly to the results I get computing "Station Pressure" using the formula above. The formula computes absolute pressure from relative pressure. One would need to invert the equation to obtain relative pressure from absolute pressure (where relative pressure is defined as stated by Steve).

Offline Cutty Sark Sailor

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