Barometer accuracy query

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Boswell
Posts: 30
Joined: 17 Dec 2015, 19:00

Barometer accuracy query

Post by Boswell » 17 Dec 2015, 19:05

I've only just started to use my weather station, and I'm curious about the accuracy of the barometer, as for my use, it's an important reading.

Looking at the Netatmo WeatherMap, within a 10 mile radius of my location, the pressure readings of other users vary by about 6 mb (i.e. from 1013-1019).

Why are the readings so variable?

In theory, the mean sea level pressure (i.e. corrected for height) should be reasonably identical within that small radius.

Is Netatmo really that inaccurate?

ballogie
Posts: 1
Joined: 29 Dec 2015, 12:00

Re: Barometer accuracy query

Post by ballogie » 29 Dec 2015, 12:02

Same problem noticed here. Variation of 4mb in a 3km radius - why the discrepancy of readings?

Boswell
Posts: 30
Joined: 17 Dec 2015, 19:00

Re: Barometer accuracy query

Post by Boswell » 29 Dec 2015, 12:31

I don't know the logic of the pressure readings we're measuring.

It could be (and only Netatmo can tell us) that the readings we are getting are actual pressure readings at the measuring site. That makes logical sense.

But they aren't corrected for altitude of the observing station (possible, but unlikely, though Google maps give elevation). In other words (aircraft jargon warning - we are measuring QFE rather than QNH).

So we aren't measuring pressure corrected for sea level, and our readings will vary with the station's altitude (at approx. 30' / mb).

If we look at Netatmo stations in the Swiss Alps, for instance, then the readings could vary a great deal (i.e. mountain resort / valley town).

I haven't got time to investigate until later.....but it could be that the pressure readings we're getting are rather irrelevant unless we know the altitude of the station we're looking at.

Later

There are a couple of crude ways of checking the accuracy of the barometer readings:

1) Find a current sea level synoptic chart and look at the approx. orientation of the isobars. Then pick a line of Netatmo stations approx. along that line - do they indicate the same pressure? No, as far as I can see, but the stations might not be at the same altitudes.

2) So pick a bunch of Netatmo stations at approx. sea level. For instance, the north west corner of France. There are a dozen or so, all approx. at sea level along the coast line. Do they indicate approx. the same pressure? Not as far as I can see.

Anyone more clued-up on this?

(This isn't a 'complaint'. I want to understand how to get an accurate pressure reading, corrected to sea level, for my own station)

Boswell
Posts: 30
Joined: 17 Dec 2015, 19:00

Re: Barometer accuracy query

Post by Boswell » 31 Dec 2015, 00:37

Looking through the information provided on the iPhone app, utilising our Netatmo base station, we are measuring the actual 'raw' pressure corrected by our altitude to give mean sea level pressure.

How does my station know its altitude? According to Netatmo this was detected by my smartphone's GPS's ability to accurately position my station, and therefore give my altitude when I setup the station (and of course, if you live on a mountain, near a precipice, this could be wildly inaccurate).

However, I didn't use my smartphone to setup the station, I used my PC, and my PC doesn't know its position, therefore my Netatmo station doesn't know its altitude.

By this logic only the stations which used a smartphone to setup the installation will be transmitting an approximately correct mean sea level pressure.

Therefore those of us who used a PC to setup our station will not be transmitting an accurate sea level pressure (unless we are on the coast, at sea level) and it could be incorrect by approx. 30' per millibar (30 feet per Hector Pascal?).

That could be the explanation for the wide difference in pressure readings in a relatively small inland area, as seen on the weather map, but relative conformity for stations along a coastline (but only in some cases, on a second examination of the Natatmo weather map).

I hope I'm wrong and Netatmo tech support will explain my faulty logic as to why we (Netatmo stations) appear to display a wide array of apparently incorrect barometric readings.

In theory, the solution is simple: we all have to use a smartphone to setup our stations?

Northender71
Posts: 10
Joined: 02 Jan 2016, 01:31

Re: Barometer accuracy query

Post by Northender71 » 02 Jan 2016, 01:42

Hi,you can set the altitude using the web app under location settings. Mine seems spot on but I also notice some others locally that seem way off.

Boswell
Posts: 30
Joined: 17 Dec 2015, 19:00

Re: Barometer accuracy query

Post by Boswell » 02 Jan 2016, 09:37

Hi Northender, thanks for the reply.

When you say yours was spot on, was that the altitude, or pressure or both?

Did it 'know' you altitude without manual tweaking?

Northender71
Posts: 10
Joined: 02 Jan 2016, 01:31

Re: Barometer accuracy query

Post by Northender71 » 02 Jan 2016, 12:39

I setup my station using a laptop and then tweaked the altitude on the web app. I think the laptop setup was close anyway, within a couple of metres, which wouldn't have any real impact on pressure readings.

I check the netatmo readings against another barometer that doesn't adjust to mean sea level and I have some tables which give adjustment figures for different altitudes. I use 1.8mb for my 50ft elevation. I also check against online charts (I found a good one but the forum will not allow me to post the url). In comparison to this other data, the netatmo readings correlate.

There is a another netatmo station a few hundred metres from me which appears to be about 5mb too low, even though its altitude looks correct.

webvan
Posts: 52
Joined: 08 Jan 2014, 10:28

Re: Barometer accuracy query

Post by webvan » 09 Jan 2017, 15:00

It seems we'd need a way to adjust the "raw reading" of the station but alas there is no such setting under advanced, only a rather useless CO2 calibration setting !

You'd think Netatmo would be interested in collecting accurate pressure data but I suppose they look at trends and the QNH is easy enough to get from a local airport...

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