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727 posts

Ultimate Geek
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Topic # 223335 24-Sep-2017 12:48
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The manual for my WS1151 says nothing useful about which is the best setting.


I previously had it set on 'Relative', but today I decided to change this to 'Absolute'.


Am I correct in assuming that 'Absolute' detects the actual pressure at the station?

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601 posts

Ultimate Geek
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  Reply # 1872189 24-Sep-2017 21:11
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From a quick squiz at the instructions, it looks like relative mode tells you how far above or below a certain point the current reading is. Default for this mode is 1013.5 hPa.




In relative mode, it should show low figures (10-20 ish max) above or below that. I suggest you put it in relative mode and have a look at an isobar map on metservice's site. You should get a reasonably close figure for where you are, depending on current temperature (and your altitude of course). The difference between the current reading from metservice and 1013.5 should be shown on your display +/- 1.5 hPa.




If you are at any kid of serious altitude your readings wont be so comparable to the isobar charts which i think are at sea level. I don't think your unit can correct for altitude.

727 posts

Ultimate Geek
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  Reply # 1872348 25-Sep-2017 11:10
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elpenguino, thanks for the info.


I've run the unit on 'relative' for 10 years or so, mainly because the manual recommends it.


But I've always wondered how that setting differed from 'absolute'.


Anyway, it's on absolute now, and my query was, does absolute mean that it's registering the 'actual' air pressure at the external sensor?


We don't get any local info from Met Service because the town - Otautau, 50k NW of Invercargill - isn't hooked into the Met network.


But I noticed yesterday that my WS1151, set on absolute, was showing a very similar pressure to Invercargill.


And today's the same, Invercargill is currently 993 hpa, while my reading is 985. I guess the 8 hpa difference could be reflecting our slight elevation from Invercargill. I have no idea what the elevation of Otautau actually is, but I'd guess at around 30 - 50m above sea level.







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Uber Geek
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  Reply # 1872379 25-Sep-2017 11:50
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According to Otautau's elevation is about 60m ~= 200ft, whereas Invercargill airport is at 2m ~= 5ft. On a standard day, you would be about 6hPa less than at Invercargill.







727 posts

Ultimate Geek
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  Reply # 1872468 25-Sep-2017 13:55
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Thanks, frankv. Makes sense.


So, that would seem an affirmative answer to my question - 'If the Station is set at 'absolute', is the the displayed pressure actually the pressure of the air surrounding the sensor?'

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Ultimate Geek
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  Reply # 1872561 25-Sep-2017 15:50
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that's right. As I mentioned, a common use for pressure readings is when they are compensated for altitude and reduced to sea level. From the manual your unit does not do that so what you are getting is the 'cistern pressure' i.e. the raw pressure at the sensing element.




If you want to to check it you could take your unit to town next time and park next to the airport and compare readings.

727 posts

Ultimate Geek
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  Reply # 1872648 25-Sep-2017 19:19
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elpenguino, ta for that, good call about the airport check.


Actually, to save confusion earlier, I didn't mention that I have two WS1151 stations.


The original unit - now about 6 years old - had always had a faulty display, although I'd never realized it. The display of black blocks (9 columns of 7 blocks) under the pressure numbers had always been stationary, but I now know that this display is supposed to be deleted and redrawn every five seconds or so, to reflect ongoing pressure changes.


When I asked the new distributor if there was a firmware upgrade that might fix the problem, he said no, but kindly sent me the new unit without charge.


However, the two units, although receiving data from the same sensor, show different absolute pressures. The old one is always 2.5 hpa higher than the new.


With Invercargill Airport currently at 994 hpa, the newer unit shows 986.7 and the old, 989.2.


Still, these WS1151 stations aren't precision instruments, are they? wink





601 posts

Ultimate Geek
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  Reply # 1872768 25-Sep-2017 22:53
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Welllll, I would take the approach that you hope to get some useful information from it. The usefulness is related to the price :-)




In that case you might be able to assess your unit over time and see if the trend information from the barometer is more accurate than the absolute accuracy.


Some kind of spreadsheet perhaps?? ( I love a good spreadsheet !)


Do the units have any ability to store an offset / correction factor?

727 posts

Ultimate Geek
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  Reply # 1873000 26-Sep-2017 11:59
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No, they're fairly basic. But they're near enough for my needs...


Cheers :-)

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