Cumulus Support

Big Daddy wrote: ↑Fri 01 May 2020 7:48 am My understanding is that pressure is always meteorogically reported as the Relative (sea level) figure, this way you get a constant "global" reading. If you report Absolute (station) then it could vary greatly even within a short distance as the house on top of the hill will report a different (lower) value to the house in the valley.

OK. But that would mean, that CumulusMX must store the sealevel figure (which apparently it does under the assumption that the weather station makes the correction):

All weather stations allow you to adjust the pressure for sea level. You should not make the adjustment within Cumulus.

Cumulus apparently stores the uncorrected pressure as measured since version 1.8.6 and not the MSLP (in sending to CWOP).

On the other hand in the Wiki article on realtime.txt it says that webtag <#press>, field 11 gives the Sea level Pressure.

I am confused and maybe it is time @mcrossley says something about this.
This seems pretty important to me. Where and when are these corrections to MSLP made? And how do we know if and how the weather station actually makes the MSL correction before being read/stored by Cumulus?

Apparently I have to tell my Davis to correct the measurement it is taking, to Sea level. Is that correct?
And btw. no matter what, talking about sea level correction when calibration is meant, seems incorrect to me.

NOTE: my confusion comes from the fact that I live just below sealevel (-1 meter for which I use 0) so I never bothered. I should have, so now I am catching up. And it is very relevant for my pwsFWI / Fire Weather Index module.

HansR wrote: ↑Fri 01 May 2020 5:59 am @galfert:
@Sadgit:
I don't really understand what you are doing / saying here (and yes I do understand the calculation that is not my point)?

My comment to Sadgit was just explaining to him that the hardware that he got would always be wrong out of the box unless he lived at sea level. Because his equipment works differently from yours. You have it much simpler. You have a Davis. With Davis you just give your console your elevation and it does the offset calculation for you (the difference between Absolute station pressure and Relative pressure). With the GW1000 there is no place to enter in your elevation directly. The user must manually calculate the elevation offset. They must do this one time...just as you set your elevation in your console one time. It would be as if your Davis console shipped with elevation set to 0 and you just plugged it in. Sure you could adjust the pressure till it matches the local METAR but then your console would still think it was at sea level (because of elevation zero), this would then throw off your station pressure (Absolute pressure) that is use for other calculations such calculating Altimeter pressure by Cumulus for use of CWOP. You wouldn't think of not giving your station your elevation and leaving it at zero. The same needs to be considered for the GW1000 but in a different way as there is no way to directly give it your elevation, so that elevation is given to the GW1000 by manually doing some math and figuring out what the Relative Offset is which matches your elevation. Just be glad you have a Davis as you don't have to worry about this.

Big Daddy wrote: ↑Fri 01 May 2020 7:48 am Thanks for the info.

I am at 16ft elevation so roughly 5m and have my Rel pressure already set to an offset of +0.6 but as you noted it does look as if my Abs pressure is 2 hpa out compared to Conningsby.

Using your information I have adjusted my pressure in line with Conningsby which is slightly different to what Windy shows but will keep making minor adjustments if needed to get it right.

Nice move...getting there. I just now checked and you are now still 0.6 hPa too high. Lower your Abs Offset by -0.6 hPa ...that means don't enter that as a value but rather subtract from what value is there now an additional -0.6 hPa from Abs Offset. (this post edited ...I originally said 0.8 hPa too high..an hour later isobars still line up and it looks more like 0.6 hPa is the correction)

The isobars right now that I checked are perfectly aligned with Coningsby RAF and your location. When this is the case is the only time when your pressure and theirs will be the same. THIS is when you calibrate! But I've now done that for you and seen that the correction is -0.6 hPa.

By the way I use Windy in inHg because the precision is higher that way. Unfortunately when you switch Windy to hPa the precision is only whole hecto Pascals.
I only need this precision to help me draw isobars, which helps me decide if now is a good time to fine tune calibrate or not. I ignore the Windy values when it comes to actual calibration numbers and use the values from the METAR sites instead as they are less delayed.

@galfert:
OK, thanks for the clear explanation. I got it after writing my remark above and you confirm it. Thanks for that.
So Cumulus always registers the relative pressure.
And yes, I'am happy with the Davis, but I set it up so long ago that I really do not remember this and I missed an important point on how it is registered and worked
Thanks.

I calibrated mine so it was the same as my Davis, as I know that does the calculation with me being 52m above seal level

Rain today davis 2.8, ecowitt 1.1, I haven't looked in the cup as it is on the garage roof and cannot be chewed to look, but I suspect it will be covered in water drops

PS thanks for the tip about FedEx as I haven't paid yet.

and I wonder if a spray and rub of wd40 would also do the rain guage some help

Sadgit wrote: ↑Fri 01 May 2020 4:21 pm I wonder if a spray and rub of wd40 would also do the rain guage some help

Sadgit wrote: ↑Fri 01 May 2020 4:21 pm it is on the garage roof

There are a range of products from WD40. Everyday WD40 would work by lowering surface tension so stopping drops sticking for a short time, but as it is a lubricating liquid, not designed as a permanent coating, it will eventually run through your gauge and/or evaporate, so you would have to climb onto that garage roof again!

Interesting discussion

@galfert - my Abs offset is currently 2.0 so you suggest lowering by -0.6 so this would make it 1.4.

@sadgit / sfws - I have some https://wd40.co.uk/specialist/anti-fric ... lubricant/ arriving middle of next week. Will give it a spray and pray for rain

Andy

i had a strange high temp of 60C at 2pm today.. 100% certain it wasn't real

Big Daddy wrote: ↑Fri 01 May 2020 6:06 pm I have some https://wd40.co.uk/specialist/anti-fric ... lubricant/ arriving middle of next week

That product is designed to set onto plastic. I have edited my post to make it clearer, I was talking about a different product to yours, Andy. There are of course many other coatings available for same purpose of making rain run off. Hope it works for you.

I am still using my original Fine Offset rain gauge (their first design with shallow lip), a decade old, it does not have as good sensitivity as I would like for the short showers of today, but with larger rainfall (like the severe flooding event in February), or longer showers of other days recently, it does match a separate rain gauge quite accurately. So I am only reading this topic to learn about new designs in case I ever need to consider them.

Sadgit wrote: ↑Fri 01 May 2020 6:16 pm i had a strange high temp of 60C at 2pm today.. 100% certain it wasn't real :D

Sounds like even under brand name of Ecowitt, Fine Offset electronics and general design is still not 100% reliable!

Actually my Abs offset was -2.0 so adjusted to -2.6

Big Daddy wrote: ↑Fri 01 May 2020 9:33 pm Actually my Abs offset was -2.0 so adjusted to -2.6

Okay, yes that is perfect. We should keep an eye on it. Wait till tomorrow around noon. Hopefully isobars will be in line...if not then we just wait for another day. But I think it should be better.

By the way where is your GW1000 located? First floor or second floor? And how high off the floor is it?

I used the guide for pressure on page 1, instead of just matching it to my Davis. The Eccowit now reports 1005.1 and the Davis 1006.60. The local METAR is showing 1005 and windy showing (29.68inhg) = 1005.08 hpa

God knows why the Davis is so high?

52m above sea level
rel offset 6.3
abs offset -2.7

I think...

galfert,
Currently sensors are on my patio table so at 16ft / 5m above seal level. They will eventually be on a 3m / 10ft pole so roughly 26ft / 8m above seal level. Using the calculation i think I then need to increase my relative offset from 0.6 to 0.96.

Once fully installed I can then look at calibrating the Abs pressure more accuratley. Its all very complicated, but interesting.

isn't it the location of the GW1000?