Cumulus Support

Big Daddy wrote: ↑Sat 02 May 2020 8:31 am 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.

Unless my understanding is wrong, because I don't have any Ecowitt stuff, the height of those sensors is irrelevant.

Surely the barometric pressure is measured within the console? I believe it is the height where you park or mount the console that matters for the adjustment.
EDIT: Sadgit has just confirmed this as I typed my longer answer.

You are absolutely right, my error, brain not in gear yet this morning. Obviously need more coffee

Sadgit wrote: ↑Sat 02 May 2020 7:22 am 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...

Looks good. Close enough.

For 52 meters I get 6.23 Rel Offset.
https://keisan.casio.com/exec/system/1224579725

Don't use Windy map for data. Only use Windy to get an idea of isobars to see if METAR lines up. Windy is delayed and there is too much averaging going on.

On your Davis after you have entered in your correct elevation, then you still need to calibrate to match up with METAR.
You can see how to do this on page 27 of the VP2 user manual:
https://www.davisinstruments.com/produc ... _06312.pdf

Big Daddy wrote: ↑Sat 02 May 2020 8:31 am Once fully installed I can then look at calibrating the Abs pressure more accuratley. Its all very complicated, but interesting.

You look spot on now. It looks like you are perfectly calibrated.

Tip - don't look at EGXC (Coningsby(RAF) pressure in mb units. Looks like they are rounding to whole hecto Pascal. Instead look at the inHg units and then convert to hPa (mb).
https://www.convertunits.com/from/hpa/to/inhg

galfert wrote: ↑Sat 02 May 2020 10:23 am Tip - don't look at EGXC (Coningsby(RAF) pressure in mb units. Looks like they are rounding to whole hecto Pascal.

It is part of the instructions for reporting METAR, that these pressure (or Q values) are truncated down to an integer. METAR dates back to communication by teletype days and aims to use very short messages.

galfert wrote: ↑Sat 02 May 2020 10:23 am Instead look at the inHg units and then convert to hPa (mb).

Do NOT do that.
The conversion to inHG on https://aviationweather.gov/metar/data? ... e=&hours=0 is no more accurate, it is based on the conversion from the value shown in brackets, and the true value might be .9 hPa higher. Both are taken from the entry in the raw METAR beginning with Q which is I believe being interpreted by that USA site as altimeter pressure QFE, i.e. at level for a aircraft cabin sitting on the runway. It may actually be a SLP, as outside North America the Q element is used for QNH (nautical height to a pilot being sea level to you or me). Believe me, I spent a lot of months studying METAR several years ago, and I don't trust sites that convert RAW METAR, they are not consistent in the assumptions they make.

The latest complete METAR at time of typing is

2020/05/02 10:50 EGXC 021050Z 28010KT 9999 SCT035 14/05 Q1009 RMK BLU

and that tells me the Q1009 is reporting SLP.


I believe there is already a link to https://cumuluswiki.org/a/Pressure_Measurement#Datum earlier in this thread, where it mention that the pressure reported is not always sea level. Some RAF stations follow UK guidance and report SLP in the international standard part of METAR for pressure that is prefixed by Q, but some were (or still are) staffed by USAF, particularly in East Anglia, and they follow US guidance reporting airfield level pressure (actually it is a standard cabin height above the runway) in that standard METAR pressure section. (US put sea level pressure in the non-standard Remark part of METAR). Because of these problems and other variations between METAR reporting stations, a proposal to replace METAR was made many years ago, and the new standard is in use; but this replacement is in XML/Geo code designed for machines to read (e.g computers flying aircraft), and it is not made available to public.

Very informative. Thank you! I guess things are different across the pond. You've spiked my interest to dig further into this.

galfert wrote: ↑Sat 02 May 2020 11:49 am You've spiked my interest to dig further into this

Perhaps you would like to start with I wrote that in 2016. It is too long ago to recall what I wrote there, but it was comprehensive and is one place where I noted my source references, the other was in respective scripts.

If that inspires you to learn more about my project, you will find all my METAR posts on this forum if you do an advance search (leave normal search box empty and hit red Search button. I do need to say, I would do the project differently if I had time to restart it, almost nobody liked my approach back then, but hindsight often gives that effect.

Very nice. Thank you for sharing, I will read through this. One thing is missing from it though.... Your name!

My WD-40® Specialist® Anti-Friction Dry PTFE Lubricant arrived 30 minutes ago, so it has been used but not dry.. and the heavens have opened... Stop rain far too early!!

Well that was a waste of 7 pounds. Water still stays on the surface of the collector. It doesn't bond to it just a film that can be wiped off. Cancel your order if you can.

Oh, thats disappointing. The impression I got was that it bonded to the surface and dried to create a clear friction free surface. Too late to cancel mine as already been dispatched and arriving in next few days. Need to have a look for an alternative.

On the upside, I managed to get the instuments permanently sited at the weekend. I need to find some bigger and better clips to hold the rain gauge in place but its all I had and didnt want to go queue at my local DIY store just for a couple of clips. The mast fixings may be a little over the top, but its a 3m x 40mm thick steel pole and quite heavy. It wont wobble in the wind though and can quite easily be extended. As its in 2 x sections I can lower it down in the brackets and take off the bottom section and then lower down to access the instruments. Likewise a new section can be added from the bottom and the whole thing pushed up.

any joy with teflon coatings? I'm struggling to find something that isn't just a lubricant!

Unfortunately not yet.

I did find this which is also sold as a dry lubricant but its £12.95 shipping so not worth the risk

https://www.scparts.co.uk/sc_en/teflon- ... gJHZPD_BwE

I was watching a few videos on youtube about the WD40 product and it does tend to show this as a product that dries. They just give it a quick blast and "it dries in seconds". May wipe mine off and try again. I did ask Ecowitt what they used in their video and was told it was just a spray on teflon but I cant find anything that looks like it will give us the same effect.

Proper Teflon looks like it needs to be baked on, on cookware that sort of thing. I did think about buying some Teflon sheets used to line baking trays and trying to make an insert for the top part of the funnel as this is where the water gets stuck.

Will keep looking.

yeah I tried again with the WD40 and it is basically like normal WD40 wet not dry.. most odd

that product is the same this is a lubricant spray just like WD40

EDIT - I have some waterproof spray for my (now hardly used due to lockdown) boots.. might give that a blast!

Just wondered but what about Rain X the stuff you put on car windscreens to repel water. No idea if it would work, just a suggestion.