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Author Topic: confused with barometer  (Read 9895 times)

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superdino

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confused with barometer
« on: April 08, 2003, 05:46:18 PM »
Hello,

well, yesterday I installed my WMR918N weather station. I am very fascinated by all the reports that Weather Display (9.70c) generates.

But I have not figured out, how the barometer pressure works right, and how it has to be set in the Weather Display software.

Let me explain:
my alitude is 560 mt over 0.
Currently CNN.com/weather shows me a barometer pressure of 1013 hPa (comes from the local airport - that is near here -- but at 220 mt over 0).

Ok - on the portable indoor unit (the white one thats indoor thermo & hygro and baro) shows me actually 945mb.
So on the main touch-screen unit I have adjusted the sealevel the way that it shows me up 1013 mb as cnn.com/weather reports. Is this correct?

We have clear weather today. Is this correct till here, how I did it?
Must I adjust/recalculate something as I am at 560mt, and not at 220 mt over sealevel?

2nd question:
When firing up Weather Display the software shows me 884 hPa. Shouldn't it bee 1013 or 945?

What's wrong?

Thanks for your help!

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confused with barometer
« Reply #1 on: April 08, 2003, 07:10:42 PM »
You need to set an offset in WD so that is shows the correct local pressure. Your WMR918N is reading 61mb low which is the same as my WMR928N does. You need to go to:

Setup | Enter Barometer offset, other offsets and Rainfall

...and then enter 61.0 into the Barometer Offset (mb) field. WD should now show the same as the reading on your console/sensor unit.

Secondly, atmospheric pressure reduces by approximately 1mb per 10m gain in height. As your local airport is 340m lower than you, the pressure there will be 34mb higher than your local reading (a more accurate calculation gives a 38-40mb reduction in pressure between the airport and you). If the airport is reading 1013mb, then you should be reading around 974mb.

However, check the time that is given for the airport pressure reading. My local airport only seems to update it's info every few hours and if CNN are only updating their details every few hours you could find the current airport reading is significantly different to the one reported on CNN.

http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/barfor.html allows you to calculate the effect of height on pressure.

Finally, WD calculates the pressure at mean sea level (MSL) if you tell it how high you are. This means that you don't need to put an offset into your console. Go to:

Setup | Display Units Options/reset times/altitude/other settings

...and enter your height (in feet) into the Height above sea level field.

Hope this helps.
Chris
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confused with barometer
« Reply #2 on: April 09, 2003, 12:52:17 AM »
the last bit, setting your altitude under the units setup, that is for if you then tick use QFE barometer
otherwise chris is correct with the rest, and just basicly set your barometer, via the offset, to your nearest airport reading (the altitude difference is not important for the final result, but any differences in actual pressure between you and the aiport needs to be taken into account, and they need to be taken at the same time.
i.e compare during light winds in a big hogh pressure , or when the isobars line up with you and the aiport...

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confused with barometer
« Reply #3 on: April 09, 2003, 08:51:41 PM »
Quote from: windy
the last bit, setting your altitude under the units setup, that is for if you then tick use QFE barometer


I didn't know that. However ticking the box doesn't seem to have done anything. My sensor, console, WD and web page (corrected to MSL value) all still display the same value. I'd have expected the 'corrected to MSL' value to be 2mb higher than local pressure because I'm approx 20m ASL.
Chris
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confused with barometer
« Reply #4 on: April 11, 2003, 06:59:38 PM »
Quote from: windy
the last bit, setting your altitude under the units setup, that is for if you then tick use QFE barometer


I don't understand what the QFE tick box does. My baro sensor and console are showing 1006mb and WD is is reporting the same value on the web page. As I'm approx 60ft ASL I'd have expected WD to report 1008mb.

I've restarted WD a couple of times since ticking the box. What am I doing wrong?
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confused with barometer
« Reply #5 on: April 11, 2003, 07:46:47 PM »
QFE uses the altitude setting and mean temperature to arrive at the field barometer reading.Its usefull for areas above 5000 feet and which get very cold temperatures.
if you use the offset to get the barometer reading in WD to be that which it should be for your altitude (under setup, units) before you tick use QFE, then after ticking that, the altitude will be taken into account, and the resulting reading will be sea level adjusted.

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confused with barometer
« Reply #6 on: April 12, 2003, 05:58:39 AM »
I'm probably making a big thing out of something insignificant because I'm not too far above MSL anyway, but I'm not sure how I should set my barometer up. Clearly the QFE setting is inapproriate in my location as I'm nowhere near 5000ft and it's a pretty temperate climate here.

The default web page says the barometer setting is corrected to MSL, so I assumed by setting the altitude/QFE I would be setting WD up to correctly adjust my local value.

Take the current situation. My baro sensor, the WMR928N console, WD on my PC and the default web page are all currently showing 1009mb. As I'm around 60ft ASL, the pressure at MSL should be around 1011mb, so I'd expect at least the web page to report that value.

I can't adjust the sensor value, so the sensor will always show the local pressure. I can set the sea level offset in the WMR928N console or I can adjust the baro offset in WD. Which of these is the preferred option, or doesn't it really matter?
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confused with barometer
« Reply #7 on: April 12, 2003, 10:27:35 AM »
adjust both the console (so its correct when you look at it), and WD (which uses the raw unadjusted reading from the WMR968)
and use your closest aiport/metar reading to set to, taking into account any isobar gradient at that time (use the real time graph to get the reading at the same time as the metar reading was)....

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confused with barometer
« Reply #8 on: April 12, 2003, 03:28:37 PM »
Quote from: windy
(so its correct when you look at it)


I think I'm trying to work out what 'correct' means. Does it mean, local pressure (as shown on the sensor) or the pressure corrected to MSL?

The local airport gives METARs (EGNH) and is only a mile and a half away, so that's a good source of readings. However the airport is 30-60 feet lower than me (and pretty close to MSL), so their reading is likely to be 1-2mb higher than mine. Do I take account of this difference and if so, where?
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confused with barometer
« Reply #9 on: April 12, 2003, 05:37:31 PM »
it does not matter what the altitude of the airport is...compared to you, as they are reporting sea level barometer pressure, and that is what you want your to report.
so you just set yours to theirs
easy as that
(and take into account any likely cange due to actual isobar pressure gradient at the time)

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« Reply #10 on: April 13, 2003, 07:22:13 AM »
Ahh, it hadn't clicked that the METAR pressure was adjusted to MSL. I've just checked and the airport is reporting 1018 to my 1016 which is the 2mb difference that I'd expected.

Thanks Brian.
Chris
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